The Transit of Venus: Tuesday 8th June, 2004
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By Lachlan MacDonald, 2004
On the afternoon of Tuesday 8th June 2004, for the first time in over 120 years, Venus’s dark silhouette will pass across the face of the Sun in of astronomy’s rarest and most famous events: A Transit of Venus.
WARNING: IT IS EXTREMELY DANGEROUS TO LOOK AT THE SUN. SERIOUS EYE DAMAGE MAY RESULT.
What Is A Transit?
A transit is when one of the two inner planets (Mercury and Venus) passes precisely between the Earth and the Sun, similar to when the Moon passes between the Earth and the Sun during a Solar Eclipse, although on a very much smaller scale. The result is a small dark spot slowly moving across the face of the Sun. As the two inner planets are much further away from the Earth, they appear much smaller, which is why the transit appears only as a small black spot, as opposed to the Moon almost entirely blocking out the Sun during a Solar Eclipse.
A transit will not always occur when Mercury or Venus passes between the Earth and the Sun, as they will usually pass either above or below the Sun, as viewed from Earth. Transits of Venus are so rare that no one living today has seen one, as they occur only twice in 8 years and then not again for over 100 years. Don’t worry too much if you miss this transit, as there will be another one on 7th June 2012, otherwise you will have to wait until either 2117 or 2125!
The 2004 Transit
The transit of Venus this year will be the first since 1882. Australian observers will be able to observe the beginning stages of the transit, however the Sun will set before the end of the transit. Observers in Western Australia, South Australia and Northern Territory will be able to view the transit through until it’s mid-point, however the Sun will also set shortly thereafter for those observers.
The table below provides times of the transit for the capital cities in Australia: (All times are local time)
Observers in Europe, Asia and most of Africa, the transit will be visible from beginning to end, taking a little over 6 hours from start to finish.
The “Black Drop” Effect
Precise timing of the second (and third) contact can be difficult due to the “black drop” effect, where a dark thread appears to join the edges of Venus and the Sun making the exact timing of second and third contact difficult to establish.
There are many explanations for the black drop effect. In previous years, it was believed by some that this effect was due to the atmosphere of Venus (thus an illusion), however, as its atmosphere is too thin to be responsible, we now know that it is not an illusion. The “black drop effect” has also recently been photographed by the TRACE spacecraft during transits of Mercury. Recent explanations suggest the black drop effect may be a combination of blurring caused by the Earth’s atmosphere, with the dimming of the Sun’s disc close to its edge.
How Do I View The Transit Safely?
WARNING - OBSERVING THE SUN IS EXTREMELY DANGEROUS. DO NOT LOOK DIRECTLY AT THE SUN, UNLESS YOU ARE USING AN APPROVED SOLAR FILTER OR WEARING PROPER "ECLIPSE SUNGLASSES" SPECIALLY DESIGNED TO PROTECT YOUR EYESIGHT.
The best way to observe the transit safely is to visit your local amateur astronomical society, observatory or planetarium. These groups are likely to be running public viewing sessions for the transit.
If you have a small telescope you can observe the transit safely yourself by using the telescope to project an image of the sun onto a large flat surface.
NOTE: DO NOT LOOK THROUGH THE TELESCOPE OR THE FINDERSCOPE but, instead, use the shadow of the telescope to aim it directly at the sun, and you should see the Sun’s bright disc projected onto the flat surface.
You may need to adjust the focus of the projected image, until the black spot of Venus can be seen clearly and sharply. The spot will only be about 1/33rd of the Sun’s diameter, however should easily be seen. Viewing the projected image using this method is quite safe, however looking through the telescope will cause instant blindness. DO NOT LEAVE THE TELESCOPE UNATTENDED ESPECIALLY WHEN CHILDREN ARE PRESENT.
What Will I See?
For observers with a proper Solar Filter, a magnified view of the Sun through a telescope will show the silhouette of Venus slowly moving across the face of the Sun. For many, the most interesting part of the transit will be the 20 minutes between first and second contact, as Venus “enters” the solar disc.
During the 2004 transit (for Australian observers) there will be only two moments when the discs of Venus comes into contact with the Sun at a single point. First contact will occur just as Venus initially touches the solar disc. Second contact is at the exact second when Venus has fully “entered” the Sun’s disc.