Celestial source

A celestial source is any astronomical source except the Earth.
Spherical astronomy
'Spherical astronomy' or 'positional astronomy' is the branch of astronomy that is used to determine the location of objects on the celestial sphere, as seen at a particular date, time, and location on the Earth. It relies on the mathematical methods of spherical geometry and the measurements of astrometry.
The primary elements of spherical astronomy are coordinate systems and time. The coordinates of objects on the sky are listed using the equatorial coordinate system, which are based on the projection of the Earth's equator onto the celestial sphere. The position of an object in this system is given in terms of right ascension (alpha) and declination (delta). The latitude and local time can then be used to derive the position of the object in the horizontal coordinate system, consisting of the altitude and azimuth.
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the celestial equator and the celestial poles. The celestial sphere is a very practical tool for positional astronomy.
The celestial sphere is divided by projecting the equator into space. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere. Likewise, one can locate the Celestial Tropic of Cancer, Celestial Tropic of Capricorn, North Celestial Pole, and South Celestial Pole. The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system.
Celestial coordinate system
In astronomy, a 'celestial coordinate system' is a coordinate system for mapping positions on the celestial sphere.
There are different celestial coordinate systems each using a system of spherical coordinates 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. For example, the fundamental plane of the geographic system is the Earth's equator. Each coordinate system is named for its choice of fundamental plane.
At right is an image showing the relationship between the various celestial coordinate systems on the celestial sphere. The Sun and Earth are not shown to scale but to indicate the Sun's orbital direction around the galactic center and Earth's orbital direction around the Sun.
Epoch
In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as celestial coordinates, or elliptical orbital elements of a celestial body, where these are (as usual) subject to perturbations and vary with time. The time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, or of the node of its orbit relative to a reference-plane, or of the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit.
The International Astronomical Union (IAU) decided at their General Assembly of 1976 that the new standard equinox of J2000.0 should be used starting in 1984. (Before that, the equinox of B1950.0 seems to have been the standard.) There have been some indications that the epoch J2000.0 is (at least currently) intended to have a longer period of usage than to mid-century.
To indicate the magnitude of change from one epoch to another, for example, according to SIMBAD, V Sagittae is located in equatorial coordinates for the epoch 1950 B1950.0 at right ascension (RA) and declination (Dec) and in J2000.0 at RA Dec .
Constellation
The celestial sphere has been divided into 88 constellations. The International Astronomical Union (IAU) modern constellations are areas of the sky. The original boundaries of the IAU modern constellations are along vertical and horizontal lines of right ascension and declination for the epoch B1875.0, which means that due to precession of the equinoxes, the borders on a modern star map (e.g., for epoch J2000) are already somewhat skewed and no longer perfectly vertical or horizontal. This skew increases over the years and centuries. However, this does not change the area of any constellation.
The total area of all constellations is the total area of the sky, which is 4π steradians, or 4π x (180/π) , or approximately 41252.961 square degrees.
Although there are only 88 IAU constellations, the sky is actually divided into 89 irregularly shaped boxes as the constellation Serpens is split into two separate sections, Serpens Caput (the snake's head) to the west and Serpens Cauda (the snake's tail) to the east:
: Serpens Caput (428.484 sq. deg.; midpoint: RA: 15 46.49, Dec: +10 58.20), and
: Serpens Cauda (208.444 sq. deg.; midpoint: RA: 18 07.60, Dec: -04 51.73).
Celestial source astrometry
Astrometry is the branch of astronomy that relates to precise measurements and explanations of the positions and movements of stars and other celestial bodies. Trigonometric parallax is a primary focus of astrometry and requires a concentrated effort. The larger annual parallactic shift occurs in right ascension. The quality of a parallax determination depends on many factors, including the quality of the available stellar reference frame and particularly on the observational coverage of the parallactic ellipse.<ref name=Dahn/> Roughly 100 observations adequately distributed over the parallactic ellipse and spanning at least three years in epoch range may be needed to produce a formal standard error for the relative parallax of ~ ±0.5 mas.<ref nameDahn/> Objects that are faint require nearly ideal observing conditions of a cloudless, dark sky (moonless and well away from twilight), with seeing better than 1.2 to 1.4 arcsec full-width at half maximum (FWHM), and exposure times of 60-90 min in length for only relatively low signal-to-noise (S/N) results.<ref nameDahn/>
Celestial mechanics
'Celestial mechanics' is the branch of astronomy that deals with the motions of celestial objects. The field applies principles of physics, historically classical mechanics, to astronomical objects such as stars and planets to produce ephemeris data. Orbital mechanics (astrodynamics) is a subfield which focuses on the orbits of artificial satellites. Lunar theory is another subfield focusing on the orbit of the Moon.
The only major celestial source that does not belong to a constellation is the Sun. The Sun travels through the 13 constellations along the ecliptic, the 12 of the Zodiac and Ophiuchus.
The Sun emits radiation. Some portion of the Earth’s atmosphere blocks a percentage of it.
Minor celestial sources such as comets and the planets of the solar system also travel through several constellations.

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