The brightest planet, Venus, is due to 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 seven 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.
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.
Why is a transit of Venus so rare?
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.
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.
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 3.4o. 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: Here’s everything you need to know about the last transit of Venus in this century on June 5-6, 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 next transit of Venus won’t be until December 11, 2117. Get ready to observe the transit now!