|The Venus transit of June 8, 2004, as viewed by NASA’s sun-observing TRACE spacecraft.|
How big is our solar system? How far away are our planetary neighbors? Back in the 18th century, astronomers had a perfect opportunity to find out, with the transit of Venus across the Sun. And now, in a few days, on June 5th 2012, you’ll be able to do the same thing. But don’t miss it; the next transit won’t occur for another 105 years, in 2117.
To celebrate this event, science centers, planetariums, and telescopes around the world will host observing parties.
Venus passes between us and the Sun every  days, but most of the time its trajectory places it either above or below the Sun from our vantage point, making it difficult to see. Only on very rare occasions are the Earth, Venus, and the Sun aligned such that Venus passes directly between us and the Sun. When it does, we can observe Venus’ dark body traveling across the bright disc of the Sun.
How does being able to see Venus in front of the Sun help us measure the size of our solar system? A bit of simple math, specifically trigonometry, a la Hipparchos. In 1716, astronomer Edmund Halley (yes, the gentleman after whom the comet is named) realized that if he took two known locations on Earth and, at precise times, measured the angles between these points and Venus, and then between these points and the Sun, it was possible to calculate the distances between all three celestial bodies.
Of course, in the 18th century it was difficult to take precise measurements. Today, astronomers rely on radar echos and satellite telescopes. Yet, Venus still holds our fascination, and still offers fertile ground for scientific investigation. But before you head out to see the show on the 5th, remember that looking directly at the Sun is dangerous and can cause permanent vision loss. Never do so with the naked eye. Instead, use a dark filter such as number 14 welder’s glasses, or filters specifically designed and certified for observing the Sun, available from most astronomy stores. Sunglasses are not enough!
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