|
Rising Of Stars
The heliacal rising ( ) of a star or a planet occurs annually when it becomes visible above the eastern horizon at dawn just before sunrise (thus becoming "the morning star"). A heliacal rising marks the time when a star or planet becomes visible for the first time again in the night sky after having set with the Sun at the western horizon in a previous sunset (its heliacal setting), having since been in the sky only during daytime, obscured by sunlight. Historically, the most important such rising is that of Sirius, which was an important feature of the Egyptian calendar and astronomical development. The rising of the Pleiades heralded the start of the Ancient Greek sailing season, using celestial navigation, as well as the farming season (attested by Hesiod in his Works and Days). Heliacal rising is one of several types of risings and settings, mostly they are grouped into morning and evening risings and settings of objects in the sky. Culmination in the evening and then morning ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sky, night; their immense distances from Earth make them appear as fixed stars, fixed points of light. The most prominent stars have been categorised into constellations and asterism (astronomy), asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated to stars. Only about 4,000 of these stars are visible to the naked eye—all within the Milky Way galaxy. A star's life star formation, begins with the gravitational collapse of a gaseous nebula of material largely comprising hydrogen, helium, and traces of heavier elements. Its stellar mass, total mass mainly determines it ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Works And Days
''Works and Days'' ()The ''Works and Days'' is sometimes called by the Latin translation of the title, ''Opera et Dies''. Common abbreviations are ''WD'' and ''Op'' for ''Opera''. is a didactic poem written by ancient Greek poet Hesiod around 700 BC. It is in dactylic hexameter and contains 828 lines. At its center, the ''Works and Days'' is a Almanac, farmer's almanac in which Hesiod instructs his brother Perses in the agricultural arts. Scholars have seen this work against a background of agrarian crisis in mainland Greece, which inspired a wave of Greek colonies, colonial expeditions in search of new land. In the poem, Hesiod also offers his brother extensive moralizing advice on how he should live his life. ''Works and Days'' is perhaps best known for its two mythological aetiology, aetiologies for the toil and pain that define the human condition—the story of Prometheus and Pandora, and the so-called Ages of Man, Myth of Five Ages. Synopsis In ''Works and Days'', Hesi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Circumpolar Star
A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets below the horizon due to its apparent proximity to one of the celestial poles. Circumpolar stars are therefore visible from said location toward the nearest pole for the entire night on every night of the year (and would be continuously visible throughout the day too, were they not overwhelmed by the Sun's glare). Others are called ''seasonal'' stars. All circumpolar stars lie within a circumpolar circle whose size is determined by the observer's latitude. Specifically, the angular measure of the radius of this circle equals the observer's latitude. The closer the observer is to the North or South Pole, the larger its circumpolar circle. Before the definition of the Arctic was formalized as the region north of the Arctic Circle which experiences the midnight sun, it more broadly meant those places where the 'bear' constellations (Ursa Major, the Great Bear, and Ursa Minor, the Little Bear) we ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Equator
The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumference, halfway between the North Pole, North and South Pole, South poles. The term can also be used for any other celestial body that is roughly spherical. In three-dimensional space, spatial (3D) geometry, as applied in astronomy, the equator of a rotating spheroid (such as a planet) is the parallel (circle of latitude) at which latitude is defined to be 0°. It is an imaginary line on the spheroid, equidistant from its geographical pole, poles, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid with the plane (geometry), plane perpendicular to its axis of rotation and midway between its geographical poles. On and near the equator (on Earth), noontime sunlight appears almost directly o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Latitude
In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at the south pole to 90° at the north pole, with 0° at the Equator. Parallel (latitude), Lines of constant latitude, or ''parallels'', run east-west as circles parallel to the equator. Latitude and longitude are used together as a coordinate pair to specify a location on the surface of the Earth. On its own, the term "latitude" normally refers to the ''geodetic latitude'' as defined below. Briefly, the geodetic latitude of a point is the angle formed between the vector perpendicular (or ''Normal (geometry), normal'') to the ellipsoidal surface from the point, and the equatorial plane, plane of the equator. Background Two levels of abstraction are employed in the definitions of latitude and longitude. In the first step the physical surface i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Archeoastronomy
