Historical Models Of The Solar System
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Historical models of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
first appeared during prehistoric periods and remain updated to this day. The models of the Solar System throughout history were first represented in the early form of cave markings and drawings,
calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single and specific day within such a system. A calendar is ...
s and astronomical symbols. Then books and written records became the main source of information that expressed the way the people of the time thought of the Solar System. New models of the Solar System are usually built on previous models, thus, the early models are kept track of by intellectuals in astronomy, an extended progress from trying to perfect the geocentric model eventually using the
heliocentric Heliocentrism (also known as the heliocentric model) is a Superseded theories in science#Astronomy and cosmology, superseded astronomical model in which the Earth and Solar System, planets orbit around the Sun at the center of the universe. His ...
model of the Solar System. The use of the Solar System model began as a resource to signify particular periods during the year as well as a navigation tool which was exploited by many leaders from the past. Astronomers and great thinkers of the past were able to record observations and attempt to formulate a model that accurately interprets the recordings. This scientific method of deriving a model of the Solar System is what enabled progress towards more accurate models to have a better understanding of the Solar System that civilization is located within


Prehistoric astronomy


Early astronomy


The Nebra Sky Disc

The Nebra Sky Disc is a bronze dish with symbols that are interpreted generally as the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
or full moon, a lunar
crescent A crescent shape (, ) is a symbol or emblem used to represent the lunar phase (as it appears in the northern hemisphere) in the first quarter (the "sickle moon"), or by extension a symbol representing the Moon itself. In Hindu iconography, Hind ...
, and stars (including a cluster of seven stars interpreted as the
Pleiades The Pleiades (), also known as Seven Sisters and Messier 45 (M45), is an Asterism (astronomy), asterism of an open cluster, open star cluster containing young Stellar classification#Class B, B-type stars in the northwest of the constellation Tau ...
). The disc has been attributed to a site in present-day Germany near Nebra,
Saxony-Anhalt Saxony-Anhalt ( ; ) is a States of Germany, state of Germany, bordering the states of Brandenburg, Saxony, Thuringia and Lower Saxony. It covers an area of and has a population of 2.17 million inhabitants, making it the List of German states ...
, and was originally dated by
archaeologists Archaeology or archeology is the study of human activity through the recovery and analysis of material culture. The archaeological record consists of Artifact (archaeology), artifacts, architecture, biofact (archaeology), biofacts or ecofacts, ...
to , based on the provenance provided by the looters who found it. Researchers initially suggested the disc is an artefact from the Bronze Age Unetice culture, although a later dating to the
Iron Age The Iron Age () is the final epoch of the three historical Metal Ages, after the Chalcolithic and Bronze Age. It has also been considered as the final age of the three-age division starting with prehistory (before recorded history) and progre ...
has also been proposed.


Firmament

The ancient Hebrews, like all the ancient peoples of the Near East, believed the sky was a solid dome with the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
,
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
,
planets A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the te ...
and
stars A star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of ...
embedded in it. In biblical cosmology, the
firmament In ancient near eastern cosmology, the firmament means a celestial barrier that separates the heavenly waters above from the Earth below. In biblical cosmology, the firmament ( ''rāqīaʿ'') is the vast solid dome created by God during the G ...
is the vast solid
dome A dome () is an architectural element similar to the hollow upper half of a sphere. There is significant overlap with the term cupola, which may also refer to a dome or a structure on top of a dome. The precise definition of a dome has been a m ...
created by God during his creation of the world to divide the primal sea into upper and lower portions so that the dry land could appear.


Babylonian interpretation

Babylonians thought the universe revolved around heaven and Earth. They used methodological observations of the patterns of planets and stars movements to predict future possibilities such as
eclipses An eclipse is an astronomical event which occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ...
. Babylonians were able to make use of periodic appearances of the Moon to generate a time source - a calendar. This was developed as the appearance of the full moon was visible every month. The 12 months came about by dividing the ecliptic into 12 equal segments of 30 degrees and were given zodiacal constellation names which were later used by the Greeks.


Chinese theories

The Chinese had multiple theories of the structure of the universe. The first theory is the ''Gaitian'' (celestial lid) theory, mentioned in an old mathematical text called ''Zhou bei suan jing'' in 100 BCE, in which the Earth is within the heaven, where the heaven acts as a dome or a lid. The second theory is the ''Huntian'' (Celestial sphere) theory during 100 BCE. This theory claims that the Earth floats on the water that the Heaven contains, which was accepted as the default theory until 200 AD. The ''Xuanye'' (Ubiquitous darkness) theory attempts to simplify the structure by implying that the Sun, Moon and the stars are just a highly dense vapour that floats freely in space with no periodic motion.


Greek astronomy

Since 600 BCE, Greek thinkers noticed the periodic fashion of the Solar System (then regarded as the "whole
universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...
") but, like their contemporaries, they were puzzled about the forward and retrograde motion of the planets, the "wanderer stars", long taken as heavenly
deities A deity or god is a supernatural being considered to be sacred and worthy of worship due to having authority over some aspect of the universe and/or life. The ''Oxford Dictionary of English'' defines ''deity'' as a God (male deity), god or god ...
. Many theories were announced during this period, mostly purely speculative, but progressively supported by
geometry Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician w ...
.
Thales of Miletus Thales of Miletus ( ; ; ) was an Ancient Greek pre-Socratic philosopher from Miletus in Ionia, Asia Minor. Thales was one of the Seven Sages, founding figures of Ancient Greece. Beginning in eighteenth-century historiography, many came to ...
alleged to have predicted the solar eclipse of 586 BCE. Around 475 BCE,
Parmenides Parmenides of Elea (; ; fl. late sixth or early fifth century BC) was a Pre-Socratic philosophy, pre-Socratic ancient Greece, Greek philosopher from Velia, Elea in Magna Graecia (Southern Italy). Parmenides was born in the Greek colony of Veli ...
claimed that the universe is spherical and moonlight is a reflection of sunlight. Shortly after, circa 450 BCE,
Anaxagoras Anaxagoras (; , ''Anaxagóras'', 'lord of the assembly'; ) was a Pre-Socratic Greek philosopher. Born in Clazomenae at a time when Asia Minor was under the control of the Persian Empire, Anaxagoras came to Athens. In later life he was charged ...
was the first philosopher to consider the Sun as a huge object (larger than the land of
Peloponnesus The Peloponnese ( ), Peloponnesus ( ; , ) or Morea (; ) is a peninsula and geographic regions of Greece, geographic region in Southern Greece, and the southernmost region of the Balkans. It is connected to the central part of the country by the ...
), and consequently, to realize how far from Earth it might be. He also suggested that the Moon is
rocky ''Rocky'' is a 1976 American independent film, independent sports drama film directed by John G. Avildsen and written by and starring Sylvester Stallone. It is the first installment in the Rocky (film series), ''Rocky'' franchise and also star ...
, thus opaque, and closer to the Earth than the Sun, giving a correct explanation of eclipses. To him, comets are formed by collisions of planets and that the motion of planets is controlled by the ''nous'' (mind).


Anaximander cosmology

Anaximander Anaximander ( ; ''Anaximandros''; ) was a Pre-Socratic philosophy, pre-Socratic Ancient Greek philosophy, Greek philosopher who lived in Miletus,"Anaximander" in ''Chambers's Encyclopædia''. London: George Newnes Ltd, George Newnes, 1961, Vol. ...
, around 560 BCE, was the first to conceive a mechanical model of the world. In his model, the Earth floats very still in the centre of the infinite, not supported by anything. Its curious shape is that of a cylinder with a height one-third of its diameter. The flat top forms the inhabited world, which is surrounded by a circular oceanic mass. At the origin, after the separation of hot and cold, a ball of flame appeared that surrounded Earth like bark on a tree. This ball broke apart to form the rest of the Universe. It resembled a system of hollow concentric wheels, filled with fire, with the rims pierced by holes like those of a flute. Consequently, the Sun was the fire that one could see through a hole the same size as the Earth on the farthest wheel, and an eclipse corresponded with the occlusion of that hole. The diameter of the solar wheel was twenty-seven times that of the Earth (or twenty-eight, depending on the sources) and the lunar wheel, whose fire was less intense, eighteen (or nineteen) times. Its hole could change shape, thus explaining
lunar phase A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. Because the Moon is tidally locked with the Earth, the same hemisphere is always facing the Earth. In common usage, the four maj ...
s. The stars and the planets, located closer, followed the same model. Anaximander was the first philosopher to present a system where the celestial bodies turned at different distances.


Pythagorean astronomical system

Around 400 BCE, Pythagoras' students believed the motion of planets is caused by an out-of-sight "fire" at the centre of the universe (not the Sun) that powers them, and Sun and Earth orbit that ''Central Fire'' at different distances. The Earth's inhabited side is always opposite to the Central Fire, rendering it invisible to people. So, the Earth rotates around itself synchronously with a daily orbit around that Central Fire, while the Sun revolves it yearly in a higher orbit. That way, the inhabited side of Earth faces the Sun once every 24 hours. They also claimed that the Moon and the planets orbit the Earth. This model is usually attributed to
Philolaus Philolaus (; , ''Philólaos''; ) was a Greek Pythagorean and pre-Socratic philosopher. He was born in a Greek colony in Italy and migrated to Greece. Philolaus has been called one of three most prominent figures in the Pythagorean tradition and ...
. This model is the first one that depicts a moving Earth, simultaneously self-rotating and orbiting around an external point (but not around the Sun), thus not being geocentrical, contrary to common
intuition Intuition is the ability to acquire knowledge without recourse to conscious reasoning or needing an explanation. Different fields use the word "intuition" in very different ways, including but not limited to: direct access to unconscious knowledg ...
. Due to philosophical concerns about the number 10 (a "
perfect number In number theory, a perfect number is a positive integer that is equal to the sum of its positive proper divisors, that is, divisors excluding the number itself. For instance, 6 has proper divisors 1, 2 and 3, and 1 + 2 + 3 = 6, so 6 is a perfec ...
" for the Pythagorians), they also added a tenth "hidden body" or
Counter-Earth The Counter-Earth is a : Hypothetical bodies of the Solar System, hypothetical body of the Solar System that orbits on the other side of the Solar System from Earth. A Counter-Earth or ''Antichthon'' () was hypothesized by the pre-Socratic philoso ...
(''Antichthon''), always in the opposite side of the invisible Central Fire and therefore also invisible from Earth.


Platonic geocentrism

Around 360 BCE when
Plato Plato ( ; Greek language, Greek: , ; born  BC, died 348/347 BC) was an ancient Greek philosopher of the Classical Greece, Classical period who is considered a foundational thinker in Western philosophy and an innovator of the writte ...
wrote in his '' Timaeus'' his idea to account for the motions. He claimed that circles and spheres were the preferred shape of the universe and that the Earth was at the centre and the stars forming the outermost shell, followed by planets, the Sun and the Moon. This is the so-called
geocentric model In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded scientific theories, superseded description of the Universe with Earth at the center. Under most geocentric m ...
. In Plato's
cosmogony Cosmogony is any model concerning the origin of the cosmos or the universe. Overview Scientific theories In astronomy, cosmogony is the study of the origin of particular astrophysical objects or systems, and is most commonly used in ref ...
, the
demiurge In the Platonic, Neopythagorean, Middle Platonic, and Neoplatonic schools of philosophy, the Demiurge () is an artisan-like figure responsible for fashioning and maintaining the physical universe. Various sects of Gnostics adopted the term '' ...
gave the primacy to the motion of Sameness and left it undivided; but he divided the motion of Difference in six parts, to have seven unequal circles. He prescribed these circles to move in opposite directions, three of them with equal speeds, the others with unequal speeds, but always in proportion. These circles are the orbits of the heavenly bodies: the three moving at equal speeds are the Sun, Venus and Mercury, while the four moving at unequal speeds are the Moon, Mars, Jupiter and Saturn. The complicated pattern of these movements is bound to be repeated again after a period called a 'complete' or 'perfect' year. So, Plato arranged these celestial orbs in the order (outwards from the center): Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, and fixed stars, with the fixed stars located on the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
. However, this did not suffice to explain the observed planetary motion.


