Question

What determines the size of a planet ?
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Answer 1

Answer:

The most common is to measure the apparent angular diameter of the planet – how big it looks against the sky – very precisely using a telescope. Combining this with a measure of its distance (deduced from its orbit around the Sun) reveals the planet's actual size.

Answer 2

Combining this with a measure of its distance (deduced from its orbit around the Sun) reveals the planet's actual size. The most common is to measure the apparent angular diameter of the planet – how big it looks against the sky – very precisely using a telescope. If you know how far away a planet is from you, you can determine its linear diameter D. The diameter of a planet D = 2p × (distance to the planet) × (the planet's angular size in degrees)/360°, where the symbol p is a number approximately equal to 3.14 (your calculator may say 3.141592653...).