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Solar observation is the scientific endeavor of studying 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 ...
and its behavior and relation to the Earth and the remainder 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 ...
. Deliberate solar observation began thousands of years ago. That initial era of direct observation gave way to telescopes in the 1600s followed by satellites in the twentieth century.


Prehistory

Stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...
data suggest that solar cycles have occurred for hundreds of millions of years, if not longer; measuring
varves A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. Of the many rhythmites in the geological record ...
in
precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...
sedimentary rock has revealed repeating peaks in layer thickness corresponding to the cycle. It is possible that the early atmosphere on Earth was more sensitive to
solar irradiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W ...
than today, so that greater glacial melting (and thicker sediment deposits) could have occurred during years with greater sunspot activity. This would presume annual layering; however, alternative explanations (diurnal) have also been proposed. Analysis of
tree rings Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed in a tree. As well as dating them, this can give data for dendroclimatology, the study of climate ...
revealed a detailed picture of past solar cycles: Dendrochronologically dated
radiocarbon Carbon-14, C-14, C or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic matter is the basis of the radiocarbon dating method pioneered by Willard Libby and coll ...
concentrations have allowed for a reconstruction of sunspot activity covering 11,400 years.


Early observations

Solar activity and related events have been regularly recorded since the time of the
Babylonians Babylonia (; , ) was an ancient Akkadian-speaking state and cultural area based in the city of Babylon in central-southern Mesopotamia (present-day Iraq and parts of Kuwait, Syria and Iran). It emerged as an Akkadian-populated but Amorite-ru ...
. In the 8th century BC, they described solar eclipses and possibly predicted them from numerological rules. The earliest extant report of sunspots dates back to the Chinese ''
Book of Changes The ''I Ching'' or ''Yijing'' ( ), usually translated ''Book of Changes'' or ''Classic of Changes'', is an ancient Chinese divination text that is among the oldest of the Chinese classics. The ''I Ching'' was originally a divination manual in ...
'', . The phrases used in the book translate to "A dou is seen in the Sun" and "A mei is seen in the Sun", where ''dou'' and ''mei'' would be darkening or obscuration (based on the context). Observations were regularly noted by Chinese and Korean astronomers at the behest of the emperors, rather than independently. The first clear mention of a sunspot in Western literature, around 300 BC, was by the
ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
scholar
Theophrastus Theophrastus (; ; c. 371 – c. 287 BC) was an ancient Greek Philosophy, philosopher and Natural history, naturalist. A native of Eresos in Lesbos, he was Aristotle's close colleague and successor as head of the Lyceum (classical), Lyceum, the ...
, student of
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 ...
and
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 ...
and successor to the latter. The ''
Royal Frankish Annals The ''Royal Frankish Annals'' (Latin: ''Annales regni Francorum''), also called the ''Annales Laurissenses maiores'' ('Greater Lorsch Annals'), are a series of annals composed in Latin in Carolingian Francia, recording year-by-year the state of ...
'' record that beginning on 17 March AD 807 a large sunspot that was visible for eight days and incorrectly conclude that it was a
transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1980 film), a 1980 Israeli film * ''Transit'' (1986 film), a Canadian short film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countrie ...
of Mercury. The earliest surviving record of deliberate sunspot observation dates from 364 BC, based on comments by
Chinese astronomer Chinese may refer to: * Something related to China * Chinese people, people identified with China, through nationality, citizenship, and/or ethnicity **Han Chinese, East Asian ethnic group native to China. **''Zhonghua minzu'', the supra-ethnic ...
Gan De Gan De (; fl. 4th century BC), also known as the Lord Gan (Gan Gong), was an ancient Chinese astronomer and astrologer born in the State of Qi. Along with Shi Shen, he is believed to be the first in history known by name to compile a star catal ...
in a
star catalogue 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 ...
. By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records. A large sunspot was observed at the time of
Charlemagne Charlemagne ( ; 2 April 748 – 28 January 814) was List of Frankish kings, King of the Franks from 768, List of kings of the Lombards, King of the Lombards from 774, and Holy Roman Emperor, Emperor of what is now known as the Carolingian ...
's death in AD 813. Sunspot activity in 1129 was described by
John of Worcester John of Worcester (died c. 1140) was an English monk and chronicler who worked at Worcester Priory. He is now usually held to be the author of the . Works John of Worcester's principal work was the (Latin for "Chronicle from Chronicles") or ...
and
Averroes Ibn Rushd (14 April 112611 December 1198), archaically Latinization of names, Latinized as Averroes, was an Arab Muslim polymath and Faqīh, jurist from Al-Andalus who wrote about many subjects, including philosophy, theology, medicine, astron ...
provided a description of sunspots later in the 12th century; however, these observations were also misinterpreted as planetary transits. The first unambiguous mention of the solar corona was by Leo Diaconus, a Byzantine historian. He wrote of the 22 December 968 total eclipse, which he experienced in Constantinople (modern-day Istanbul, Turkey): The earliest known record of a
sunspot drawing Sunspot drawing or sunspot sketching is the act of drawing sunspots. Sunspots are darker spots on the Sun's photosphere. Their prediction is very important for radio communication because they are strongly associated with solar activity, which can ...
was in 1128, by
John of Worcester John of Worcester (died c. 1140) was an English monk and chronicler who worked at Worcester Priory. He is now usually held to be the author of the . Works John of Worcester's principal work was the (Latin for "Chronicle from Chronicles") or ...
. Another early observation was of solar prominences, described in 1185 in the Russian Chronicle of Novgorod.


