In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric with the Earth. All objects in the sky can be thought of as lying upon the sphere. Projected, from their corresponding terrain equivalents, are the celestial equator and the celestial poles.
Many ancient societies believed that the stars were equidistant from the Earth and that this sphere was a real model of the universe. This model is a useful abstraction, but not correct. Everything we see in the sky is so very ...
An armillary sphere (also known as a spherical astrolabe, armilla, or armil) is a model of the celestial sphere, invented by Eratosthenes in 255 BC. Its name comes from the Latin armilla (circle, bracelet), since it has a skeleton made of graduated metal circles linking the poles and representing the equator, the ecliptic, meridians and parallels. Usually a ball representing the Earth or, later, the Sun is placed in its center. It is used t ...
Throughout Chinese history, astronomers have created celestial globes to assist the observation of the stars.
The earliest celestial globe was dated back to 52 BC in the West Han dynasty by the astronomers Geng Shou-chang (耿壽昌) and Luo-xia Hong (落下閎). The first water powered celestial globe was created by Zhang Heng in the East Han dynasty.
Then Li Chun-feng (李淳風) of the Tang dynasty created one in 633 AD with three spherical layers to cali ...
The geocentric model (in Greek: geo = Earth and centron = center) of the universe is a paradigm which places the Earth at its center. Common in ancient Greece after the discovery of the approximately spherical shape of Earth, it was believed by both Aristotle and Ptolemy. Most Greeks assumed that the Sun, Moon, stars, and planets orbit Earth. Similar ideas were held in ancient China.
The geocentric model assumes a spherical Earth; thus it is not the same as the older flat Earth model. Also, according to this model, ...
The celestial equator is a great circle on the imaginary celestial sphere, which is actually the plane of the terrestrial equator extended out into the universe (i.e., it could be constructed by extrapolating the Earth's equator until it touches the celestial sphere). The celestial equator is inclined by ~23.5°, with respect to the ecliptic plane; a result of axial tilt. Celestial objects near the celestial equator are visible worldwide, b ...
The two celestial poles are the imaginary points where the Earth's spin axis intersects the celestial sphere.
The sky appears to drift overhead from east to west, completing a full circuit around the sky in 24 (sidereal) hours. This phenomenon is due to the spinning of the Earth on its axis. The Earth's spin axis intersects the celestial sphere at two points. These points are the celestial poles. As the Earth spins, they remain fixed in the sky, and all other points seem to rotate around them. The celestial poles are also the p ...
The antisolar point is the imaginary point on the celestial sphere exactly opposite the sun. It is where anticrepuscular rays appear to converge, and on a moonless night away from city lights, it is often possible to see the gegenschein there.
Category: Spherical astronomy
Other related archivesSpherical astronomy, anticrepuscular rays, celestial sphere, gegenschein, sun
Right ascension (RA; symbol α: Greek letter alpha; celestial longitude) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.
Right ascension - Explanation.
RA is comparable to longitude, measured from a zero point known as the vernal equinox point. RA is measured in hours, minutes, and seconds. Being closely tied with sidereal time, it is both a uni ...
The First Point of Aries, also called the vernal equinox point, is one of the two points on the celestial sphere where the celestial equator intersects the ecliptic. It is defined as the position of the Sun on the celestial sphere at the time of the vernal equinox in the Northern Hemisphere. It was named after the constellation in which it occurred in ancient times - Aries. However due to precession, the point gradually moves around the ecliptic. It entered the constellation Pisces in about 500 A.D. (or around 100 A.D. if the m ...
In astronomy, a celestial coordinate system is a coordinate system for mapping positions in the sky. There are different celestial coordinate systems each using a coordinate grid projected on the celestial sphere, in analogy to the geographic coordinate system used on the surface of the Earth. The coordinate systems differ only in their choice of the fundamental plane, which divides the sky into two equal hemispheres along a great circle. (The fundamental plane of the geographic system is the Earth's equator). Each coordinate system i ...
