This photographic plate was taken by a team of astronomers from the U.S. Naval Observatory in 1882. It shows Venus, during the transit of 1882, against the backdrop of the Sun. Venus is the black dot near the top of the photo, just left of center. Click on image for full size (19K JPEG) Image courtesy the U.S. Naval Observatory Library.

Transits of Venus are extremely rare astronomical phenomena. They occur in pairs, separated by eight years, with more than a century elapsing between successive pairs of transits. There will be two Venus transits in the 21st century, on June 8, 2004 and June 6, 2012.

Prior to the 2004 transit, only five Venus transits have been observed by humans. Those five occurred on December 4, 1639; June 5, 1761; June 3, 1769; December 8, 1874; and December 6, 1882. One other Venus transit took place since people first began looking skyward with telescopes in 1609. That transit was in 1631, but there is no record that anyone observed it.

The first observations of a transit of Venus were made in 1639 by Jeremiah Horrocks and William Crabtree. They refined Kepler's original predictions of Venus' orbit in order to determine the correct date of the 1639 transit.

Edmond Halley published a paper in 1716 describing how transits could be used to determine the distance from Earth to the Sun. At that point, astronomers didn't know how far we are from the Sun or the scale of distances within our Solar System in general.

In order to determine the Earth-Sun distance (called the Astronomical Unit, or AU) using a transit, astronomers needed to make observations of the transit from different locations on Earth. Ideally, such measurements should be made at locations as far apart as possible. During the pairs of transits in the 1700s and 1800s, astronomers organized expeditions to the far "corners" of the world to make the detailed observations required. The scientists successfully gathered the required data, and were thus able to make the first accurate estimates of the size of our Solar System and of the distance from Earth to the Sun. The values they calculated were very close to the currently accepted figures, which have been determined by bouncing radar signals off nearby planets.

In case you're planning for a very long life, the next pair of Venus transits after the 2012 event will occur on December 11, 2117 and December 8, 2125!

This photograph was taken in 1882. It shows Venus during the transit of 1882. The big white circle is the Sun. Venus is the black dot on the Sun. Venus is near the top of the Sun, just left of center. Click on image for full size (19K JPEG) Image courtesy the U.S. Naval Observatory Library.

Sometimes Venus passes between Earth and the Sun. This event is called a transit of Venus. Transits of Venus don't happen very often. There is a pattern in the time between transits of Venus. The pattern goes like this: after one transit there is another one eight years later, then more than 100 years go by, then another transit and then another eight years after that, then another wait of over 100 years, and so on.

The last Venus transit was in 1882. There are two coming up soon, on June 8, 2004 and June 6, 2012. After that there will be a long wait until the next transits. The next ones after 2012 will be on December 11, 2117 and December 8, 2125

There have only been six Venus transits so far since people started using telescopes to look at the sky. Those six were in the years 1631, 1639, 1761, 1769, 1874, and 1882.

A couple hundred years ago people didn't really know how far the Earth is from the Sun or how big our Solar System is. Astronomers were able to find out those distances by making measurements during Venus transits. They had to make measurements from many different places on Earth. During the Venus transits in the 1700s and 1800s, scientists made those measurements from many places on Earth. They were able to figure out that Earth is about 150 million kilometers (93 million miles) from the Sun.

This picture was made in 1882. It shows Venus during the transit of 1882. The big white circle is the Sun. Venus is the black dot on the Sun. Venus is near the top of the Sun. Click on image for full size (19K JPEG) Image courtesy the U.S. Naval Observatory Library.

Sometimes the planet Venus goes between Earth and the Sun. From Earth it looks like a black dot moves across the Sun. Astronomers call this a transit of Venus.

Transits of Venus don't happen very often. Sometimes the wait between transits is about eight years. Sometimes the wait is more than 100 years!

There will be two Venus transits pretty soon. One is very soon. It will be on June 8, 2004. The next one will be eight years later on June 6, 2012. After that, you will have to wait a very long time before another Venus transit. The next one won't be until the year 2117!

People have only seen five transits of Venus so far. Those transits were in 1639, 1761, 1769, 1874, and 1882.

In the past, people didn't know how far the Sun is from Earth. We didn't know how big the rest of our Solar System is either. Astronomers made some measurements during Venus transits. They used those measurements to figure out how far away the Sun is. The Sun is about 150 million kilometers (93 million miles) from Earth. That is a very, very long way!