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Bruce BettsJune 2, 2012

Transit of Venus June 5: Why Should You Care and How to Observe


Don't miss the transit of Venus on June 5 (or 6th depending on time zone). A Venus transit occurs when Venus comes directly between the Earth and Sun. Venus appears as a small dark disk that over many hours crosses the disk of the Sun. 

Here are my top reasons why you should care about the little black spot on the Sun Tuesday:

  1. It looks moderately spiffy.
  2. The next Venus transit won't occur until the year 2117. The mere fact that one won't occur again in your lifetime, in all probability, provides both a sense of mortality (I hate that) and a feeling that now is your only chance. I will also try to attend the last Rolling Stones tour, currently scheduled for 2041. By the way, Venus transits appear in pairs. The last one was in 2004 (not visible from North America). But those pairs are separated from the next pair by about a century. 
  3. Venus transits have figured prominently in history amazingly enough. If you measure the exact timing of the transit from widely separated locations on the Earth, you can use that information along with Kepler's Laws of Motion to calculate the distance from the Earth to the Sun. That was a key unknown in understanding the scale of the solar system. Once you have that distance, it can be extrapolated to give a scale for the solar system. This motivated voyages to see the transits in the 1700s and 1800s, for example motivating Captain Cook's voyage to Tahiti and the South Pacific in 1769. When you observe a Venus transit, you metaphorically reach back in history.
  4. At least for me, things like transits and eclipses help to visualize the three dimensional nature of the solar system. It is easy for our brains to, without thinking, kind of picture every object out there as equidistant on the celestial sphere, even if we know better. But, when one object moves in front of another, the 3D solar system slaps your perceptions around.
  5. It is a nice example of one technique we now use to find planets around other stars. This technique, oddly enough, is called the transit method. The Kepler spacecraft has found over 2000 exoplanet candidates using this technique which measures the tiny drops in light that occur when a planet passes in front of the star. With enough sensitivity, an astronomer in another solar system could discover Venus from Venus transits.

So, even if all you do is pull up live images on the Internet, don't miss this transit. And, think about the history and the 3D solar system while you are at it.

Here are the basics of the transit. The transit will be best viewed from the Pacific Ocean. North America will be able to see the start of the transit before sunset, while southern Asia, northern Africa and most of Europe will see the end of it. Here is a more detailed plot of visibility.

The transit gets underway about 22:09 UT or 15:09 PDT. The detailed timing of the transit is defined by contacts, as shown in the following diagram. It is the recording of the variations in timing of those contacts from different locations on the globe that can allow the triangulation to be done to determine the Sun Earth distance.

Here are the timings of the contacts:

Event  UT PDT
Contact I  22:09 15:09
Contact II  22:27 15:27
Greatest 01:29 18:29
Contact III 04:31 21:31
Contact IV 04:49 21:49

You can observe the transit using proper filters for your eyes (Venus should be visible for those with normal vision), using properly filtered telescopes, by projecting an image of the Sun with a pinhole or binoculars, or by checking any number of live telescope feeds on the web. See the links below for more. And, hey, be safe, don't look at the Sun without true, proper filters (see links below for more detail). Sunglasses are NOT good enough, and even your dog's tail probably isn't, well, depending on breed. Seriously, read some of the safetly links in the links below.

While you are thinking about looking up, there is also a partial lunar eclipse on Sunday night (Monday morning) June 3/4. It will be visible from most of Asia, Australia, the Pacific Ocean and the Americas. It starts about 10:00 UT (3:00 PDT) and lasts for about two hours. At maximum, about one third of the Moon will be in eclipse.


A good general site to find everything from transit details, to links to live telescope feeds you can check during the transit, to information on how to safely observe the Sun, to local events, is NASA's Transit of Venus site.

For details on more gory details on timings and locations a good site is NASA's eclipses and transits site.

Some other good sites for general information are: has all sorts of information.
Sky and Telescope's Venus Transit viewing guide and links therein.
And, if you want to recreate the measuring of the Earth Sun distance with a bunch of other people see this Astronomers without Borders site and their phone app.
Over the next few days, I will be tweeting Random Space Facts related to Venus Transits on twitter @RandomSpaceFact

And, stay tuned, the next transit of Mercury is in 2016. It is a smaller little black spot, but its transits occur more frequently.

Read more: history, the Sun, astronomy, Venus, transit of Venus

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society
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