17th and 18th centuries

Giordano Bruno Giordano Bruno ( , ; ; born Filippo Bruno; January or February 1548 – 17 February 1600) was an Italian philosopher, poet, alchemist, astrologer, cosmological theorist, and esotericist. He is known for his cosmological theories, which concep ...
and
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 ...
suggested the idea that the sun rotated on its axis. Sunspots were first observed telescopically on 18 December 1610 (Gregorian calendar, not yet adopted in England) by English astronomer
Thomas Harriot Thomas Harriot (; – 2 July 1621), also spelled Harriott, Hariot or Heriot, was an English astronomer, mathematician, ethnographer and translator to whom the theory of refraction is attributed. Thomas Harriot was also recognized for his con ...
, as recorded in his notebooks. On 9 March 1611 (Gregorian calendar, also not yet adopted in East Frisia) they were observed by Frisian medical student Johann Goldsmid (latinised name
Johannes Fabricius Johann Goldsmid, better known by his Latinisation of names, Latinized name Johann(es) Fabricius (8 January 1587 – 19 March 1616), eldest son of David Fabricius (1564–1617), was a Frisians, Frisian astronomer and a modern era discoverer of suns ...
) who subsequently teamed up with his father
David Fabricius David Fabricius (9 March 1564 – 7 May 1617) was a Frisian pastor who made two major discoveries in the early days of telescopic astronomy, jointly with his eldest son, Johannes Fabricius (1587–1615). David Fabricius (Latinization of his prope ...
, a pastor and astronomer, to make further observations and to publish a description in a pamphlet in June 1611. The Fabricius' used
camera obscura A camera obscura (; ) is the natural phenomenon in which the rays of light passing through a aperture, small hole into a dark space form an image where they strike a surface, resulting in an inverted (upside down) and reversed (left to right) ...
telescopy to get a better view of the solar disk, and like Harriot made observations shortly after sunrise and shortly before sunset. Johann was the first to realize that sunspots revealed solar rotation, but he died on 19 March 1616, aged 26 and his father a year later. Several scientists such as
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 ...
, Simon Marius, and
Michael Maestlin Michael Maestlin (; also ''Mästlin'', ''Möstlin'', or ''Moestlin''; 30 September 1550 – 26 October 1631) was a German astronomer and mathematician, best known as the mentor of Johannes Kepler. A student of Philipp Apian, Maestlin is recogniz ...
were aware of the Fabricius' early sunspot work, and indeed Kepler repeatedly referred to it his writings. However, like that of Harriot, their work was otherwise not well known.
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 ...
almost certainly began telescopic sunspot observations around the same time as Harriot, given he made his first telescope in 1609 on hearing of the Dutch patent of the device, and that he had managed previously to make naked-eye observations of sunspots. He is also reported to have shown sunspots to astronomers in Rome, but we do not have records of the dates. The records of telescopic observations of sunspots that we do have from Galileo do not start until 1612, for when they are of unprecedented quality and detail as by then he had developed the telescope design and greatly increased its magnification. Likewise
Christoph Scheiner Christoph Scheiner (25 July 1573 (or 1575) – 18 June 1650) was a Jesuit priest, physicist and astronomer in Ingolstadt. Biography Augsburg/Dillingen: 1591–1605 Scheiner was born in Markt Wald near Mindelheim in Swabia, earlier margravate Burg ...
had probably been observing the spots using an improved
helioscope A helioscope is an instrument used in observing the Sun and sunspots. The helioscope was first used by Benedetto Castelli (1578–1643) and refined by Galileo Galilei (1564–1642). The method involves projecting an image of the sun onto a white ...
of his own design. Galileo and Scheiner, neither of whom knew of the work of Harriot or Fabricius vied for the credit for the discovery. In 1613, in
