Have you been watching Venus in the evening sky? It’s beautiful – especially when it’s near the moon.

Venus and Jupiter above the moon as darkness falls on February 24

Moon and Venus closest for the month on February 25

Here’s something fun to think about as you gaze at Venus. By June 2012, as Venus leaves the evening sky to enter the morning sky, this brightest of planets will pass right in front of the sun, to stage one of the rarest of predictable astronomical phenomena: a transit of Venus across the sun’s face. This upcoming transit of Venus will be the last one for the 21st century. It will take place across a period of nearly 7 hours on June 5-6, 2012. During the transit, Venus will appear in silhouette as a small, dark dot moving in front of the solar disk. This exceedingly rare astronomical event – a transit of Venus – won’t happen again until December 11, 2117.

Transit of Venus June 8, 2004. Image Credit: Wikimedia Commons

The last transit of Venus was June 8, 2004. But don’t be fooled by that proximity in time. Transits of Venus are very rare, plus transits tend to occur in pairs. They occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of up to 121.5 years. Before 2004, the last pair of transits were in December 1874 and December 1882.

A transit of Venus is, in effect, a tiny sort of eclipse of the sun. Venus, after all, crosses in front of the sun, just as the moon does during a total eclipse. Thus proper eye protection is absolutely essential for watching a transit of Venus, else you risk blindness or permanent eye damage.

Observing solar eclipses safely

Who will see the June 5-6, 2012 transit of Venus?

Depending on where you live worldwide, the transit of Venus will happen on June 5 or 6, 2012. If you live in the world’s Western Hemisphere (North America, northwestern South America, Hawaii, Greenland or Iceland), the transit will start in the afternoon hours on June 5. In the world’s Eastern Hemisphere (Europe, Africa, Asia, Australia or New Zealand), the transit will first be seen at sunrise or in the morning hours on June 6.

Check out an observatory or astronomy club near you to see if it is hosting a public presentation of the event. Or, look for a webcam. As the time for the transit approaches, return to this post. We’ll be gathering info on public presentations and webcams.

Image credit: NASA Eclipse Web Site. Click here for a larger map

From the mainland U.S., the sun will set as the transit is still taking place (on June 5), though the West Coast will get to see more hours of the transit than the East Coast does. Here in Austin, Texas, we’ll see the first half of the transit, while the second half will take place after the sun goes beneath our horizon. In North America, it’ll be to our advantage to find a level western horizon, as the sun will be low in the west at the time of the transit.

As for the world’s Eastern Hemisphere, it’s as equally important to find a level eastern horizon on June 6. For Africa, Europe, much of Asia and western Australia, the sun will rise (on June 6) as the transit is taking place. Elsewhere in the Eastern Hemisphere, the transit will start in the morning hours on June 6.

Local clock times for the Venus transit in your time zone

Many people find it difficult to translate the Universal Time of astronomical events to the clock time in their own time zones. Fortunately, we are happy to link you to a site that lets you find out the clock times of the transit for your own time zone, with absolutely no need to convert from Universal Time.

Local transit times for your time zone (no conversion necessary!)

Sunrise/sunset times for your sky

Everything should be pretty straight forward at this handy-dandy site – except, perhaps, the terminology being used to define Venus’ major contacts with the sun. To help out, we provide an illustration of the transit of Venus, with the contact points defined below.

Venus goes from east to west (left to right) across the solar disk

Image credit: Fred Espenak

The contact times (I, II, greatest transit, III and IV) on the above illustration are given in Universal Time. It is important to note that these times are for an imaginary observer at the Earth’s center – not the Earth’s surface. So if you translate from Universal Time to the clock time in your time zone, it’ll give you a ballpark reference of your local transit times, which won’t be off by any more than a maximum of plus or minus seven minutes.

But make it easy on yourself. Click here to know the precise transit times without having to do any figuring whatsoever.

Why is a transit of Venus so rare?

Venus, the second planet outward from the sun and next planet inward from Earth, swings between the Earth and sun (at a point called inferior conjunction) five times every eight years, or one time in every 584 or so days. (See the Diagram of Venus’ orbit around the sun below.) More often than not, Venus passes above or below the solar disk at inferior conjunction – that point in its orbit where Venus passes out Earth’s evening sky and into Earth’s morning sky.

Diagram of Venus’ orbit around the sun

Venus passes in between the Earth and sun at inferior conjunction

If Venus and Earth revolved around the sun on the same plane, there would be five inferior conjunctions – and five transits – of Venus every eight years. However, Venus’ orbital plane is inclined to Earth’s orbital plane by 3.4^o. Because the orbital planes of the two planets don’t quite mesh, a combination of factors is necessary for a transit of Venus to take place in Earth’s sky.

For half of Venus’ orbit, Venus travels south of the Earth’s orbital plane, and for the other half of Venus’ orbit, Venus travels north of the Earth’s orbital plane. At two places in Venus’ orbit, Venus crosses the Earth’s orbital plane at points called nodes. If Venus is going from south to north, it’s called an ascending node, or if going from north to south, it’s called a descending node.

If Venus at inferior conjunction closely coincides with one of its nodes, then a transit of Venus is in the works. On June 5-6, 2012, Venus swings to inferior conjunction and sufficiently close to its descending node to present the last transit of Venus until December 11, 2117.

Bottom line: The last transit of Venus in this century will take place June 5 or 6, 2012. The exact date will depend on your hemisphere on Earth. During the transit, Venus will appear in silhouette as a small, dark dot moving in front of the solar disk. The last transit was June 8, 2004, and the next one won’t be until December 11, 2117. It’s essential to use proper eye protection when trying to observe a transit of Venus.