Archaeoastronomy (also spelled archeoastronomy) is the Interdisciplinarity, interdisciplinary or multidisciplinary study of how people in the past "have understood the phenomenon, phenomena in the sky, how they used these phenomena and what role the sky played in their cultures". Clive Ruggles argues it is misleading to consider archaeoastronomy to be the study of ancient astronomy, as modern astronomy is a scientific discipline, while archaeoastronomy considers symbolically rich cultural interpretations of phenomena in the sky by other cultures. It is often twinned with ''ethnoastronomy'', the anthropology, anthropological study of skywatching in contemporary societies. Archaeoastronomy is also closely associated with historical astronomy, the use of historical records of heavenly events to answer astronomical problems and the history of astronomy, which uses written records to evaluate past astronomical practice. Archaeoastronomy uses a variety of methods to uncover evidence ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Precession Of The Equinoxes
In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's Rotation around a fixed axis, rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In particular, axial precession can refer to the gradual shift in the orientation of Earth's axis of rotation in a cycle of approximately 26,000 years.Hohenkerk, C.Y., Yallop, B.D., Smith, C.A., & Sinclair, A.T. "Celestial Reference Systems" in Seidelmann, P.K. (ed.) ''Explanatory Supplement to the Astronomical Almanac''. Sausalito: University Science Books. p. 99. This is similar to the precession of a spinning top, with the axis tracing out a pair of Cone (geometry), cones joined at their Apex (geometry), apices. The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis—astronomical nutation, nutation and polar motion—are much smaller in magnitude ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Zodiac
The zodiac is a belt-shaped region of the sky that extends approximately 8° north and south celestial latitude of the ecliptic – the apparent path of the Sun across the celestial sphere over the course of the year. Within this zodiac belt appear the Moon and the brightest planets, along their orbital planes. The zodiac is divided along the ecliptic into 12 equal parts, called " signs", each occupying 30° of celestial longitude. These signs roughly correspond to the astronomical constellations with the following modern names: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. The signs have been used to determine the time of the year by identifying each sign with the days of the year the Sun is in the respective sign. In Western astrology, and formerly astronomy, the time of each sign is associated with different attributes. The zodiacal system and its angular measurement in 360 sexagesimal degree ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Proper Motion
Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable reference such as the International Celestial Reference Frame (ICRF). Patterns in proper motion reveal larger structures like stellar streams, the general rotation of the Milky Way disk, and the random motions of stars in the Galactic halo. The components for proper motion in the equatorial coordinate system (of a given epoch, often J2000.0) are given in the direction of right ascension (''μ''α) and of declination (''μ''δ). Their combined value is computed as the ''total proper motion'' (''μ''). It has dimensions of angle per time, typically arcseconds per year or milliarcseconds per year. Knowledge of the proper motion, distance, and radial velocity allows calculations of an object's motion from the Solar System's frame of reference an ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Axial Precession
In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In particular, axial precession can refer to the gradual shift in the orientation of Earth's axis of rotation in a cycle of approximately 26,000 years.Hohenkerk, C.Y., Yallop, B.D., Smith, C.A., & Sinclair, A.T. "Celestial Reference Systems" in Seidelmann, P.K. (ed.) ''Explanatory Supplement to the Astronomical Almanac''. Sausalito: University Science Books. p. 99. This is similar to the precession of a spinning top, with the axis tracing out a pair of cones joined at their apices. The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis— nutation and polar motion—are much smaller in magnitude. Earth's precession was historically called the precession of the equinoxes, b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sidereal Year
A sidereal year (, ; ), also called a sidereal orbital period, is the time that Earth or another planetary body takes to orbit the Sun once with respect to the fixed stars. Hence, for Earth, it is also the time taken for the Sun to return to the same position relative to Earth with respect to the fixed stars after apparently travelling once around the ecliptic. It equals for the J2000.0 epoch, or a little over 366 sidereal days. The sidereal year differs from the solar year, "the period of time required for the ecliptic longitude of the Sun to increase 360 degrees", due to the precession of the equinoxes. The sidereal year is 20 min 24.5 s longer than the mean tropical year at J2000.0 . At present, the rate of axial precession corresponds to a period of 25,772 years, so sidereal year is longer than tropical year by 1,224.5 seconds (20 min 24.5 s, ~365.24219*86400/25772). Before the discovery of the precession of the equinoxes by Hipparchus in the Hellenistic peri ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Solar Year
A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronomical seasons. For example, it is the time from vernal equinox to the next vernal equinox, or from summer solstice to the next summer solstice. It is the type of year used by tropical solar calendars. The tropical year is one type of astronomical year and particular orbital period. Another type is the sidereal year (or sidereal orbital period), which is the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars, resulting in a duration of 20 minutes longer than the tropical year, because of the precession of the equinoxes. Since antiquity, astronomers have progressively refined the definition of the tropical year. The entry for "year, tropical" in the '' Astronomical Almanac Online G ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