Letters on Sunspots '' Letters on Sunspots'' (''Istoria e Dimostrazioni intorno alle Macchie Solari'') was a pamphlet written by Galileo Galilei in 1612 and published in Rome by the Accademia dei Lincei in 1613. In it, Galileo outlined his recent observation of dark ...
, Galileo refuted Scheiner's 1612 claim that sunspots were planets inside Mercury's orbit, showing that sunspots were surface features. Although the physical aspects of sunspots were not identified until the 20th century, observations continued. Study was hampered during the 17th century due to the low number of sunspots during what is now recognized as an extended period of low solar activity, known as the Maunder Minimum. By the 19th century, then-sufficient sunspot records allowed researchers to infer periodic cycles in sunspot activity. In 1845,
Henry Henry may refer to: People and fictional characters * Henry (given name), including lists of people and fictional characters * Henry (surname) * Henry, a stage name of François-Louis Henry (1786–1855), French baritone Arts and entertainmen ...
and
Alexander Alexander () is a male name of Greek origin. The most prominent bearer of the name is Alexander the Great, the king of the Ancient Greek kingdom of Macedonia who created one of the largest empires in ancient history. Variants listed here ar ...
observed the Sun with a
thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoel ...
and determined that sunspots emitted less radiation than surrounding areas. The emission of higher than average amounts of radiation later were observed from the solar
faculae Facula ( faculae) is a bright spot on the surface of a planet or a star. It may refer to * Solar facula * * Bright spots on Ceres * * * * {{disambig ...
. Sunspots had some importance in the debate over the nature 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 ...
. They showed that the Sun rotated, and their comings and goings showed that the Sun changed, contrary to Aristotle, who had taught that all celestial bodies were perfect, unchanging spheres. Sunspots were rarely recorded between 1650 and 1699. Later analysis revealed the problem to be a reduced number of sunspots, rather than observational lapses. Building upon
Gustav Spörer Friederich Wilhelm Gustav Spörer (23 October 1822 – 7 July 1895) was a German astronomer. He is noted for his studies of sunspots and sunspot cycles. In this regard he is often mentioned together with Edward Maunder. Spörer was the first to ...
's work, the wife-and-husband team of
Annie Maunder Annie Scott Dill Maunder (; 14 April 1868 – 15 September 1947) was an Irish-British astronomer, who recorded the first evidence of the movement of the sunspot emergence from the poles toward the equator over the 11-year solar cycle, finding t ...
and Edward Maunder suggested that the Sun had changed from a period in which sunspots all but disappeared to a renewal of sunspot cycles starting in about 1700. Adding to this understanding of the absence of solar cycles were observations of
aurorae An aurora ( aurorae or auroras), also commonly known as the northern lights (aurora borealis) or southern lights (aurora australis), is a natural light display in Earth's sky, predominantly observed in high-latitude regions (around the Arc ...
, which were absent at the same time, except at the very highest magnetic latitudes The lack of a
solar corona In astronomy, a corona (: coronas or coronae) is the outermost layer of a star's Stellar atmosphere, atmosphere. It is a hot but relatively luminosity, dim region of Plasma (physics), plasma populated by intermittent coronal structures such as so ...
during
solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season i ...
s was also noted prior to 1715. The period of low sunspot activity from 1645 to 1717 later became known as the " Maunder Minimum". Observers such as
Johannes Hevelius Johannes Hevelius Some sources refer to Hevelius as Polish: * * * * * * * Some sources refer to Hevelius as German: * * * * *of the Royal Society * (in German also known as ''Hevel''; ; – 28 January 1687) was a councillor and mayor of Danz ...
,
Jean Picard Jean Picard (21 July 1620 – 12 July 1682) was a French astronomer and priest born in La Flèche, where he studied at the Jesuit Collège Royal Henry-Le-Grand. He is principally notable for his accurate measure of the size of the Earth, ...
and Jean Dominique Cassini confirmed this change.