The ecliptic plane is the geometric plane that contains the orbit of the Earth. The ecliptic is the intersection of the ecliptic plane and the celestial sphere. A more intuitive definition would be to say that the ecliptic is the apparent path of the Sun during a year as seen from Earth. The orbits of most planets in the Solar System lie very close to it. Seen from the Earth, this is a bisecting great circle, superimposed upon the celestial sphere, which contains the different points of the Sun's path, relative to the backgroun ...
The zenith, in astronomy, is the point in the sky which appears directly above the observer. More precisely, it is the point on the sky with an altitude of +90 degrees, and it is the pole of the horizontal coordinate system. Geometrically, it is the point on the celestial sphere intersected by a line drawn from the center of the Earth through your location on the Earth's surface. The point opposite the zenith is the nadir.
The zenith is, by ...
The seven climes (klima, plural klimata, meaning "inclination", referring to the angle between the axis of the celestial sphere and the horizon) was a notion of dividing the Earth into zones in Classical Antiquity. The lists of klimata found in early geographers vary in their extension, but by convention, they numbered seven, counted from south to north. This number was taken up by Ptolemy who in his Geography divided the northern tempe ...
A constellation is a group of stars visibly related to each other in a particular configuration.
Constellation - Explanation.
In three-dimensional space, most of the stars we see have little relation to one another, but can appear to be grouped on the celestial sphere of the night sky. Humans excel at finding patterns and throughout history have grouped stars that appear close to one another into constellations.
An "unofficial" constellation, that is, one that may be widely known but is not recognize ...
The sidereal year is the time for the Sun to return to the same position in respect to the stars of the celestial sphere. The sidereal year is the orbital period of Earth. A sidereal year equals 365.25636042 mean solar days. The sidereal year is 20 minutes and 24 seconds longer than the tropical year.
As the Sun and the stars cannot be seen at the same time, this requires a little explanation. If you look every dawn at the eastern sky, the last stars you see appearing are not always the same. In a week or so you notice an upwar ...
According to medieval Christian theologians, the Angels are organized into several orders, or Angelic Choirs. The most influential of these classifications was that put forward by Pseudo-Dionysius the Areopagite in the Fourth or Fifth century, in his book The Celestial Hierarchy.
In this work, the author drew on passages from the New Testament, specifically Ephesians 6:12 and Colossians 1:16, to construct a schema of three Hierarchies, Spheres or Triads of angels, with each Hierarchy containing three ...
Musica universalis or music of the spheres is a medieval philosophical concept that regards the proportions in the movements of the celestial bodies - the Sun, Moon and planets - as a form of musica (the medieval Latin name for music). This music was not thought of as an audible sound, but simply as a mathematical concept. The Greek philosopher Pythagoras was frequently credited with originating the concept, which stemmed from his semi-mystical, semi-mathematical philosophy and its associated system of numerology. (See Pythagor ...
Boötes, a name deriving from Egypt, is one of the 88 modern constellations and was also one of the 48 constellations listed by Ptolemy. Boötes is generally referred to as the Bear Watcher, since it appears to be watching over the constellations Ursa Major and Ursa Minor. It contains the third brightest star in the night sky, Arcturus. Note that the "ö" in the name is a diaeresis, not an umlaut.
The constellation is located between 0° and +60° declination, 13 and 16 hours of right ascension on the celestial sphere.
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In the context of spacecraft, attitude control is control of the angular position and rotation of the spacecraft, either relative to the object that it is orbiting, or relative to the celestial sphere.
In flight dynamics, the orientation is often described using three angles called yaw, pitch, and roll.
Attitude control - Sensors.
Inertial guidance system
The most conventional sensor. It tracks the current position and angle based on earlier position, and acceleration. Mea ...
A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). The precise length of time depends on which point of the ecliptic one chooses: starting from the (northern) vernal equinox, one of the four cardinal points along the ecliptic, yields the vernal equinox year; averaging over all starting points on the ecliptic yields the mean tropical year. At J 2000.0 it was 365.242190517 days or 365 d., 5 h., 48 min. and about 45.26 s.Including:
