Date: Fri Aug 22 10:26:18 2008

Author: Anthony Lapinski

Subject: celestial motion


I'm teaching astronomy this fall, and I was thinking about the motions of
the Sun and Moon to begin the course.

Roughly, the Moon's orbital speed is about 2000 mph. It's own diameter is
4 times less than Earth's, or about 2000 miles. Thus, the Moon is the
only known satellite that moves through space at approximately its own
diameter each hour. Is this why it takes about an hour for solar/lunar
eclipses to go from start to totality?

What I want to know is the Sun's angular speed in the sky. The Sun is
about 0.5 across. So in a 12-hour day, it moves 180 from east to west.
This is 15/hr, or 0.25/min. When we look at a sunset, it already
happened. Since light takes about 8 min to go from the Sun to Earth, the
Sun has moved 2 in this time -- about 4 solar diameters.! This seems a
bit large, or did I make an error?