On
the afternoon of 8th June the planet Venus will pass across the Sun's disc as a
small black spot. Venus will enter from the top right
at approximately 3.15 pm and will slowly move across towards the top left.
The sun will have set before the transit is complete.
On
6th June 2012 almost the whole
transit will be visible from Melbourne but it will mean a crack of dawn start!. The transit
will just have started at dawn. Venus will be a small black dot at the bottom
of the sun's disc. The transit will continue until Venus leaves the sun's disc
half way up the left hand side at 2.50 pm.
To get a prevue of these transits visit
J. Giesen's Transit Web Site. It takes a little time to load but the wait is worth it. The web site allows you to make all manner of observations but the essential thing to do is:
Click the position of Melbourne on the world map, set the Time Offset to be UT +10Hrs and then click on Watch 2004 Transit. Set the Local time at 14.45. Now step through the time to watch the transit. Notice the sun sets before the transit is over!
Transits
of Venus have a special place in astronomical history and in the history of Australia.
Copernicus
had persuaded us that the planets went round the Sun and Kepler's Laws enabled
us to calculate the sizes of the orbits of the planets. To calculate them,
that is in terms of the Astronomical Unit (AU). The Astronomical Unit being
the distance from the Sun to the Earth. Thus Venus is 0.723 AU from the Sun, Mercury is 0.387 AU
and Mars is 1.524 AU. The problem is that despite the pioneering work of the
Greeks, Aristarchus and Eratosthenes, no one was very sure of the size of the
AU.
The first observation of the Transit of Venus was made in 1639 by
Jeremiah Horrocks
from his home near Liverpool in England. On the basis of his single set of observations he was able to make an estimate of the the distance between the Sun and the Earth, the Astronomical Unit (the AU). In 1716 English
astronomer Edmund Halley realized that by observing a transit of
Venus simultaneously from several parts of the world one would be able to
calculate a much more accurate value for the AU. Halley knew
that he would not live to see either of the next pair of transits (1761 and
1769). However he made a reasonable attempt. Sir Edmund Halley lived to the age of 86; he died in 1742.
Halley
left detailed instructions for future generations. In both 1761 and 1769 teams
of astronomers were establish in remote and far flung parts of the world. The
major effort was contributed by the British and the French. One huge complication
was that in1761 Britain and France were at war. Typically the scientists would have
none of this nonsense. They were not going to miss this opportunity. Special
permits were granted by both sides to allow the observations to take place.
The data obtained in 1761 was valuable, nevertheless the scientists were
convinced that, given this experience, they would do even better next time.
Fortunately by 1769 the war was over and international travel was less
hazardous. One of the British parties sailed in the Endeavor captained by
James Cook. The Endeavor's
mission was to circumnavigate the globe and explore the southern Pacific Ocean. On the way, they were to put in at the island of Tahiti to make their observations. This they
followed by a four month cruise up the east coast of Australia making detailed
maps of Terra Australis.
When at last all the data from the two transits was
pulled together and the calculations performed, a value of 153 million
kilometres was obtained for the Astronomical Unit. This is within 1% of the
modern accepted value. What a triumph!
Observing the Transit and
not harming your eyes.
Under no circumstances must
you look directly at the Sun. Even a setting Sun. The intensity of light is
so strong that when focused on to your retina it can cause serious and lasting
burns.
The rules for observing the
transit are similar to the rules for observing an eclipse of the Sun. In fact,
a transit is an eclipse, the only difference being is that it is Venus ( not the Moon) passing
in front of the Sun. However, whereas in an eclipse of the sun the moon typically covers a large part of the sun's disc in the case of the transit of Venus the small dot covers only 2% of the disc. Observing the transit is more like observing sunspots.
This is a task for the experts. The projection techniques commonly used for observing eclipses will not work (resolution too low). Filters should not be trusted (even if they are dark enough they are easily scratched).
Fortunately experts are at hand and several people will be setting up webcams to observe the transit. Our advice is to gather round the computer and enjoy this very special event in total safety.
Personal post script: I was in London for the day. The weather was magnificent and I was able to set up a screen and projection lenses. My grandson was born two days before the transit. I tried my best to interest him in this very special event but he was totally unimpressed. He is a great disappointment to his grandfather.
