Astronomy Question: Calculating Distance Between Planets at a Given Time…

Elham in Iran asks:

“I wanna know if there is any exact formula for distance between planets at the right time !”

Distances between planets vary, as orbits themselves are slightly elliptical.
If you know the orbits of two planets, and the position each planet is in each orbit,
yes, you could calculate the distance between two planets.

Much as I dislike wikipedia, this article may help you learn more:
http://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion

You could chart out the exact positions of each planet mathematically given a certain time/date.
For instance, when a probe is launched to say, Mars or Titan, you have to know where your destination is in relation to the earth…, otherwise your probe will miss the destination entirely!

Here’s a few links to help you understand more about Kepler’s laws of planetary motion:

http://www.davidcolarusso.com/astro/

This site will help show you the distance between earth and any planet in the solar system:
http://www.fourmilab.ch/cgi-bin/Solar

Here are some detailed information on calculating the exact position of a planet at a given time:
http://stjarnhimlen.se/comp/tutorial.html
http://www.stargazing.net/kepler/ellipse.html