How Do We Know How Far Away the Planets Are?

COOL DIVYA wants to know how we know that Jupiter is the largest planet, and I think that this is the best place to start answering that.

In The Beginning

The first good measurements of the distances to planets were done using a method called parallax, which I have mentioned in a few other posts.

You can experience parallax for yourself, right now: Look out of a window, or if you're feeling daring, actually go outside. Look at a distant tree or building or something. Close one eye and hold up your thumb so that it covers the thing you're looking at. Keeping your head still and your thumb steady, close your eye and open the other one. Your thumb is now no longer covering up the distant object even though you haven't moved anything.

This is parallax in action: viewing two objects from two different positions makes the closer object look like it has moved even if it has actually stayed still all along. Try the experiment again, but this time hold your thumb up closer to your face. Notice that when your thumb is closer, the distance it appears to move compared to the tree or building it's covering up is greater.

It's this effect that allows us to calculate distances to the closer object: We take measurements of a planet's position in the sky relative to more distant stars, wait six months for the Earth to move round to the opposite point in its orbit, and then take measurements of the same planet relative to the same stars.

To cut a long story short (which you can read more about here), you can use trigonometry* to work out a distance to the closer object which is, in this case, whichever planet we were looking at.

Repeated measurements give us a good idea of the direction and distance a planet is away from us, and we can eventually plot its motion around the Sun.

These Days

Nowadays we can bounce radar signals off other planets and time how long it takes them to get back to us. Because radar signals travel at the speed of light we can work out how far they must have travelled. Also, if we send a spacecraft out to another planet we can time how long its transmissions take to get back to us and work out how far they must have travelled in the same way.

* That SOH CAH TOA stuff.


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