how do we measure the size of planets and other entities like black holes that are extremely big?

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how do we measure the size of planets and other entities like black holes that are extremely big?

In: Earth Science

We can measure how big it looks in the sky, and then using its distance calculate its size. We know the relationship between distance and relative size. For example, if someone is 6 feet tall we know how big they’ll appear to be if they are 100 feet away, 1000 feet away, a mile away, etc

Like most things in astronomy, it is determined indirectly.

Basically we determine the mass of the object through interactions with other objects

We determine it’s density through observations of it’s spectral signature.

Knowing the mass and density, we can calculate it’s volume and size.

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The specific steps can vary depending on the type of object, but it always comes down to thsoe steps.