Venus and the Sun

2012 sees the second of the transits of Venus in this 8-year transit pair. 2004 was the first. The last pair occurred in 1874 and 1882 and the next will occur on 1 December 2117 and 8 December 2125. Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that repeats every 243 years, with pairs of transits eight years apart, falling currently every 121.5 years then 105.5 years.

The next pairing is in 105.5 years and the last was 121.5. The half-year is because they alternate in a polarity from one to the next on opposite sides. This is currently June and December or Gemini and Sagittarius. This time frame can vary within the 243 year cycle with only single viewing in the cycle, which was the case until the beginning of the current paired transit cycle in 1518.

A Venus transit occurs when Venus moves directly between the Sun and earth and as you can see is a very rare event. A transit is similar to a solar eclipse by the moon but with Venus being so far away from earth the sun is not blocked but a small black circle is seen to pass across the face of the sun. This year (2012) the transit occurs on June 6 in the east and June 5 for those west of the International Date Line.

In Australia we are lucky enough to see the full transit from its first contact at 08:15 am until leaving at around 2:45 pm.

As it is not safe to look directly at the sun though, the only way to observe it first hand is with the aid of special viewing glasses, grade 14 welding glasses or with a telescope that has a special solar filter. A simple pinhole viewer is also suitable. Place a pin hole in the centre of one piece and hold it so the light from the sun shines through the hole and onto the 2nd piece of cardboard. I remember doing this as a young child in primary school. We all had our pieces of cereal boxes and stood on the school oval to watch a solar eclipse. The CSIRO has instructions on how to make your own here

Transits of Venus in front of the Sun have been used by astronomers to refine their calculation of the distance between the Sun and the Earth. This was important in terms of calculating distance in space, which is measured as astronomical units or AU. The distance between the earth and sun is the base measurement of 1 AU. Everything else is measured by how many times the distance between the Sun and earth would fit from here to there or equal to about 149,597,870.7 kilometres (92,955,807.3 mi). In order to get accurate measurement of distance in space it was important to have an accurate AU. For example Jupiter is 5.2 AU from the sun. This means it is 5 times the distance from the Sun that the earth is.

Image above is is a Ford Maddox Brown engraving of William Crabtree viewing the 1639 transit
using a telescope as a projector

This same measurement is used to define the size of distant stars. The mean diameter of Betelgeuse is 5.5 AU or 822 800 000 km. So Betelgeuse is so big that if it happened to drift our way it would not be able to squeeze between the Sun and Jupiter without taking out everything in between along with a little of our sun and Jupiter as well.

It can also be used to measure time in space with one AU being equal to 8.317 light minutes, which is the time it takes the light from the sun to reach us. So when something is measured in light years away, it is really a long way from us. It also shows that time and space really are the same thing.

For more information and times in your area check the NASA site here

© Sherrynne Dalby 2011

images courtesy wikimedia commons