Your Guide to the Transit of Venus

Posted by Brian Ventrudo

On June 5-6 you get your last chance to see one of the rarest of astronomical events, the Transit of Venus, during which the black disk of Venus passes across the glowing disk of the Sun.  This transit has happened just seven times since the invention of the telescope more than 400 years ago.  The last transit was in 2004.   There won’t be another until December 2117.  Here’s how and where to see next month’s transit for yourself…

Where To See The Transit
The maps below shows where the 2012 transit of Venus is visible…

Worldwide visibility map for the 2012 transit of Venus (click to enlarge).

The western and central Pacific, including most of Australia, New Zealand, Fiji, and Hawaii can see the entire transit.  Western and southern Africa, Spain and Portugal, and eastern South America will not see the transit  at all because it occurs when the sun has set.  And the rest of the world can see some of the transit after the Sun rises or before it sets.

The June 5-6 transit begins at 22:09 UT (GMT) on June 5, and ends at 04:50 UT on June 6.  You can convert from GMT to your local time here: http://wwp.greenwichmeantime.com/gmt-converter/

Why It Happens
Like a solar eclipse, a transit occurs when Venus, rather than the Moon, passes between Earth and the Sun.  And like a solar eclipse, the transit requires careful alignment of the Sun, Earth, and Venus.  As seen from Earth, Venus usually passes over or under the Sun every 584 days, on average.  But the geometry and periods of the orbits of two planets cause Venus to pass in front of the Sun at well-defined intervals of 121.5 and 101.5 years in either June or December.  And the transits occur in pairs separated by eight years.  The last transit occurred on June 8, 2004.   The last pair of transits were on December 1874 and December 1882.

The Transit of Venus once held the key to understanding the size of the solar system.  In the early 18th century, Edmund Halley determined a way to measure the distance from the Earth to the Sun by precisely timing the transit of Venus from widely separated parts of the Earth.  Once this distance was known, the distances to other planets could be determined through Kepler’s Laws.

These transits were so important that most advanced nations sent astronomers around to world to measure the events of 1761 and 1769.  The transit of Venus in 1761 yielded few conclusive results despite hundreds of attempted measurements.  But the transit of 1769 was measured precisely by, among others, the team led by Lieutenant James Cook, RN, who witnessed the event from Tahiti before sailing on to claim Australia for England.  Astronomers used Cook’s measurements to calculate a distance to the Earth of 150 million kilometers, close to the now-accepted value of 149,597,870.7 kilometers.

How To See The Transit
For this June 5-6 transit, Venus will traverse the northern half of the Sun’s disk (see below)…

You’ll get the best view of the transit with a telescope, but a telescope is not required. Telescope or not, you’ll need a safe solar filter.  Here’s some advice on finding a solar filter suitable for observing this event.  If you don’t have your own filter, check if your local astronomy club is holding a public event during the transit.  They’ll have properly equipped scopes and other hardware to help you enjoy this rare event.

The transit of Venus unfolds in four stages.  First, the leading edge of the planet contacts the Sun.  Then the trailing edge makes contact, which is hard to time exactly because of the “black drop effect” that bleeds darkness from the limb of the planet as it moves onto the solar disk.  The same two stages reverse themselves as the planet leaves the solar disk.   The June 5-6, 2012 transit takes about 6 hours, which is a long time compared to the scant few minutes of a solar eclipse.  This link gives you precise timing for the four stages of the transit as seen from 121 cities throughout the world…

http://eclipse.gsfc.nasa.gov/transit/venus/city12-1.html

During the transit, the black disk of Venus, just 33x smaller than the solar disk, blocks enough light to measurably decrease the Sun’s brightness.  NASA’s Kepler observatory, in fact, uses this same idea… a transiting planet blocking light from its home star… to look for Earth-like planets around nearby stars.  Astronomers will use the 2012 transit of Venus to test new measurement techniques to find extra-solar planets using space-based telescopes.

The history and the rarity and the beauty of this event make it a compelling and memorable sight.  Please… observe it for yourself if you can.