Transit of Venus

Ken Tapping, June 6, 2012

In the sky this week…

  • Mars and Saturn dominate the southern sky.
  • The Moon will reach Last Quarter on June 11.

On June 6, 2012, Venus will pass between the Earth and the Sun. This planet, which is just a tiny bit smaller than ours, will be visible as a small, dark disc against the much bigger, brighter, solar disc. Venus is the second planet out from the Sun while we live on the third; because its orbit is closer to the Sun, Venus also travels faster than the Earth. This means that Venus overtakes us on the inside lane roughly every 600 days. It usually passes by above or below the solar disc – only rarely does it pass exactly between us and the Sun. These “Transits of Venus” happen in pairs, about eight years apart, with the pairs separated by long gaps of 105.5-121.5 years. This summer’s transit is the second of the pair; the first was in 2004, so it is unlikely that most of us will get another opportunity to see one of these events.  Since international astronomy uses Universal Time as a standard, this event starts late on June 5, and then continues onto June 6. First contact is at 22:09 UT; Venus is fully on the solar disc by 22:27 UT, starts to leave at 04:31 UT the next day and it is all over by 04:49 UT. However, for us in the Western Hemisphere, without time zones running behind Universal Time, the transit happens on the fifth. Here are the times for the Eastern and Pacific Time Zones, on June 5. First contact (18:09 EDT, 15:09 PDT), fully on (18:27 EDT, 15:27 PDT). In the EDT time zone the Sun will set before Venus reaches the other edge of the disc, so the remaining times are PDT: beginning to leave (21:31:39 PDT), all over (21:49:35 PDT), very close to sunset.

The problem with events like this is on one hand, one should not miss such a rare opportunity; but on the other hand, any sort of visual observation involving the Sun can be very dangerous unless you know exactly how to do it. If young, enthusiastic astronomers are involved, you have to be extra careful and vigilant.

It’s not that eclipses or transits of Venus are particularly dangerous. Observing the Sun is what can do the damage. Over our many years of evolution, our bodies have evolved instinctive precautions for protecting our eyes. On sunny days, without our even needing to think about it, we avoid looking at the Sun and if the Sun is unavoidably in our field of view, we instinctively blink and move our eyes to prevent damage. However, curiosity can overcome those protective instincts. The danger in eclipses, transits and other solar phenomena, is that it makes us want to stare at the Sun. Looking through a telescope that is pointed at the Sun can  be dangerously naive. If you want to enjoy this event of a lifetime, the best way is to see what the local astronomical society is up to. Many amateur groups are setting up Transit Observing Parties, with appropriate observing equipment, where you can go observe in safety with experts around to explain what is going on. Look at the website of the Royal Astronomical Society of Canada, at for observing sessions near you. This is an astronomical event of a lifetime, which astronomers used to travel to the other side of the world to observe. Don’t miss it.

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

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