Chasing Venus: Observing the Transits of Venus 1631-2004
What is a Transit of Venus?
Discovering the Transits: The 1631 and 1639 Transits
Measuring the Universe: The 1761 and 1769 Transits
New Possibilities: The 1874 and 1882 Transits
The Transits of 2004 and 2012
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Education and Events
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What is a Transit of Venus

The Many Faces of Venus

Although Venus is the closest planet to Earth, it has long remained a mystery due to the thick clouds that completely hide its surface.

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What Is a Transit of Venus?

A transit of Venus occurs when the planet Venus passes directly between Earth and the Sun. Venus appears as a black dot moving across the surface of the Sun.

Only six transits of Venus occurred between 1600 and 2000, but two will take place this century. Click here to see the timeline.

Lining Things Up

Venus actually passes between Earth and the Sun every 584 days. Usually when this happens, Venus appears to us to be above or below the Sun, because Venus's orbit is tilted slightly with respect to Earth's orbit.

On very rare occasions, however, Venus lines up directly between us and the Sun, just as the Moon does during a solar eclipse. But because Venus covers only a tiny part of the Sun, we don't notice any change in the Sun's brightness. Unless we knew where to look, we would miss the transit entirely.

A Yardstick to the Sun

For ages, astronomers looked for a way to measure the distance between Earth and the Sun. In the 1700s they realized that the transit of Venus might help them do it.

By comparing the different times of the start and/or end of a transit of Venus from various locations on Earth, astronomers could use surveying methods to measure the distance from Earth to Venus. Since Kepler in the 1600s had determined the relative distances among the planets and the Sun, astronomers could use the Earth-Venus distance to calculate the Earth-Sun distance.

The Earth-Sun distance - called the astronomical unit (AU) - was a yardstick that could measure the entire solar system. In pursuit of the AU, astronomers began chasing the transit of Venus in earnest.


The key to finding the Earth-Sun distance using the transit of Venus was in timing the exact moment when the edge of Venus's disk appeared to make contact with the edge of the Sun.
There were four contact points that astronomers could measure as Venus passed in front of the Sun. However, an optical trick called the black drop effect prevented astronomers from obtaining the precise time of contact.

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