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.
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, 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.
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.