19th century


Solar spectroscopy

After the detection of
infra-red radiation Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...
by
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 ...
in 1800 and of
Ultraviolet radiation Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...
by
Johann Wilhelm Ritter Johann Wilhelm Ritter (16 December 1776 – 23 January 1810). was a German chemist, physicist and philosopher. He was born in Samitz (Zamienice) near Haynau (Chojnów) in Silesia (then part of Prussia, since 1945 in Poland), and died in Muni ...
, solar spectrometry began in 1817 when
William Hyde Wollaston William Hyde Wollaston (; 6 August 1766 – 22 December 1828) was an English chemist and physicist who is famous for discovering the chemical elements palladium and rhodium. He also developed a way to process platinum ore into malleable i ...
noticed that dark lines appeared in the solar spectrum when viewed through a glass
prism PRISM is a code name for a program under which the United States National Security Agency (NSA) collects internet communications from various U.S. internet companies. The program is also known by the SIGAD . PRISM collects stored internet ...
.
Joseph von Fraunhofer Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a German physicist and optical lens manufacturer. He made optical glass, an achromatic telescope, and objective lenses. He developed diffraction grating and also invented the ...
later independently discovered the lines and they were named
Fraunhofer lines The Fraunhofer lines are a set of spectral absorption lines. They are dark absorption lines, seen in the optical spectrum of the Sun, and are formed when atoms in the solar atmosphere absorb light being emitted by the solar photosphere. The l ...
after him. Other physicists discerned that properties of the solar atmosphere could be determined from them. Notable scientists to advance spectroscopy were
David Brewster Sir David Brewster Knight of the Royal Guelphic Order, KH President of the Royal Society of Edinburgh, PRSE Fellow of the Royal Society of London, FRS Fellow of the Society of Antiquaries of Scotland, FSA Scot Fellow of the Scottish Society of ...
,
Gustav Kirchhoff Gustav Robert Kirchhoff (; 12 March 1824 – 17 October 1887) was a German chemist, mathematician, physicist, and spectroscopist who contributed to the fundamental understanding of electrical circuits, spectroscopy and the emission of black-body ...
,
Robert Wilhelm Bunsen Robert Wilhelm Eberhard Bunsen (; 30 March 1811 – 16 August 1899) was a German chemist. He investigated emission spectra of heated elements, and discovered caesium (in 1860) and rubidium (in 1861) with the physicist Gustav Kirchhoff. The Buns ...
and
Anders Jonas Ångström Anders Jonas Ångström (; ; 13 August 1814 – 21 June 1874) was a Swedish physicist and one of the founders of the science of spectroscopy.P.Murdin (2000): "Angstrom" chapter in ''Encyclopedia of Astronomy and Astrophysics''. Ångström is a ...
.


Solar cycle

The cyclic variation of the number of sunspots was first observed by
Samuel Heinrich Schwabe Samuel Heinrich Schwabe (25 October 1789 – 11 April 1875) was a German amateur astronomer remembered for his work on sunspots. He observed sunspots and made drawings of them from 1825 to 1867 and suggested in 1838 that there may be a ten-year cy ...
between 1826 and 1843.
Rudolf Wolf Johann Rudolf Wolf (7 July 1816 – 6 December 1893) was a Swiss astronomer and mathematician best known for his research on sunspots. Wolf was born in Fällanden, near Zurich. He studied at the universities of Zurich, Vienna, and Berlin. Encke ...
studied the historical record in an attempt to establish a history of solar variations. His data extended only to 1755. He also established in 1848 a relative sunspot number formulation to compare the work of different astronomers using varying equipment and methodologies, now known as the Wolf (or Zürich) sunspot number.
Gustav Spörer Friederich Wilhelm Gustav Spörer (23 October 1822 – 7 July 1895) was a German astronomer. He is noted for his studies of sunspots and sunspot cycles. In this regard he is often mentioned together with Edward Maunder. Spörer was the first to ...
later suggested a 70-year period before 1716 in which sunspots were rarely observed as the reason for Wolf's inability to extend the cycles into the 17th century. Also in 1848,
Joseph Henry Joseph Henry (December 17, 1797– May 13, 1878) was an American physicist and inventor who served as the first secretary of the Smithsonian Institution. He was the secretary for the National Institute for the Promotion of Science, a precursor ...
projected an image of the Sun onto a screen and determined that sunspots were cooler than the surrounding surface. Around 1852, Edward Sabine, Wolf, Jean-Alfred Gautier and Johann von Lamont independently found a link between the solar cycle and geomagnetic activity, sparking the first research into interactions between the Sun and the Earth. In the second half of the nineteenth century
Richard Carrington Richard Christopher Carrington (26 May 1826 – 27 November 1875) was an English astronomer whose 1859 astronomical observations demonstrated the existence of solar flares as well as suggesting their electrical influence upon the Earth and its ...
and Spörer independently noted the migration of sunspot activity towards the solar equator as the cycle progresses. This pattern is best visualized in the form of the so-called butterfly diagram, first constructed by
Edward Walter Maunder Edward Walter Maunder (12 April 1851 – 21 March 1928) was a British astronomer. His study of sunspots and the solar magnetic cycle led to his identification of the period from 1645 to 1715 that is now known as the Maunder Minimum. Early and ...
and Annie Scott Dill Maunder in the early twentieth century (see graph). Images of the Sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce a time-series diagram. Half a century later, the father-and-son team of Harold and
Horace Babcock Horace Welcome Babcock (September 13, 1912 – August 29, 2003) was an American astronomer. He was the son of Harold D. Babcock. Career Babcock invented and built several astronomical instruments and was the first to propose adaptive optic ...
showed that the solar surface is magnetized even outside of sunspots; that this weaker magnetic field is to first order a
dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: * An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...
; and that this dipole undergoes polarity reversals with the same period as the sunspot cycle (see graph below). These observations established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole.


Photography

The Sun was photographed for the first time, on 2 April 1845, by French physicists Louis Fizeau and
Léon Foucault Jean Bernard Léon Foucault (, ; ; 18 September 1819 – 11 February 1868) was a French physicist best known for his demonstration of the Foucault pendulum, a device demonstrating the effect of Earth's rotation. He also made an early measuremen ...
. Sunspots, as well as the
limb darkening Limb darkening is an optical effect seen in stars (including the Sun) and planets, where the central part of the disk appears brighter than the edge, or '' limb''. Its understanding offered early solar astronomers an opportunity to construct mode ...
effect, are visible in their daguerrotypes. Photography assisted in the study of solar prominences,
granulation Granulation is the process of forming grains or granules from a powdery or solid substance, producing a granular material. It is applied in several technological processes in the chemical and pharmaceutical industries. Typically, granulation inv ...
and spectroscopy. Charles A. Young first captured a prominence in 1870. Solar eclipses were also photographed, with the most useful early images taken in 1851 by Berkowski and in 1860 by De la Rue's team in Spain.


Rotation

Early estimates of the Sun's rotation period varied between 25 and 28 days. The cause was determined independently in 1858 by Richard C. Carrington and Spörer. They discovered that the latitude with the most sunspots decreases from 40° to 5° during each cycle, and that at higher latitudes sunspots rotate more slowly. The Sun's rotation was thus shown to vary by latitude and that its outer layer must be fluid. In 1871 Hermann Vogel, and shortly thereafter by
Charles Young Charles is a masculine given name predominantly found in English and French speaking countries. It is from the French form ''Charles'' of the Proto-Germanic name (in runic alphabet) or ''*karilaz'' (in Latin alphabet), whose meaning was ...
confirmed this spectroscopically.
Nils Dúner Nils is a Scandinavian given name, a chiefly Norwegian, Danish, Swedish and Latvian variant of Niels, cognate to Nicholas. People and animals with the given name * Nils Elias Anckers (1858–1921), Swedish naval officer * Nils Beckman (1902–19 ...
's spectroscopic observation in the 1880s showed a 30% difference between the Sun's faster equatorial regions and its slower polar regions.