Concentric spheres

Eudoxus of Cnidus Eudoxus of Cnidus (; , ''Eúdoxos ho Knídios''; ) was an Ancient Greece, ancient Greek Ancient Greek astronomy, astronomer, Greek mathematics, mathematician, doctor, and lawmaker. He was a student of Archytas and Plato. All of his original work ...
, student of Plato in around 380 BCE, introduced a technique to describe the motion of the planets called the ''method of exhaustion''. Eudoxus reasoned that since the distances of the stars, the Moon, the Sun and all known planets do not appear to be changing, they are fixed in a sphere in which the bodies move around the sphere but with a constant radius and the Earth is at the centre of the sphere. To explain the complexity of the movements of the planets, Eudoxus thought they move as if they were attached to a number of concentrical, invisible spheres, every of them rotating around its own and different axis and at different paces. Eudoxus’ model had twenty-seven homocentric spheres with each sphere explaining a type of observable motion for each celestial object. Eudoxus assigns one sphere for the fixed stars which is supposed to explain their daily movement. He assigns three spheres to both the Sun and the Moon with the first sphere moving in the same manner as the sphere of the fixed stars. The second sphere explains the movement of the Sun and the Moon on the ecliptic plane. The third sphere was supposed to move on a “latitudinally inclined” circle and explain the latitudinal motion of the Sun and the Moon in the cosmos. Four spheres were assigned to Mercury, Venus, Mars, Jupiter, and Saturn, the only known planets at that time. The first and second spheres of the planets moved exactly like the first two spheres of the Sun and the Moon. According to Simplicius, the third and fourth sphere of the planets were supposed to move in a way that created a curve known as a
hippopede In geometry, a hippopede () is a plane curve determined by an equation of the form :(x^2+y^2)^2=cx^2+dy^2, where it is assumed that and since the remaining cases either reduce to a single point or can be put into the given form with a rotation. ...
. The hippopede was a way to try and explain the retrograde motions of planets. Eudoxus emphasised that this is a purely mathematical construct of the model in the sense that the spheres of each celestial body do not exist, it just shows the possible positions of the bodies. Around 350 BCE
Aristotle Aristotle (; 384–322 BC) was an Ancient Greek philosophy, Ancient Greek philosopher and polymath. His writings cover a broad range of subjects spanning the natural sciences, philosophy, linguistics, economics, politics, psychology, a ...
, in his chief cosmological treatise ''De Caelo'' (On the Heavens), modified Eudoxus' model by supposing the spheres were material and crystalline. He was able to articulate the spheres for most planets, however, the spheres for Jupiter and Saturn crossed each other. Aristotle solved this complication by introducing an unrolled sphere in between, increasing the number of spheres needed well above Eudoxus'. Historians are unsure about how many spheres Aristotle thought there were in the cosmos with theories ranging from 43 up to 55. Aristotle also tried to determine whether the Earth moves and concluded that all the celestial bodies fall towards Earth by natural tendency and since Earth is the centre of that tendency, it is stationary.


Incipient heliocentrism

By 330 BCE,
Heraclides of Pontus Heraclides Ponticus ( ''Herakleides''; c. 390 BC – c. 310 BC) was a Greek philosopher and astronomer who was born in Heraclea Pontica, now Karadeniz Ereğli, Turkey, and migrated to Athens. He is best remembered for proposing that the Earth ...
said that the
rotation of the Earth Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star ...
on its axis, from west to east, once every 24 hours, explained the apparent daily motion of the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
. Simplicius says that Heraclides proposed that the irregular movements of the planets can be explained if the Earth moves while the Sun stays still, but these statements are disputed. Around 280 BCE,
Aristarchus of Samos Aristarchus of Samos (; , ; ) was an ancient Greek astronomer and mathematician who presented the first known heliocentric model that placed the Sun at the center of the universe, with the Earth revolving around the Sun once a year and rotati ...
offers the first definite discussion of the possibility of a heliocentric cosmos, and uses the size of the Earth's
shadow A shadow is a dark area on a surface where light from a light source is blocked by an object. In contrast, shade occupies the three-dimensional volume behind an object with light in front of it. The cross-section of a shadow is a two-dimensio ...
on the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
to estimate the Moon's orbital radius at 60 Earth radii, and its physical radius as one-third that of the Earth. He made an inaccurate attempt to measure the distance to the Sun, but sufficient to assert that the Sun is bigger than Earth and it is further away than the Moon. So the minor body, the Earth, must orbit the major one, the Sun, and not the opposite. Following the heliocentric ideas of Aristarcus (but not explicitly supporting them), around 250 BCE
Archimedes Archimedes of Syracuse ( ; ) was an Ancient Greece, Ancient Greek Greek mathematics, mathematician, physicist, engineer, astronomer, and Invention, inventor from the ancient city of Syracuse, Sicily, Syracuse in History of Greek and Hellenis ...
in his work ''
The Sand Reckoner ''The Sand Reckoner'' (, ''Psammites'') is a work by Archimedes, an Ancient Greek mathematician of the 3rd century BC, in which he set out to determine an upper bound for the number of grains of sand that fit into the universe. In order to do ...
'' computes the diameter of the universe centered around the Sun to be about 1014 stadia (in modern units, about 2
light year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distance, astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by t ...
s, , ).Archimedes, The Sand Reckoner 511 R U, by Ilan Vardi
accessed 28-II-2007.
In Archimedes' own words: However, Aristarchus' views were not widely adopted, and the geocentric notion will remain for centuries.


