To Them That Runs the Ship:
A long time ago I had a friend who kept Hubble telescope photos on the walls at work and just generally "knew stuff." Anyway, one day he walks up to me and sez, "Ya see the moon there? It's half full. And that is where the Earth was in space three hours ago." Wow, I sez. And being the trusting fool I am, I have spread this "fact" about to many people. But I ask myself, is this true?
-- Jeffrey Fox, the net
And now, sensibly, you ask us. You can't be any more amazed than we are to learn that this "fact" you've spread around is absolutely true. Or at least as absolute as anything in astronomy is. Get out a pencil and paper. Draw a dot for the sun, then draw the Earth's orbit around it. Next, draw a circle for the moon's orbit around earth. Note that the orbits intersect. As it happens, even in three dimensions, the orbits (more or less) intersect since they're (more or less) in the same plane. Sez our resident astronomer, "Because the Earth moves eastward around the sun at about 66,000 miles per hour, when the moon lies near its last quarter phase (rises at midnight and is visible due south at dawn) we cross that distance in about 3.6 hours." This Saturday is the next half moon; try lunaroutreach.org for dates and times (past and future) of moon phases.