Space weather

The first modern, and clearly described, accounts of a solar flare and coronal mass ejection occurred in 1859 and 1860 respectively. On 1 September 1859, Richard C. Carrington, while observing sunspots, saw patches of increasingly bright light within a group of sunspots, which then dimmed and moved across that area within a few minutes. This event, also reported by R. Hodgson, is a description of a solar flare. The widely viewed total solar eclipse on 18 July 1860 resulted in many drawings, depicting an anomalous feature that corresponds with modern CME observations. For many centuries, the earthly effects of solar variation were noticed but not understood. E.g., displays of
auroral An aurora ( aurorae or auroras), also commonly known as the northern lights (aurora borealis) or southern lights (aurora australis), is a natural light display in Earth's sky, predominantly observed in polar regions of Earth, high-latitude ...
light have long been observed at high latitudes, but were not linked to the Sun. In 1724,
George Graham George Graham (born 30 November 1944) is a Scottish former football player and manager. Nicknamed "Stroller", he made 455 appearances in England's Football League as a midfielder or forward for Aston Villa, Chelsea, Arsenal, Manchester Unite ...
reported that the needle of a
magnetic compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with m ...
was regularly deflected from
magnetic north The north magnetic pole, also known as the magnetic north pole, is a point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downward (in other words, if a magnetic compass needle is allowed t ...
over the course of each day. This effect was eventually attributed to overhead electric currents flowing in the ionosphere and magnetosphere by
Balfour Stewart Balfour Stewart (1 November 182819 December 1887) was a Scottish physicist and meteorologist. His studies in the field of radiant heat led to him receiving the Rumford Medal of the Royal Society in 1868. In 1859 he was appointed director of ...
in 1882, and confirmed by
Arthur Schuster Sir Franz Arthur Friedrich Schuster (12 September 1851 – 14 October 1934) was a German-born British physicist known for his work in spectroscopy, electrochemistry, optics, X-radiography and the application of harmonic analysis to physics. S ...
in 1889 from analysis of magnetic observatory data. In 1852, astronomer and British major general
Edward Sabine Sir Edward Sabine (; 14 October 1788 – 26 June 1883) was an Irish physicist, geodesist,astronomer, geophysicist, ornithologist, polar explorer, soldier, and the 30th president of the Royal Society. He led the effort to establish a system o ...
showed that the probability of the occurrence of magnetic storms on Earth was correlated with the number of
sunspots Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are one of the most recognizable Solar phenomena and despite the fact that they are mostly visible in the solar photosphere they usually affe ...
, thus demonstrating a novel solar-terrestrial interaction. In 1859, a great
magnetic storm A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere that is driven by interactions between the magnetosphere and large-scale transient plasma and magnetic field structures that originate ...
caused brilliant auroral displays and disrupted global
telegraph Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas ...
operations.
Richard Carrington Richard Christopher Carrington (26 May 1826 – 27 November 1875) was an English astronomer whose 1859 astronomical observations demonstrated the existence of solar flares as well as suggesting their electrical influence upon the Earth and its ...
correctly connected the storm with a
solar flare A solar flare is a relatively intense, localized emission of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and ot ...
that he had observed the day before in the vicinity of a large sunspot group—thus demonstrating that specific solar events could affect the Earth.
Kristian Birkeland Kristian Olaf Bernhard Birkeland (born 13 December 1867 – 15 June 1917) was a Norway, Norwegian space physics, space physicist, inventor, and professor of physics at the University of Oslo, Royal Fredriks University in Oslo. He is best remembe ...
explained the physics of aurora by creating artificial aurora in his laboratory and predicted the
solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
.


20th century


Observatories

Early in the 20th century, interest in astrophysics grew in America, and multiple observatories were built. Solar telescopes (and thus, solar observatories), were installed at
Mount Wilson Observatory The Mount Wilson Observatory (MWO) is an Observatory#Astronomical observatories, astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson (California), Mount Wilson, a peak in the San Gabrie ...
in California in 1904, and in the 1930s at
McMath–Hulbert Observatory The McMath-Hulbert Solar Observatory is a solar observatory in Lake Angelus, Michigan, USA. It was established in 1929 as a private observatory by father and son Francis Charles McMath and Robert Raynolds McMath and their friend, Judge Henry H ...
. Interest also grew in other parts of the world, with the establishment of the
Kodaikanal Solar Observatory The Kodaikanal Solar Observatory is a solar observatory owned and operated by the Indian Institute of Astrophysics. It is on the southern tip of the Palani Hills from Kodaikanal. The Evershed effect was first detected at this observatory in J ...
in India at the turn of the century, the Einsteinturm in Germany in 1924, and the Solar Tower Telescope at the National Observatory of Japan in 1930. Around 1900, researchers began to explore connections between solar variations and Earth's weather.
Smithsonian Astrophysical Observatory The Smithsonian Astrophysical Observatory (SAO) is a research institute of the Smithsonian Institution, concentrating on Astrophysics, astrophysical studies including Galactic astronomy, galactic and extragalactic astronomy, cosmology, Sun, solar ...
(SAO) assigned
Abbot Abbot is an ecclesiastical title given to the head of an independent monastery for men in various Western Christian traditions. The name is derived from ''abba'', the Aramaic form of the Hebrew ''ab'', and means "father". The female equivale ...
and his team to detect changes in the radiation of the Sun. They began by inventing instruments to measure solar radiation. Later, when Abbot was SAO head, they established a solar station at
Calama, Chile Calama is a city and commune in the Atacama Desert in northern Chile. It is the capital of El Loa Province, part of the Antofagasta Region. Calama is one of the driest cities in the world with average annual precipitation of just . The River L ...
to complement its data from
Mount Wilson Observatory The Mount Wilson Observatory (MWO) is an Observatory#Astronomical observatories, astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson (California), Mount Wilson, a peak in the San Gabrie ...
. He detected 27 harmonic periods within the 273-month
Hale cycle The Solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surface. Over ...
s, including 7, 13, and 39-month patterns. He looked for connections to weather by means such as matching opposing solar trends during a month to opposing urban temperature and precipitation trends. With the advent of
dendrochronology Dendrochronology (or tree-ring dating) is the scientific method of chronological dating, dating tree rings (also called growth rings) to the exact year they were formed in a tree. As well as dating them, this can give data for dendroclimatology, ...
, scientists such as Glock attempted to connect variation in tree growth to periodic solar variations and infer long-term secular variability in the
solar constant The solar constant (''GSC'') measures the amount of energy received by a given area one astronomical unit away from the Sun. More specifically, it is a flux density measuring mean solar electromagnetic radiation ( total solar irradiance) per un ...
from similar variations in millennial-scale chronologies.


