Posted by **Judy** on Monday, September 18, 2006 at 9:08am.

I'm having trouble understanding the formulas for determining a planet's distance and mass.

A planet's distance is related to the period of revolution and the mass of the central star (e.g, the sun). The relationship is called Kepler's Third law. Distance can also be determined applying trigonometry, using parallax measurement.

The mass of a planet can be determined easily if it has satellites, by again using Kepler's third law. A planet's mass can sometimes be obtained by measuring its "perturbation" effect upon the orbit of a nearby planet or a space probe flying by.

Let us know which formula you are having trouble understanding and one of us w3ill try to provide more specific help.

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