Horizontal (Alt-Azimuth) Coordinates
Horizontal or alt-azimuth coordinates are so called because they are based on the position of a celestial object relative to the observer’s horizon.
The angle subtended by the object above the horizon is known as the Altitude, and the angle between the north point and the perpendicular projection of the object’s position down onto the horizon is known as the Azimuth.
Horizontal coordinates are the easiest way to locate an object in the sky from the observer’s location. However, as the Earth rotates on its axis, the horizontal coordinates of a celestial object will continuously change as it moves across the sky.
An object’s azimuth is given relative to the north point of the horizon, such that an object lying directly above, below or on the north point of the horizon will have an azimuth of 0 degrees. Objects directly to the south have an azimuth of 180 degrees, while an object to the East or West will have an azimuth of 90 degrees or 270 degree, respectively.
The altitude coordinate is the angle above the horizon such that an object on the horizon has an altitude of zero degrees and an object directly overhead (at the zenith) has an altitude of 90 degrees. Negative values can be used to indicate an object is hidden below the horizon, with -90 degrees indicating a position at the observer’s nadir – i.e. the point directly below the observer.
Because altitude and azimuth are angular measurements, fractions of a degree are often stated in minutes and seconds rather than decimals.
Equatorial Coordinates
Equatorial coordinates are so called because they are based on the position of an object relative to the celestial equator.
The angle subtended by the object above or below the equatorial plane of the Earth is known as the Declination (often abbreviated as dec or δ), and the angle between the first point of Aries (the position of the vernal equinox) and the perpendicular projection of the object’s position onto the celestial equator is known as the Right Ascension (often abbreviated as RA or α).
Knowing an object’s equatorial coordinates is useful for tracking its movement across the sky with the rotation of the Earth, when using an equatorial mount.
As with altitude and azimuth, an object’s declination is often stated in degree, minutes and seconds. Negative value are usually used to indicate a declination angle is to the south of the celestial equator. Therefore, declination values are usually between 90 degrees at the north celestial pole and -90 degrees at the south celestial pole, with 0 degrees indicating a position on the celestial equator.
Right ascension, however, is usually stated in hours (h), minutes (m), and seconds (s), with 24 hour being equivalent to a full circle, meaning that negative values aren’t used.
Note that, due to precession of the Earth’s axis of rotation, the equatorial coordinates of a fixed celestial object will change over time. Equatorial coordinates are therefore given with reference to a particular year, known as the epoch.
A standard epoch year is usually chosen when providing equatorial coordinates. The currently used standard epoch is J2000.0, meaning that equatorial coordinates are provided as at the 1st of January in the year 2000 at 12 noon (midday) TT (terrestrial time). The J in J2000.0 refers to the use of the Julian calendar.
Note that to precisely determine the equatorial coordinates of a star, at any given time, its proper motion as well as its parallax must also be taken into account.