One of the astronomical highlights of the century will occur before sunset on June 5, when the planet Venus moves across or transits the face of sun. This rare celestial event has helped astronomers understand the size of the solar system.

Venus and Mercury are the only two planets for which we can see transits because they are the only two planets that orbit between Earth and the sun. When these two planets pass directly in line between Earth and the sun, a transit occurs.

If Earth and Venus orbited the sun in the same orbital plane, transits would happen about every 1.6 years. However, the orbit of Venus is tilted with respect to Earth's orbit, so when Venus passes between the sun and Earth, Venus is usually a little bit above or below the sun. As a result, there have been only seven transits of Venus in the past 400 years, and the next transit won't occur until 2117.

In 1610, Galileo, with his primitive refracting telescope, was the first human to actually see Venus as more than just a bright point of light in the sky. Meanwhile, Johannes Kepler, with astronomical data assembled by Tycho Brahe, accurately predicted the motions of the planets, thus calculating that Venus would pass in front of the sun in 1631. Unfortunately, that transit was not visible from Europe. Using Kepler's data from the 1631 transit, British astronomer Jeremiah Horrocks became the first person to see the transit, on Dec. 4, 1639.

Scientists discovered they could use the transit to figure out one of the chief astronomical puzzles of the 18th century, the size of the solar system. By careful observation of the transit of 1882, they calculated the distance between the sun and Earth at about 93 million miles, thereby setting the scale for the entire solar system and the cosmos beyond.

First Published: May 24, 2012, 4:00 a.m.
Updated: May 24, 2012, 4:38 a.m.