Models of the Sky - Celestial Sphere We'll now go back to the coordinates discussion. How about the East-West designations? We used latitude for the North-South system, so let's see if we can do the same with longitude for the East-West coordinates. Unfortunately, we can't use the longitude system since the objects in the sky move too quickly - they aren't located constantly over the same location on the Earth. Instead, astronomers use a system based on a clock. This is logical since it appears that the sky gets back in the same configuration after 24 hours (again, you should remember that this is only how it appears; the sky isn't really rotating!). Astronomers divide the sky into 24 units known as hours, which go all the way around the Celestial Sphere. To make your life complicated the name for the East-West coordinate system is known as Right Ascension (R.A. for short). Values of R.A. increase as you go further to the east. There is a location labeled 0 h ; further east is 1 h , then 2
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