3.1 Two coordinate systems
The coordinate system that you will primarily use in this course is the Equatorial coordinate system. This is based on the concept of a celestial sphere which is attached to the sky rather than to the surface of the Earth. This equatorial system has coordinates known as Right ascension and Declination and is used for controlling advanced telescopes such as COAST and for specifying an object’s position uniquely.
We will also briefly consider a second system: the Altitude–Azimuth (Alt-Az) system. This is mostly used when working with simple telescopes but it is also useful for determining when an object is well placed for viewing from a given location. It is important to understand both systems as they serve different purposes, so we will look briefly at this Altitude-Azimuth system before considering the Equatorial system in more detail.
Why would it not be possible to use ground-based latitude and longitude coordinates to specify positions of celestial objects?
Because the Earth is rotating a given celestial object will not always be directly above the same point on the Earth’s surface. For astronomical purposes a system of coordinates attached to fixed directions in space rather than to fixed points on the Earth is required.