Ask A Scientist© Astronomy Archive

Author:      doug a foster
How do scientists measure the size of the other planets and their distances 
from the Earth?

Response #:  1 of 1
Author:      jlu
Kepler, essentially, did it, and it was not simple.  The idea is basically
that if I know a planet's orbit around the Sun, and I know where the planet
appears to be at two different positions of the Earth in the Earth's orbit,
then I can figure the distance to the planet by triangulation.  Once I know
the distance, figuring the size is straight trigonometry based upon the
planet's apparent size.  It all checks, because the planet's distance from
the Sun can be calculated from the length of its year.