Further developments

Around 210 BCE,
Apollonius of Perga Apollonius of Perga ( ; ) was an ancient Greek geometer and astronomer known for his work on conic sections. Beginning from the earlier contributions of Euclid and Archimedes on the topic, he brought them to the state prior to the invention o ...
shows the equivalence of two descriptions of the apparent retrograde planet motions (assuming the geocentric model), one using eccentrics and another
deferent and epicycle In the Hipparchian, Ptolemaic, and Copernican systems of astronomy, the epicycle (, meaning "circle moving on another circle") was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, ...
s. The latter will be a key feature for future models. The epicycle is described as a small orbit within a greater one, called the ''deferent'': as a planet orbits the Earth, it also orbits the original orbit, so its trajectory resembles an epitrochoid, as shown in the illustration at left. This could explain how the planet seems to move as viewed from Earth. In the following century, measures of sizes and distances of the Earth and the Moon are improved. Around 200 BCE
Eratosthenes Eratosthenes of Cyrene (; ;  – ) was an Ancient Greek polymath: a Greek mathematics, mathematician, geographer, poet, astronomer, and music theory, music theorist. He was a man of learning, becoming the chief librarian at the Library of A ...
determines that the radius of the Earth is roughly 6,400 km. Circa 150 BCE
Hipparchus Hipparchus (; , ;  BC) was a Ancient Greek astronomy, Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equinoxes. Hippar ...
uses
parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
to determine that the distance to the Moon is roughly 380,000 km. The work of Hipparchus about the Earth-Moon system was so accurate that he could forecast solar and lunar eclipses for the next six centuries. Also, he discovers the
precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...
of the
equinox A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...
es, and compiles a
star catalog A star catalogue is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the year ...
of about 850 entries. During the period 127 to 141 AD,
Ptolemy Claudius Ptolemy (; , ; ; – 160s/170s AD) was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine science, Byzant ...
deduced that the Earth is spherical based on the fact that not everyone records the solar eclipse at the same time and that observers from the North can not see the Southern stars. Ptolemy attempted to resolve the Planetary motion dilemma in which the observations were not consistent with the perfect circular orbits of the bodies. Ptolemy adopted the Apollonius' epicycles as solution. Ptolemy emphasised that the epicycle motion does not apply to the Sun. His main contribution to the model was the
equant Equant (or punctum aequans) is a mathematical concept developed by Claudius Ptolemy in the 2nd century AD to account for the observed motion of the planets. The equant is used to explain the observed speed change in different stages of the plane ...
points. He also re-arranged the heavenly spheres in a different order than Plato did (from Earth outward): Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn and fixed stars, following a long astrological tradition and the decreasing orbital periods. Ptolemy's work ''
Almagest The ''Almagest'' ( ) is a 2nd-century Greek mathematics, mathematical and Greek astronomy, astronomical treatise on the apparent motions of the stars and planetary paths, written by Ptolemy, Claudius Ptolemy ( ) in Koine Greek. One of the most i ...
'' cemented the
geocentric model In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded scientific theories, superseded description of the Universe with Earth at the center. Under most geocentric m ...
in the West, and it remained the most authoritative text on astronomy for more than 1,500 years. His methods were accurate enough to keep them largely undisputed.


Medieval astronomy


Capellas' astronomical model

Around 420 AD
Martianus Capella Martianus Minneus Felix Capella () was a jurist, polymath and Latin literature, Latin prose writer of late antiquity, one of the earliest developers of the system of the seven liberal arts that structured early medieval education. He was a native ...
describes a modified geocentric model, in which the Earth is at rest in the center of the universe and circled by the Moon, the Sun, three planets and the stars, while Mercury and Venus circle the Sun. His model was not widely accepted, despite his authority; he was one of the earliest developers of the system of the seven
liberal arts Liberal arts education () is a traditional academic course in Western higher education. ''Liberal arts'' takes the term ''skill, art'' in the sense of a learned skill rather than specifically the fine arts. ''Liberal arts education'' can refe ...
, the
trivium The trivium is the lower division of the seven liberal arts and comprises grammar, logic, and rhetoric. The trivium is implicit in ("On the Marriage of Philology and Mercury") by Martianus Capella, but the term was not used until the Carolin ...
(
grammar In linguistics, grammar is the set of rules for how a natural language is structured, as demonstrated by its speakers or writers. Grammar rules may concern the use of clauses, phrases, and words. The term may also refer to the study of such rul ...
,
logic Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the study of deductively valid inferences or logical truths. It examines how conclusions follow from premises based on the structure o ...
, and
rhetoric Rhetoric is the art of persuasion. It is one of the three ancient arts of discourse ( trivium) along with grammar and logic/ dialectic. As an academic discipline within the humanities, rhetoric aims to study the techniques that speakers or w ...
) and the
quadrivium From the time of Plato through the Middle Ages, the ''quadrivium'' (plural: quadrivia) was a grouping of four subjects or arts—arithmetic, geometry, music, and astronomy—that formed a second curricular stage following preparatory work in th ...
(
arithmetic Arithmetic is an elementary branch of mathematics that deals with numerical operations like addition, subtraction, multiplication, and division. In a wider sense, it also includes exponentiation, extraction of roots, and taking logarithms. ...
,
geometry Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician w ...
,
music Music is the arrangement of sound to create some combination of Musical form, form, harmony, melody, rhythm, or otherwise Musical expression, expressive content. Music is generally agreed to be a cultural universal that is present in all hum ...
,
astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
), that structured early medieval education. Nonetheless, his single encyclopedic work, '' De nuptiis Philologiae et Mercurii'' ("On the Marriage of Philology and Mercury"), also called ''De septem disciplinis'' ("On the seven disciplines") was read, taught, and commented upon throughout the early Middle Ages and shaped European education during the early medieval period and the
Carolingian Renaissance The Carolingian Renaissance was the first of three medieval renaissances, a period of cultural activity in the Carolingian Empire. Charlemagne's reign led to an intellectual revival beginning in the 8th century and continuing throughout the 9th ...
. This model implies some knowledge about the transits of Mercury and
Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...
in front of the Sun, and the fact they pass also behind it periodically, which cannot be explained with Ptolemy's model. But it is unclear how that knowledge could be achieved by that times, due to the difficult to see these transits at naked eye; indeed, there is no evidence that any ancient culture knew of the transits. Alternatively, as seen from Earth, Mercury never departs from the Sun, neither East nor West, more than 28° and Venus no more than 47°, facts both known from antiquity that also could not be explained by Ptolemy. So it could be inferred that they orbit the Sun, and hence, there should be these transits.


