Transit of Venus

Ken Tapping, March 28, 2012

In the sky this week…

  • Mars is in the east, and Saturn rises about 10:00 p.m.
  • The Moon will reach First Quarter on March 30.

There is no way anyone can miss seeing the planet Venus in the western sky after sunset. It looks like an escaped aircraft landing light. That bright but definitely fainter object nearby is Jupiter.

Venus is the second planet out from the Sun; we live on the third. As Venus circles the Sun, we see it oscillating either side of the Sun in the sky. When Venus is far enough from the Sun to be above the horizon when the Sun is below, we see it shining brightly in the sky before dawn as the “Morning Star,” or in the sky after sunset as the “Evening Star.” Obviously there will be times when Venus is in line with the Sun, either on the near side or the far side. However, because its orbit is at a small angle compared with the Earth’s, Venus is only very rarely exactly in line with the Sun. Such events, called transits, occur in pairs, eight years apart, every hundred years or so. This June, we will have the second of the latest pair. For most of us this will be our last chance to see one of these events!

On March 27 Venus reaches its greatest angle from the Sun; after that, as the planet moves between us and the Sun, it will drop back into the sunset glare. Then, on June 5 for us in the Western Hemisphere, it will pass between us and the Sun. It will be visible as a small, black disc crossing the Sun’s face.  It will start to cut into the edge of the Sun at 18:09:38 EDT (15:09:38 PDT) and be fully on the disc at 18:27:38 (15:27:38). It will touch the opposite edge at 00:31:39 (21:31:39) and be fully off at 00:49:35 EDT (21:49:35 PDT). Unfortunately for most of Canada, this means the Sun will set before the transit is over. However, those in the Canadian Arctic will see the whole show.

Before going further, here is a big note of caution. One should never, ever look through a telescope that is pointed at the Sun. The heat and light will destroy your sight permanently in far less than a second. The safe way is to point the telescope at the Sun and then project the image from the eyepiece onto some stiff, white card. Point the telescope at the Sun by using its shadow on the card. If you have any doubts, don’t try it. A good option would be to contact the local amateur astronomy group. They are likely to have set up an organized session to observe the event.

Transits of Venus have been critically important events in astronomy. They can be used to measure the distance between the Earth and Sun. We simply measure the start and end times of the transit, when Venus is in line with the edges of the solar disc, from widely-separated locations on Earth. If we know the distance between those locations, simple triangulation will give us the Sun’s distance.

The value of these measurements led past astronomers to travel to distant locations on the Earth to make these observations. One poor individual travelled to the South Seas, saw nothing because of clouds, and decided to stick around until the last one of the pair, which would occur eight years later. Then, eight years later he was clouded out again. On the plus side, eight years on a tropical island, expenses paid, cannot be all bad. Astronomy is not like that anymore.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton,  BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mailken.tapping@nrc-cnrc.gc.ca

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