When one celestial body appears to move across the face of a larger celestial body, it's called a transit. A moon can be seen transiting its planet, or a planet might be observed transiting the sun.
The small dot (lower right) is Mercury making its way across the face of the sun on November 8, 2006.
The Hubble Space Telescope captured this image of four of Saturn's moons transiting the planet in February 2009. On the left are the icy moons Enceladus, Dione and their shadows. Titan, Saturn's largest moon, is on the upper right, and, at the far right, is Mimas. The shadows of Titan and Mimas have already passed off the face of Saturn.
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: M.H. Wong (STScI/UC Berkeley) and C. Go (Philippines)
From the earth, the only planetary transits we can see are those of Mercury and Venus, because those are the only planets that can be between us and the sun. And these are rare events. Transits of Mercury occur about a dozen times a century. The last one was in 2006, and the next will be in 2016. Venus transits are less frequent still; they occur in pairs, with eight years separating the two transits, and then more than a century will pass before there's another pair. The most recent transit of Venus was in 2004, and the next will be in 2012. After that, we'll have to wait until 2117 and 2125.
Transits don’t provide the spectacular display that a solar eclipse does, but they’re useful for scientific research. In the 1700s, Edmond Halley (of comet fame) realized that transits could be used to calculate our distance from the sun—and from that, we could understand the dimensions of the entire solar system. Using the two transits of Venus that occurred that century, astronomers found a good approximate value for the sun-earth distance. Observations of those transits of Venus also suggested that Venus has an atmosphere, which we now know to be the case.
For more information about the historical importance of Venus transits, and to watch our Transit of Venus 2004 Webcast, visit our transit of Venus Web site.
To watch our Transit of Mercury 2006 Webcast, and to view videos and still images from the 1999 transit, visit our transit of Mercury Web site.
Today, astronomers are looking at transits farther afield: planetary transits of other stars in the Milky Way. The planets themselves are too dark and distant to be seen directly, but if they transit their star, very slightly dimming its light, scientists can infer their existence. In March 2009, NASA launched the Kepler spacecraft into sun orbit, from where it will be able to keep its telescope eye on 100,000 stars for a planned three-and-a-half year period. It's anticipated that it will discover Jupiter-like giants in the early part of the mission, but the ultimate goal is to track down planets the size of the earth. In particular, astronomers are interested in planets that orbit stars similar to our sun and at a distance that would make it possible for liquid water to exist on their surface, potentially making them habitable. The Kepler mission should go a long way to help answer the question "Are we alone?"
In April 2009, Kepler discovered a "hot Jupiter," a giant gaseous planet that orbits close to its star. Named TrES-2, it races around its star (circled) every two-and-a-half days. The image shows only a fraction of the stars Kepler can see.
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