Which kinds of Coordiates do we use to descirbe the motion of the sky?
Our Cosmic Address Our place in the universe. This figure illustrates our cosmic address. The Earth is one of the nine planets in our solar systems, our solar system is one among more than 100 billion star systems in the Milky Way Galaxy; the Milky Way is one of the two largest of about 30 galaxies in the Local Group; the Local Group lies near the outskirts of the Local Supercluster; and the local Supercluster fades into the background of structure throughout the universe. How do we describe the motion of the sky? – Coordinates Some Definitions To see how some of the spherical geometry can be used, let’s look at an everyday example – the measurement of position and distance on the surface of the Earth. The rotation of the Earth on its axis presents us with an obvious means of defining a coordinate system for the surface of the Earth. The two points where the rotation axis meets the surface of the Earth are known as the north pole and the south pole and the great circle (a circle on the