Islamic astronomy

The
Islamic golden age The Islamic Golden Age was a period of scientific, economic, and cultural flourishing in the history of Islam, traditionally dated from the 8th century to the 13th century. This period is traditionally understood to have begun during the reign o ...
period in
Baghdad Baghdad ( or ; , ) is the capital and List of largest cities of Iraq, largest city of Iraq, located along the Tigris in the central part of the country. With a population exceeding 7 million, it ranks among the List of largest cities in the A ...
, picking off from Ptolemy's work, had more accurate measurements taken followed by interpretations. In 1021 A.D, Ibn Al Haytham adjusted Ptolemy's
geocentric model In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded scientific theories, superseded description of the Universe with Earth at the center. Under most geocentric m ...
to his specialty in optics in his book ''Al-shukuk 'ata Batlamyus'' which translates to "Doubts about Ptolemy". Ibn al-Haytham claimed that the epicycles Ptolemy introduced are inclined planes, not in a flat motion which settled further conflicting disputes. However, Ibn Al Haytham agreed with the Earth being in the centre of the Solar System at a fixed position. Nasir al-Din, during the 13th century, was able to combine two possible methods for a planet to orbit and as a result, derived a rotational aspect of planets within their orbits.
Copernicus Nicolaus Copernicus (19 February 1473 – 24 May 1543) was a Renaissance polymath who formulated a mathematical model, model of Celestial spheres#Renaissance, the universe that placed heliocentrism, the Sun rather than Earth at its cen ...
arrived to the same conclusion in the 16th century.
Ibn al-Shatir ʿAbu al-Ḥasan Alāʾ al‐Dīn bin Alī bin Ibrāhīm bin Muhammad bin al-Matam al-Ansari, known as Ibn al-Shatir or Ibn ash-Shatir (; 1304–1375) was an Arab astronomer, mathematician and engineer. He worked as '' muwaqqit'' (موقت, timek ...
, during the 14th century, in an attempt to resolve Ptolemy's inconsistent lunar theory, applied a double epicycle model to the Moon which reduced the predicted displacement of the Moon from the Earth. Copernicus also arrived at the same conclusion in the 16th century.


Chinese Astronomy

In 1051,
Shen Kua Shen Kuo (; 1031–1095) or Shen Gua, courtesy name Cunzhong (存中) and Art name#China, pseudonym Mengqi (now usually given as Mengxi) Weng (夢溪翁),Yao (2003), 544. was a Chinese polymath, scientist, and statesman of the Song dynasty (960 ...
, a Chinese scholar in
applied mathematics Applied mathematics is the application of mathematics, mathematical methods by different fields such as physics, engineering, medicine, biology, finance, business, computer science, and Industrial sector, industry. Thus, applied mathematics is a ...
, rejected the circular planetary motion. He substituted it with a different motion described by the term ‘willow-leaf’. This is when a planet has a circular orbit but then it encounters another small circular orbit within or outside the original orbit and then returns to its original orbit which is demonstrated by the figure on the right.


Renaissance


Copernicus's Heliocentric Model

During the 16th century
Nicholas Copernicus Nicolaus Copernicus (19 February 1473 – 24 May 1543) was a Renaissance polymath who formulated a mathematical model, model of Celestial spheres#Renaissance, the universe that placed heliocentrism, the Sun rather than Earth at its cen ...
, in reflecting on Ptolemy and Aristotle's interpretations of the Solar System, believed that all the orbits of the planets and Moon must be a perfect uniform circular motion despite the observations showing the complex retrograde motion. Copernicus introduced a new model which was consistent with the observations and allowed for perfect circular motion. This is known as the
Heliocentric model Heliocentrism (also known as the heliocentric model) is a superseded astronomical model in which the Earth and planets orbit around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed th ...
where the Sun is placed at the centre of the universe (hence, the Solar System) and the Earth is, like all the other planets, orbiting it. The heliocentric model also resolved the varying brightness of planets problem. Copernicus also supported the spherical Earth theory with the idea that nature prefers spherical limits which are seen in the Moon, the Sun, and also the orbits of planets. Copernicus furthermore believed that the universe had a spherical limit. Copernicus contributed further to practical Astronomy by producing advanced techniques of observations and measurements and providing instructional procedure. The heliocentric model implies that the Earth is also a planet, the third from the Sun after Mercury and Venus, and before Mars, Jupiter and Saturn. And also implicitly, that planets are "worlds", like Earth is, not "stars". But the Moon still orbits the Earth.