Coronagraph

Until the 1930s, little progress was made on understanding the Sun's corona, as it could only be viewed during infrequent total solar eclipses.
Bernard Lyot Bernard Ferdinand Lyot (2 7 February 1897 in Paris – 2 April 1952 in Cairo) was a French astronomer. Biography An avid reader of the works of Camille Flammarion, he became a member of the Société Astronomique de France in 1915 and made ...
's 1931 invention of the
Coronagraph A coronagraph is a telescopic attachment designed to block out the direct light from a star or other bright object so that nearby objects – which otherwise would be hidden in the object's bright glare – can be resolved. Most coronagr ...
– a telescope with an attachment to block out the direct light of the solar disk – allowed the corona to be studied in full daylight.


Spectroheliograph

American astronomer
George Ellery Hale George Ellery Hale (June 29, 1868 – February 21, 1938) was an American astrophysicist, best known for his discovery of magnetic fields in sunspots, and as the leader or key figure in the planning or construction of several world-leading ...
, as an
MIT The Massachusetts Institute of Technology (MIT) is a private research university in Cambridge, Massachusetts, United States. Established in 1861, MIT has played a significant role in the development of many areas of modern technology and sc ...
undergraduate, invented the
spectroheliograph The spectroheliograph is an instrument used in astronomy which captures a photography, photographic image of the Sun at a single wavelength of light, a monochromatic image. The wavelength is usually chosen to coincide with a optical spectrum, spec ...
, with which he made the discovery of solar
vortices In fluid dynamics, a vortex (: vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
. In 1908, Hale used a modified spectroheliograph to show that the spectra of hydrogen exhibited the
Zeeman effect The Zeeman effect () is the splitting of a spectral line into several components in the presence of a static magnetic field. It is caused by the interaction of the magnetic field with the magnetic moment of the atomic electron associated with ...
whenever the area of view passed over a sunspot on the solar disc. This was the first indication that sunspots were basically magnetic phenomena, which appeared in opposite polarity pairs. Hale's subsequent work demonstrated a strong tendency for east-west alignment of magnetic polarities in sunspots, with mirror symmetry across the solar equator; and that the magnetic polarity for sunspots in each hemisphere switched orientation from one solar cycle to the next. This systematic property of sunspot magnetic fields is now commonly referred to as the '' Hale–Nicholson law'', or in many cases simply ''Hale's laws''.


Solar radio bursts

The introduction of radio revealed periods of extreme static or noise. Severe
radar jamming Radar jamming and deception is a form of electronic countermeasures (ECMs) that intentionally sends out radio frequency signals to interfere with the operation of radar by saturating its receiver with noise or false information. Concepts that blan ...
during a large solar event in 1942 led to the discovery of solar radio bursts.


