How far away from the Sun is Jupiter?

Question posed by mamajoe.

Well, it's about 5 Astronomical Units (AU), but it'd be no fun if I just said that, would it?

We say that Jupiter is 5 AU from the Sun because that's a nice, round, easy to understand number to quote, but it's not the full story. The thing is, orbits aren't perfectly circular. They're elliptical, which is a bit like the shape you get if you take a circle and squash it a bit. That means over time Jupiter (and all the other planets in our solar system) get a little bit further away from and then a little bit closer to the Sun than the number we quote for their distance.

Jupiter, then, gets a little closer than 4.95 AU to the Sun, and then swings back out to be just over 5.45 AU away. This might not sound like a big change, and it isn't really in astronomical terms, but it equates to a difference of more than 74 million kilometres, or nearly 46 million miles.

So how far away is Jupiter from the Earth?
Given that Earth and Jupiter are both orbiting the Sun, and doing so at different speeds, the answer at any particular moment is a complicated one. So let's simplify it, and look at the extremes.

Jupiter, as we've already looked at above, varies between 4.95 AU and 5.45 AU from the Sun. The Earth wanders back and forth from a bit more than 0.98 AU to nearly 1.02 AU*.

When Earth and Jupiter are at their closest, they're on the same side of the Sun, Earth at its most distant, Jupiter at its closest, and the three objects forming a straight line. This puts Jupiter and Earth at about 3.93 AU apart.

When Earth and Jupiter are most distant, they're on opposite sides of the Sun, with Earth and Jupiter both at their most distant, and all three bodies forming a straight line, only with the Sun in the middle this time. When this happens, Earth and Jupiter are about 6.47 AU apart.

And that is why distances in our solar system are usually quoted as distances from the Sun rather than distances from the Earth.

* In case you haven't read this post, the Earth's distance from the Sun is what defines 1 AU.


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