Re: [Phys-l] celestial motion

["Dan Crowe" <Dan.Crowe@Loudoun.K12.VA.US>]

The phrase "moves through space" is vague.  You should specify that the
origin of the reference frame moves with the center of the Earth and the
coordinate axes maintain their directions relative to the "fixed"
stars.
 
The light that we see when the Sun is on the horizon was emitted ~8
minutes earlier.  Ignoring the refraction of sunlight in the Earth's
atmosphere, the Sun appeared ~2 degrees higher at the time that the
light was emitted.  By the time that the light reaches the Earth, the
Earth has rotated ~2 degrees, and the light appears to be coming from
the horizon, rather than from ~2 degrees higher.
 
Daniel Crowe
Loudoun County Public Schools
Academy of Science
dan.crowe@loudoun.k12.va.us 

> > > "Anthony Lapinski" <Anthony_Lapinski@pds.org> 8/22/2008 10:24 AM
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?