Satellites

Many satellites in Earth orbit or in the heliosphere have deployed solar telescopes and instruments of various kinds for ''in situ'' measurements of particles and fields.
Skylab Skylab was the United States' first space station, launched by NASA, occupied for about 24 weeks between May 1973 and February 1974. It was operated by three trios of astronaut crews: Skylab 2, Skylab 3, and Skylab 4. Skylab was constructe ...
, a notable large solar observational facility, grew out if the impetus of the
International Geophysical Year The International Geophysical Year (IGY; ), also referred to as the third International Polar Year, was an international scientific project that lasted from 1 July 1957 to 31 December 1958. It marked the end of a long period during the Cold War w ...
campaign and the facilities of
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
. Other spacecraft, in an incomplete list, have included the OSO series, the
Solar Maximum Mission The Solar Maximum Mission satellite (or SolarMax) was designed to investigate Solar phenomena, particularly solar flares. It was launched on February 14, 1980. The SMM was the first satellite based on the Multimission Modular Spacecraft bus man ...
, Yohkoh, Solar and Heliospheric Observatory, SOHO, Advanced Composition Explorer, ACE, TRACE, and Solar Dynamics Observatory, SDO among many others; still other spacecraft (such as MESSENGER, Fermi Gamma-ray Space Telescope, Fermi, and NuSTAR) have contributed solar measurements by individual instruments. Modulation of solar bolometric radiation by magnetically active regions, and more subtle effects, was confirmed by satellite measurements of the total solar irradiance (TSI) by the ACRIM1 experiment on the
Solar Maximum Mission The Solar Maximum Mission satellite (or SolarMax) was designed to investigate Solar phenomena, particularly solar flares. It was launched on February 14, 1980. The SMM was the first satellite based on the Multimission Modular Spacecraft bus man ...
(launched in 1980). The modulations were later confirmed in the results of the ERB experiment launched on the Nimbus program, Nimbus 7 satellite in 1978. Satellite observation was continued by ACRIMSAT, ACRIM-3 and other satellites.Active Cavity Radiometer Irradiance Monitor (ACRIM
total solar irradiance monitoring 1978 to present
(Satellite observations of total solar irradiance); access date 2012-02-03


Measurement proxies

Direct irradiance measurements have been available during the last three cycles and are a composite of multiple observing satellites. However, the correlation between irradiance measurements and other proxies of solar activity make it reasonable to estimate solar activity for earlier cycles. Most important among these proxies is the record of sunspot observations that has been recorded since ~1610. Solar radio emissions at 10.7 cm wavelength provide another proxy that can be measured from the ground, since the atmosphere is transparent to such radiation. Other proxy data – such as the abundance of cosmogenic isotopes – have been used to infer solar magnetic activity, and thus likely brightness, over several millennia. Total solar irradiance has been claimed to vary in ways that are not predicted by sunspot changes or radio emissions. These shifts may be the result of inaccurate satellite calibration. A long-term trend may exist in solar irradiance.


Other developments

The Sun was, until the 1990s, the only star whose surface had been resolved. Other major achievements included understanding of: * X-ray-emitting loops * Corona and solar wind * Variance of solar brightness with level of activity and verification of this effect in other solar-type stars * The intense Fibril state of the magnetic fields at the visible surface of a star like the sun * The presence of magnetic fields of 0.5×105 to 1×105 gauss at the base of the conductive zone, presumably in some fibril form, inferred from the dynamics of rising azimuthal flux bundles. * Low-level Electron neutrino emission from the Sun's core.


21st century

The most powerful flare observed by satellite instrumentation began on 4 November 2003 at 19:29 UTC, and saturated instruments for 11 minutes. Region 486 has been estimated to have produced an X-ray flux of Solar flare#Classification, X28. Holographic and visual observations indicate significant activity continued on the far side of the Sun. Sunspot and infrared spectral line measurements made in the latter part of the first decade of the 2000s suggested that sunspot activity may again be disappearing, possibly leading to a new minimum. From 2007 to 2009, sunspot levels were far below average. In 2008, the Sun was spot-free 73 percent of the time, extreme even for a solar minimum. Only 1913 was more pronounced, with no sunspots for 85 percent of that year. The Sun continued to languish through mid-December 2009, when the largest group of sunspots to emerge for several years appeared. Even then, sunspot levels remained well below those of recent cycles. In 2006, NASA predicted that the next sunspot maximum would reach between 150 and 200 around the year 2011 (30–50% stronger than cycle 23), followed by a weak maximum at around 2022. Instead, the sunspot cycle in 2010 was still at its minimum, when it should have been near its maximum, demonstrating its unusual weakness. Cycle 24's minimum occurred around December 2008 and the next maximum was predicted to reach a sunspot number of 90 around May 2013. The monthly mean sunspot number in the northern solar hemisphere peaked in November 2011, while the southern hemisphere appears to have peaked in February 2014, reaching a peak monthly mean of 102. Subsequent months declined to around 70 (June 2014). In October 2014, sunspot AR 12192 became the largest observed since 1990. The flare that erupted from this sunspot was classified as an X3.1-class solar storm. Independent scientists of the National Solar Observatory (NSO) and the Air Force Research Laboratory (AFRL) predicted in 2011 that Cycle 25 would be greatly reduced or might not happen at all.


References


External links

* * * * * NOAA / NESDIS / NGDC (2002
Solar Variability Affecting Earth
NOAA CD-ROM NGDC-05/01. This CD-ROM contains over 100 solar-terrestrial and related global data bases covering the period through April 1990. * *

updated every Monday {{The Sun Solar phenomena Vortices Space physics Solar System