The Tychonic system

The heliocentric model was not immediately adopted.
Conservatism Conservatism is a Philosophy of culture, cultural, Social philosophy, social, and political philosophy and ideology that seeks to promote and preserve traditional institutions, Convention (norm), customs, and Value (ethics and social science ...
along to numerous observational, philosophical and religious concerns prevented it for more than a century. In 1588,
Tycho Brahe Tycho Brahe ( ; ; born Tyge Ottesen Brahe, ; 14 December 154624 October 1601), generally called Tycho for short, was a Danish astronomer of the Renaissance, known for his comprehensive and unprecedentedly accurate astronomical observations. He ...
publishes his own
Tychonic system The Tychonic system (or Tychonian system) is a model of the universe published by Tycho Brahe in 1588, which combines what he saw as the mathematical benefits of the Copernican heliocentrism, Copernican system with the philosophical and "physic ...
, a blend between the Ptolemy's classical geocentric model and Copernicus' heliocentric model, in which the Sun and the Moon revolve around the Earth, in the center of universe, and all other planets revolve around the Sun. It was an attempt to conciliate his religious beliefs with heliocentrism. This was the so-called geoheliocentric model, and it was adopted by some astronomers during the geocentrism vs heliocentrism disputes.


Kepler's model

In 1609,
Johannes Kepler Johannes Kepler (27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, Natural philosophy, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best know ...
, an advocate of the heliocentric model, using his patron Tycho Brahe's accurate measurements, noticed the inconsistency of a heliocentric model where the Sun is exactly in the centre. Instead Kepler developed a more accurate and consistent model where the Sun is located not in the centre but at one of the two foci of an
elliptic orbit In astrodynamics or celestial mechanics, an elliptical orbit or eccentric orbit is an orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. Some orbits have been referre ...
. Kepler derived the three laws of planetary motion which changed the model of the Solar System and the orbital path of planets. These three laws of planetary motion are: # All planets orbit the Sun in elliptical orbits (image on the right) and not perfectly circular orbits. # The radius vector joining the planet and the Sun has an equal area in equal periods. # The square of the period of the planet (one revolution around the Sun) is proportional to the cube of the average distance from the Sun. In modern notation, :\frac=\frac where ''a'' is the radius of the orbit, ''T'' is the period, ''G'' is the
gravitational constant The gravitational constant is an empirical physical constant involved in the calculation of gravitational effects in Sir Isaac Newton's law of universal gravitation and in Albert Einstein's general relativity, theory of general relativity. It ...
and ''M'' is the mass of the Sun. The third law explains the periods that occur during the year which relates the distance between the Earth and the Sun. Along with unprecedent accuracy, the Keplerian model also allows put the Solar System into scale. If a reliable measure between planetary bodies would be taken, the whole size of the system could be computed. By this time, the Solar System started to be conceived as something smaller than the rest of the universe. (Yet, up to 1596 Kepler himself still believed in the sphere of fixed stars, as it was illustrated in his book ''Mysterium Cosmopgraphicum''.)


Galileo's discoveries

With the help of the
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
providing a closer look into the sky,
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
proved the most part of the heliocentric model of the Solar System. Galileo observed the phases of Venus's appearance with the telescope and was able to confirm Kepler's first law of planetary motion and Copernicus's heliocentric model, of which Galileo was an advocate. Galileo claimed that the Solar System is not only made up of the Sun, the Moon and the planets but also comets. By observing movements around Jupiter, Galileo initially thought that these were the actions of stars. However, after a week of observing, he noticed changes in the patterns of motion in which he concluded that they are moons, four moons. Shortly after, it was proved by Kepler himself that the Jupiter's moons move around the planet the same way planets orbit the Sun, thus making Kepler's laws universal.


Enlightenment to Victorian Era


Newton's interpretation

After all these theories, people still did not know what made the planets orbit the Sun, nor why the Moon tracks the Earth. Until the 17th century when
Isaac Newton Sir Isaac Newton () was an English polymath active as a mathematician, physicist, astronomer, alchemist, theologian, and author. Newton was a key figure in the Scientific Revolution and the Age of Enlightenment, Enlightenment that followed ...
introduced The Law of Universal Gravitation. He claimed that between any two masses, there is an attractive force between them proportional to the inverse of the distance squared. :F = G \frac where m1 is the mass of the Sun and m2 is the mass of the planet, G is the
gravitational constant The gravitational constant is an empirical physical constant involved in the calculation of gravitational effects in Sir Isaac Newton's law of universal gravitation and in Albert Einstein's general relativity, theory of general relativity. It ...
and r is the distance between them. This theory was able to calculate the force on each planet by the Sun, which consequently, explained the planets elliptical motion. The term "
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
" entered the English language by 1704, when
John Locke John Locke (; 29 August 1632 (Old Style and New Style dates, O.S.) – 28 October 1704 (Old Style and New Style dates, O.S.)) was an English philosopher and physician, widely regarded as one of the most influential of the Enlightenment thi ...
used it to refer to the Sun, planets, and comets as a whole. By then it had been established beyond doubt that planets are other worlds, and stars are other distant suns, so the whole Solar System is actually only a small part of an immensely large universe, and definitively something distinct.


