How Do We Know How Big the Planets Are?

COOL DIVYA wants to know how we know that Jupiter is the largest planet.

If we know how far away a planet is, then finding out how big it is is relatively easy.

In order to find out a planet's actual size we need to measure how big it looks from here. Specifically, we measure its 'angular diameter'. To understand what one of those is, go outside with a pencil. Stand some distance away from something big (a tree, a house, the local monster) and point your pencil at one edge of it. Now, keeping your body still, move your arm until the pencil is pointing at the opposite edge. If you measured the angle that your arm travelled through, you'd have the object's angular size.

Once we've got this info for a planet, and we've found out how far away it is, we have enough information to work out its actual size using... trigonometry*!

Doing this with all the planets helps us to work out all of their actual sizes and then compare them. That's how we know that Jupiter is the biggest planet in our solar system!




* And you always thought that trig was useless, didn't you?

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