What is the distance to the Sun? And do we know? These were questions that puzzled astronomers for centuries. Besides the answers we provided below, there are also lots of books in the market about the Solar System.
First, I’ll just give you the quick answer, so you can fill out your homework assignment and move on. The average distance to the Sun is 149,597,887.5 km. Astronomers call this distance 1 astronomical unit (or AU).
But this number isn’t entirely accurate, because the Earth travels an elliptical orbit around the Sun. At some points in its orbit, the distance from the Earth to the Sun is shorter, and other times its further.
The closest distance to the Sun, which astronomers call perihelion, is 147,098,074 km. And the furthest distance to the Sun – called aphelion – is 152,097,701 km.
Now… how do we know the distance to the Sun so accurately? Geometry. Early astronomers calculated the angles to Mars during opposite points of the Earth’s orbit around the Sun. This method allowed them to calculate that the Earth was about 140 million km from the Sun. A more accurate measurement was made in 1761 when Venus made a rare transit in front of the Sun. During the most recent transit of Venus in 2004, astronomers from around the world worked together to refine the current value for the distance to the Sun.
Here’s an article from NASA that talks about the story of Venus transits. Pretty exciting stuff.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.