SCIENCE Transits | The AU | The AU using transit method | The AU using radar

What is a Transit?

When an astronomical object gets between the Earth and another object, one of two things can happen. If the closer object appears to be about the same size or larger, we see what is known as an eclipse. For example, the Moon is 450 times smaller than the sun, but 450 times closer, making it appear the same size and when the Moon passes between the Earth and the Sun we experience a solar eclipse. When the Earth passes between the Moon and the Sun, the moon gets plunged into darkness and that is when we get a lunar eclipse.

If the intervening object is smaller however, we get what is known as a transit. As a result of its motion, it appears to move across the face of the object behind it and so we can track its path and deduce a lot of things from this. Transits of Venus occur on an eight-year/122-year shift pattern, due to Venus's motion out of the plane of the Earth-Sun system (i.e. if the orbit of the Earth round the sun was on the edge of a flat plate, Venus would sometimes be slightly above, slightly below, and occasionally on the plate depending on the time). Transits of Mercury happen more frequently, but are much more difficult to spot, as Mercury is smaller and a lot further away.