How far away is the Sun, in miles?

How to get close to this number

If you remember only one number in astronomy, make it this one, because everything else in the solar system is quoted relative to it. It is the definition of the astronomical unit, so Mars, Jupiter and the rest are all given as multiples of this distance. Learn it once and you can reconstruct the scale of the entire solar system from memory.

The cross-check that catches a bad guess is light travel time: sunlight takes about eight minutes to reach us. Combine that with the speed of light and you should land on the right answer — and if your guess is off by a factor of ten, this is the check that tells you immediately. Two independent routes to the same figure is how you know an estimate is sound rather than lucky.

As with the Moon, the orbit is an ellipse, so the real distance swings either side of this average over the course of a year. Counterintuitively for anyone in the northern hemisphere, the Earth is closest to the Sun in January: seasons are driven by axial tilt, not by distance. The figure itself is now fixed by international agreement rather than remeasured, which is why it does not drift between sources.