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Thanks, Jen.

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A Venus transit occurs when Venus crosses the disk of the Sun as seen by an observer.

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It's like a solar eclipse in that Venus is located on the line between the Sun and the

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Earth and therefore blocks some of the Sun's light.

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However, in a Venus transit, the amount of sunlight blocked is very small compared to

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a solar eclipse, and so the observer who is unaware will never notice it.

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Venus's circular shadow is much, much smaller than our Moon's shadow.

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Even though Venus is nearly the size of the Earth, it is much farther away than the Moon.

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In clear weather, Venus transits are visible with the naked eye or with a small telescope,

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which is why they became popular in the 1600s.

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Before the advent of radar, Venus transits were used mainly for the measurement of the

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astronomical unit, or the Sun-Earth distance, as you've heard earlier.

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The biggest activity surrounding the June 2004 Venus transit will be the International

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Network of Amateur Astronomers.

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These astronomers will measure the astronomical unit with the Venus transit using the same

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techniques as used by the early observers.

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An innovative aspect this time, however not available in 1882, is the widespread use of

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the Internet to organize international participation and the ease of access to the tools needed

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to make the parallax calculations.

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There also will be a few astronomical researchers who will try to exploit state-of-the-art observing

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tools to see what can be learned about the use of transits to investigate planets around

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other stars.

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Transits are currently being used to search for such planets.

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Perhaps this Venus transit will lead to some new technique or measurement that will allow

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future researchers to further study the terrestrial planets during long-range planet-finding missions.

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The Venus transit will also serve to remind us of Earth's place in the cosmos.

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The tiny dot crossing the solar disk is a terrestrial planet with an atmosphere, and

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yet it is far from an Earth.

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Venus was once called a twin Earth, in part because of its similar size and distance from

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the Sun.

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It is now known to be a place that is extremely hostile to life for reasons that are still

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under study.

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One can speculate how our own pale blue dot would look to some distant alien astronomer

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as it passed across the Sun in transit, and whether it has ever been so observed.

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Maybe one day humans will be able to observe the Earth transit.

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To learn more about the planet Venus and the Venus transit, check out the Sun-Earth Connection

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Education Forum website.

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Take it away, Jennifer!