One of the basic needs of astronomy, as well as other physical sciences, is to give reasonable descriptions for the positions of objects relative to each other. Scientifically, this is done in mathematical language, by properly assigning numbers to each position in space; these numbers are called coordinates and the system defined by this procedure a coordinate system.
We specify locations and directions on the Earth with the cardinal directions, North, East, South, West. The term"meridian"generally refers to any line that runs pole to pole, on the surface of a sphere.
Compared to the size of Earth, all celestial objects (with the exception of some satellites and meteorites in Earth's atmosphere) are far away. Viewing them, they look all at far distance, which can not be distinguished easily, so that they look as being positioned on a far-away sphere. Thus each observer can look at the skies as being manifested on the interior of a big sphere, the so-called celestial sphere.
Equatorial coordinates, also called simply celestial coordinates, are the standard coordinate system for the sky. It is meant for the fixed sphere of the stars within which the Earth and Sun move. The equatorial system is almost the same as Earth's coordinate system, since it is also defined by the Earth's axis and equator. The celestial equator is the projection of Earth's equator onto the sky, and the North and South celestial poles (NCP and SCP) are the points on the sky directly above Earth's North and South poles.
The horizon system is defined locally for each observer, or site, on Earth (or another celestial body). Its origin is the observer's location, its reference axis is the local vertical or plumb line (defined e.g. by the local gravitational field), and its reference plane is the apparent horizon or simply horizon perpendicular to it at the observer's location. The direction directly, or vertically, above the observer, or its intersection point with the virtual celestial sphere, is called zenith, the opposite direction or point, vertically below the observer, is called nadir.
Stars and other celestial bodies appear to rotate around the celestial poles (as actually Earth rotates and carries the observer away below them), i.e. move along circles of constant declination in the co-rotating equatorial system. By doing so, stars will cross the local meridian (defined e.g. by zero hour angle HA) twice a day; these events are called transits or culminations, i.e., the upper and the lower transit, or the upper and the lower culmination. These events also mark the maximal and minimal altitude a the objects can reach in the observer's sky, and may both take place above or below the horizon of the observer, depending on the declination Dec of the object and the geographic latitude B of the observer.
Astronomical Coordinate System
ISHAN K PATEL
B.TECH-AEROSPACE ENG./5TH SEM
THE ULTIMATE GOAL
• Position of the Objects
• Relative Motion
• Dimensions and Characteristics
TRANSFORMATION FROM HORIZONTAL
SYSTEM TO EQUATORIAL SYSTEM
Measured observed coordinates in the horizontal system, azimuth A
and altitude a, can be transformed to (co-rotating) equatorial ones,
hour angle HA and declination Dec, for an observer at geographical
latitude B, by the transformation formulae:
cos Dec * sin HA = cos a * sin A
sin Dec = sin B * sin a + cos B * cos a * cos A
cos Dec * cos HA = cos B * sin a + sin B * cos a * cos A
EFFECT OF EARTH’S ROTATION
• Objects seem to rotate around the celestial poles
• Move along the circle of constant declination
• Culmination of Circumpolar objects