Derived measurements

In 1672
Jean Richer Jean Richer (1630–1696) was a French astronomer and assistant (''élève astronome'') at the French Academy of Sciences, under the direction of Giovanni Domenico Cassini. Between 1671 and 1673 he performed experiments and carried out celestial ...
and
Giovanni Domenico Cassini Giovanni Domenico Cassini (8 June 1625 – 14 September 1712) was an Italian-French mathematician, astronomer, astrologer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the County of Nice, part of the Savoyard sta ...
measure the
astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...
(AU), the mean distance Earth-Sun, to be about 138,370,000 km, (later refined by others up to the current value of 149,597,870 km). This gave for first time ever a well estimated size of the then known Solar System (that is, up to Saturn), following the scale derived from Kepler's third law. In 1798
Henry Cavendish Henry Cavendish ( ; 10 October 1731 – 24 February 1810) was an English experimental and theoretical chemist and physicist. He is noted for his discovery of hydrogen, which he termed "inflammable air". He described the density of inflammable a ...
accurately measures the gravitational constant in the laboratory, which allows the mass of the Earth to be derived thru Newton's law of universal gravitation, and hence the masses of all bodies in the Solar System.


New members of the Solar System

Telescopic observations found new moons around Jupiter and Saturn, as well as an impressive
ring system A ring system is a disc or torus orbiting an astronomical object that is composed of solid material such as dust, meteoroids, planetoids, moonlets, or stellar objects. Ring systems are best known as planetary rings, common components of sate ...
around the latter. In 1705
Edmond Halley Edmond (or Edmund) Halley (; – ) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720. From an observatory he constructed on Saint Helena in 1676–77, Hal ...
asserted that the comet of 1682 is periodical with a highly elongated elliptical orbit around the Sun, and predicts its return in 1757. Johann Palitzsch observed in 1758 the return of the comet that Halley had anticipated. The interference of Jupiter's orbit had slowed the return by 618 days. Parisian astronomer La Caille suggests it should be named "Halley's Comet". Comets became a popular target for astronomers, and were recognized as members of the Solar System. In 1766 Johann Titius found a numeric progression for planetary distances, published in 1772 by
Johann Bode Johann Elert Bode (; 19 January 1747 – 23 November 1826) was a German astronomer known for his reformulation and popularisation of the Titius–Bode law. Bode determined the orbit of Uranus and suggested the planet's name. Life and career Bo ...
, the so-called Titius-Bode rule. When in 1781
William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...
discovered a new planet,
Uranus Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...
, it was found it lies at a distance beyond Saturn that approximately matches that predicted by the Titius-Bode rule. That rule observed a gap between
Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
and
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
void of any known planet. In 1801 Giuseppe Piazzi discovered Ceres, a body that filled the gap and was regarded as a new planet, and in 1802 Heinrich Wilhelm Olbers discovered
Pallas Pallas may refer to: Astronomy * 2 Pallas asteroid ** Pallas family, a group of asteroids that includes 2 Pallas * Pallas (crater), a crater on Earth's moon Mythology * Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena * Pa ...
, at roughly the same distance to the Sun than Ceres. He proposed that the two objects were the remnants of a destroyed planet, and predicted that more of these pieces would be found. Due their star-like apparience, William Herschel suggested Ceres and Pallas, and similar objects if found, be placed into a separate category, named
asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s, although they were still counted among the planets for some decades. In 1804
Karl Ludwig Harding Karl Ludwig Harding (29 September 1765 – 31 August 1834) was a German astronomer, who discovered 3 Juno, Juno, the third asteroid of the main-belt in 1804. Life and career Harding was born in Lauenburg. From 1786–1789, he was educated a ...
discovered the asteroid Juno, and in 1807 Olbers discovered the asteroid Vesta. In 1845
Karl Ludwig Hencke Karl Ludwig Hencke (8 April 1793 – 21 September 1866) was a German amateur astronomer and discoverer of minor planets. He is sometimes confused with Johann Franz Encke, another :German astronomers, German astronomer. Biography Hencke was born ...
discovered a fifth body between Mars and Jupiter,
Astraea In ancient Greek religion and mythology, Astraea (; ), also spelled Astrea or Astria, is a daughter of Astraeus and Eos. She is the virgin goddess of justice, innocence, purity, and precision. She is closely associated with the Greek goddess of ...
, and in 1849 Annibale de Gasparis discovers the asteroid Hygiea, the fourth largest asteroid in the Solar System by both volume and mass. As new objects of that kind were found there at an accelerating rate, counting them among the planets became increasingly cumbersome. Eventually, they were dropped from the planet list (as first suggested by
Alexander von Humboldt Friedrich Wilhelm Heinrich Alexander von Humboldt (14 September 1769 – 6 May 1859) was a German polymath, geographer, natural history, naturalist, List of explorers, explorer, and proponent of Romanticism, Romantic philosophy and Romanticism ...
in the early 1850s) and Herschel's coinage, "asteroids", gradually came into common use. Since then, the region they occupy between Mars and Jupiter is known as the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...
. Alexis Bouvard detected irregularities in the
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
of Uranus in 1821. Later, between 1845 and 1846,
John Adams John Adams (October 30, 1735 – July 4, 1826) was a Founding Fathers of the United States, Founding Father and the second president of the United States from 1797 to 1801. Before Presidency of John Adams, his presidency, he was a leader of ...
and
Urbain Le Verrier Urbain Jean Joseph Le Verrier (; 11 March 1811 – 23 September 1877) was a French astronomer and mathematician who specialized in celestial mechanics and is best known for predicting the existence and position of Neptune using only mathematics. ...
separately predicted the existence and location of a new planet from irregularities in the orbit of Uranus. This new planet was finally found by Johann Galle and eventually named
Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
, following the predicted position gave to him by Le Verrier. This fact marked the climax of the Newtonian mechanics applied to astronomy, but the Neptune's orbit does not fit with the Titius-Bode rule, so it has been deprecated from then on. Eventually, new moons were discovered also around Uranus starting in 1787 by Herschel, around Neptune starting in 1846 by
William Lassell William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer.around Mars in 1877 by
Asaph Hall Asaph Hall III (October 15, 1829 – November 22, 1907) was an American astronomer who is best known for having discovered the two moons of Mars, Deimos and Phobos, in 1877. He determined the orbits of satellites of other planets and of doubl ...
.


