The Earth is further from the Sun than Venus, but how much further? Twice as far? Ten times?

When we measure something we use a scale : we consider the size of one thing in terms of another.

Make a fist with your hand (it's about the size of a large orange isn't it?), and then locate a point about 10m away from you.

If your fist was the Sun, the Earth would be more than 10 metres away and less than 1 mm in diameter (the tip of a ball-point pen).

Venus orbits the Sun three times in roughly two Earth years. On rare occasions Venus can be seen (from Earth) passing across the face of the Sun, this is called the Transit of Venus.

Usually Venus appears to pass either above or below the Sun because the plane of Venus' orbit is slightly tilted to the Earth's own orbit around the Sun. The Transit of Venus has only been observed and recorded six times since telescopes became available early in the 17th century.

Perhaps you remember The Transit of Venus happening in 2004, it will happen again in 2012.

Although scientists can measure distances across space using radio waves, that's a recent ability, before that the only measurement that could be made was angle (the change in direction of view through a telescope) and time.

The Transit of Venus was a valuable observation because it provided the data with which the Earth to Sun distance could be calculated. This distance is called the Astronomical Unit and is used like a scale for the Solar System.

But even without a Transit of Venus to provide data astronomers could know the ratio between the Venus to Sun distance and the Earth to Sun distance. What could they observe and what calculation would they need to do?

Google will give you all sorts of interesting things connected with the Astronomical Unit and the Transit of Venus, you may also enjoy looking at this link to the Motivate Project , but before you do that imagine you are living in the 17th century and don't have the internet, how might this Venus to Sun : Earth to Sun ratio be known?