dipper@utastro.UUCP (Debbie Byrd) (01/14/86)
You can get a sense that you're standing on a ball in space by looking at the moon. More on the moon as a mirror -- after this. January 14 The Moon as a Mirror The Earth and moon are alike in that they're both globes in space -- with one half always in sunlight -- and one half always facing away. If you could see these worlds from a vantagepoint outside the Earth/moon system, they'd look like twin balls -- one larger and more colorful than the other -- but both alike in phase. Both would be crescents, both half lighted, or both full -- depending on how you looked at them. From the surface of the Earth, you can sometimes get a sense of our world's orientation toward the sun -- by looking at the globe of the moon. When there's a crescent moon like you'll see in the next few evenings, the moon is in the west -- with its sunlit side facing mostly away from you. The Earth is facing the sun in exactly the same way as the moon. In the evening, Earth's sunlit side has just moved out from under you as our world spins on its axis. So Earth's sunlit side is facing in the same direction as the sunlit side of the moon you see this evening -- both facing the sun -- below your western horizon in the evening hours. Take a look at the crescent moon this evening. Try to picture the moon with respect to the Earth and sun -- and think about what makes the moon appear as a crescent. Then realize that the world you're standing on mirrors the moon in space -- two globes -- each always half lighted. Script by Deborah Byrd. (c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin