Transit of Venus

A transit of Venus occurs when Venus passes directly between the Earth and the Sun, forming a small black circle that obscures a portion of the Sun as it goes across. Since Venus is closer to the Sun, and orbits almost twice as quickly as Earth, you would expect to see almost two of these events a year. But transits of Venus are actually very rare, only occurring in pairs every 100 years or so.

Why is there such a long break when Venus passes between the Earth and the Sun almost twice a year? It’s because the orbit of Venus is inclined at an angle of 3.4% relative to Earth. When Venus is directly in between the Earth and the Sun, it’s either above or below the Sun in the sky, and is just lost in the glare.

Earth and Venus have an orbital dance that follows a 243 year cycle. Let’s say that you have a transit of Venus at the beginning of this cycle. Then 8 years later you’ll see a second transit. This if followed by a gap of 121.5 years with no transits. Then you get another pair of transits broken up by 8 years. Then another long delay of 105.5 years. Then you’re back to the beginning of the cycle.

The length of time that Venus transits the Sun depends on the path it crosses the Sun with, but a transit usually takes 5-6 hours and is visible from whatever hemisphere of the Earth has the Sun in the sky when the transit occurs.

The most recent transit of Venus was on June 8, 2004, and the next one will occur on June 5, 2012. Then there won’t be another transit until December 10, 2117 and its pair on December 8, 2125.

We wrote several articles about the last Venus transit back in June, 2004. Here’s an article, and here’s an article about a transit of Mercury.

Want more information on Venus? Here’s a link to Hubblesite’s News Releases about Venus, and here’s NASA’s Solar System Exploration Guide to Venus.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Venus. Listen to it here, Episode 50: Venus.