# Distance to the Sun

The mean Earth-Sun distance is very important in astronomy. It is called 1 Astronomical Unit or 1 AU.

It is rather easy to determine distances within the Solar System if you measure them in AU's.

• The planets move around the Sun in orbits.
• The orbits are really ellipses, but for our purposes we can consider them to be circles.
• Call the radius of the orbit a.
• This is the distance from the planet to the Sun.
• Let us measure a in AU.
• Thus for the Earth, a = 1.
• By observing the position of a planet in the sky over a period of months, we can determine the radius a of its orbit.
• Here is an example.

• This involves trigonometry, which is beyond the knowledge base assumed for this course.
• But you can see that you could measure the distance to Venus this way by simpling measuring the angle shown in the sky, making a scale drawing on a piece of paper, and measuring the distance between the Sun and Venus on your paper.
• Knowing the distance to Venus in AU, we could measure how big 1 AU is in km if we could measure the distance to Venus in km.
• The most accurate way to do that is to bounce a radar signal off of Venus.