Celestial Navigation 101: Lesson 5, Objects in Motion (Introduction)

So far, we have looked at celestial objects as if they were more or less motionless in the sky. But with the exception of Polaris, nothing could be farther from the truth. The earth rotates on its own axis, causing the entire sky to appear to spin around it once every day. The earth revolves around the sun in an elliptical orbit once each year, and the other planets also revolve around the sun. The moon revolves around the earth, moving easterly relative to the stars. The stars themselves move relative to the earth and to one another. From our perspective this motion of the stars is minor, but not so minor that we can ignore it for our navigation.

I'll be on the water the next few days; when I'm back in front of a real computer we'll tackle the problem of how to determine the geographic position of a body in motion on a different body in motion.