20th century add-ons

In 1919 Arthur Stanley Eddington uses a solar eclipse to successfully test
Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...
's General Theory of Relativity, which in turn explains the observed irregularities in the orbital motion of Mercury, and disproves the existence of the hypothesized inner planet Vulcan. General Theory of Relativity supersedes Newton's celestial mechanics. Instead of forces of attraction, gravity is seen as a ''bend'' of the tissue of the continuum space-time produced by the bodies' masses. Clyde Tombaugh discovered
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
in 1930. It was regarded for decades as the ninth planet of the Solar System. In 1978 James Christy discovers Charon, the large moon of Pluto. In 1950 Jan Oort suggested the presence of a cometary reservoir in the outer limits of the Solar System, the
Oort cloud The Oort cloud (pronounced or ), sometimes called the Öpik–Oort cloud, is scientific theory, theorized to be a cloud of billions of Volatile (astrogeology), icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 A ...
, and in 1951
Gerard Kuiper Gerard Peter Kuiper ( ; born Gerrit Pieter Kuiper, ; 7 December 1905 – 23 December 1973) was a Dutch-American astronomer, planetary scientist, selenographer, author and professor. The Kuiper belt is named after him. Kuiper is consi ...
argued for an annular reservoir of comets between 40 and 100
astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...
s from the Sun having formed early in the Solar System's evolution, but he did not think that such a belt still existed today. Decades later, this region was named after him, the
Kuiper belt The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
. New asteroid populations were discovered, as
Trojan Trojan or Trojans may refer to: * Of or from the ancient city of Troy * Trojan language, the language of the historical Trojans Arts and entertainment Music * '' Les Troyens'' ('The Trojans'), an opera by Berlioz, premiered part 1863, part 18 ...
s since 1906 by
Max Wolf Maximilian Franz Joseph Cornelius Wolf (21 June 1863 – 3 October 1932) was a German astronomer and a pioneer in the field of astrophotography. He was the chairman of astronomy at the University of Heidelberg and director of the Heidelberg-K ...
, and
Centaur A centaur ( ; ; ), occasionally hippocentaur, also called Ixionidae (), is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse that was said to live in the mountains of Thessaly. In one version o ...
s since 1977 by Charles Kowal, among many other. From 1957 on,
technology Technology is the application of Conceptual model, conceptual knowledge to achieve practical goals, especially in a reproducible way. The word ''technology'' can also mean the products resulting from such efforts, including both tangible too ...
allowed for direct
space exploration Space exploration is the process of utilizing astronomy and space technology to investigate outer space. While the exploration of space is currently carried out mainly by astronomers with telescopes, its physical exploration is conducted bo ...
of the Solar System's bodies. To date, all their known main bodies have been visited at least once by
robotic Robotics is the interdisciplinary study and practice of the design, construction, operation, and use of robots. Within mechanical engineering, robotics is the design and construction of the physical structures of robots, while in computer s ...
spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
, providing firsthand scientific data and closeup imaging. In some instances, robotic probes and rovers have landed on satellites, planets, asteroids and comets. And even some samples have been returned.


Current model

The
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
is a lone,
G-type main-sequence star A G-type main-sequence star (spectral type: G-V), also often, and imprecisely, called a yellow dwarf, or G star, is a main sequence, main-sequence star (luminosity class V) of stellar classification, spectral type G. Such a star has about 0.9 to ...
inside the
galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
of the
Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
, surrounded by eight major
planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
s orbiting the star by the influence of gravity, most of them with a cohort of satellites, or moons, orbiting them. The biggest planets also have rings, consisting of a multitude of tiny solid objects and dust. The planets are, in order of distance from the Sun: Mercury,
Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...
,
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
,
Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
,
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
,
Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
,
Uranus Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...
and
Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
. There are three main belts of minor bodies: * The
asteroid belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...
, between Mars and Jupiter. * The
Kuiper belt The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
beyond Neptune, followed by the
scattered disc The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small Solar System bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc obj ...
. * The
Oort cloud The Oort cloud (pronounced or ), sometimes called the Öpik–Oort cloud, is scientific theory, theorized to be a cloud of billions of Volatile (astrogeology), icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 A ...
in the boundaries of the Solar System. The biggest of these minor bodies are regarded as dwarf planets: Ceres in the asteroid belt, and
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
, Eris,
Haumea Haumea ( minor-planet designation: 136108 Haumea) is a dwarf planet located beyond Neptune's orbit. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory, and formally announced in 2005 by a team heade ...
,
Makemake Makemake ( minor-planet designation: 136472 Makemake) is a dwarf planet and the largest of what is known as the classical population of Kuiper belt objects, with a diameter approximately that of Saturn's moon Iapetus, or 60% that of Pluto. It ...
,
Gonggong Gonggong () is a Chinese water god who is depicted in Chinese mythology and folktales as having a copper human head with an iron forehead, red hair, and the body of a serpent, or sometimes the head and torso are human, with the tail of a serpen ...
, Quaoar, Sedna, and Orcus (along with other candidates) in the Kuiper belt.


See also

* Discovery and exploration of the Solar System * Timeline of Solar System astronomy * Timeline of Solar System exploration *
History of astronomy The history of astronomy focuses on the contributions civilizations have made to further their understanding of the universe beyond earth's atmosphere. Astronomy is one of the oldest natural sciences, achieving a high level of success in the sec ...
* Timeline of astronomy * Fixed stars * Dynamics of the celestial spheres


References


External links


An accurate web-based scroll map of the Solar System scaled to the Moon being 1 pixel
{{Solar System models History of astronomy Solar System models Solar System