Ask A Scientist© Astronomy Archive

Author:      cindy c loc
1. Why do planets appear to glow?
2. How did they measure the distance between the planets?
3. Is their any relationship between the distance of the planets and the
circumference of the planets?  

Response #:  1 of 1
Author:      richard a gerber
1. The planets do not produce any light of their own.  The Sun shines on
them, and we see the reflected light.
2. The most accurate method in use today uses radio waves, which travel at
the speed of light.  We can send radio waves from the Earth, which bounce
off the planets and then return to us.  Since we can measure the time delay
between sending the signal and receiving it, and we know how fast the waves
are traveling, we can figure out the distance to the planet.
3. The planets are arranged in such a way that the smaller planets are
located nearest the Sun (except for , which may be an exceptional case).  I
do not think anyone really knows for sure why this is the case.