A planet is said to transit the Sun when it passes in front of the Sun from our point of view. From Earth it is possible to witness two planets transiting the Sun, Mercury and Venus, since these planets orbit closer to the Sun than the Earth; from Mars it is also possible to see Earth transiting the Sun, etc. Venus, seen from Earth, transits the Sun four times every 243 years, on a cycle of 125.5 years, eight years, 105.5 years, eight years, 125.5 years, and so on. The most recent of these transits occurred on 5-6 June 2012), provoking excitement among both amateur and professional astronomers. This will not happen again till December 2117.
NASA's Solar Dynamic Observatory constantly observes the Sun from a geosynchronous orbit 36 000 above the Pacific Ocean. This placed it in an excellent position to observe the transit of Venus, untroubled by the clouds which bedeviled many Earth-bound observers.
Composite image of Venus transiting the Sun at a wavelength of 171 Å. NASA/Solar Dynamic Observatory.
As well as providing us with dramatic images, this also allows the Solar Dynamic Observatory to perform useful science. The Observatory observes the Sun at a number of different wavelengths. All gasses are opaque at some wavelengths and transparent at others. Therefore by calculating how much light is absorbed by the atmosphere of Venus at different wavelengths, scientists hope to be able to learn more about the composition of that atmosphere.
Film of the 2012 transit of Venus at a wavelength of 171 Å. NASA/Solar Dynamic Observatory.
This space-based view of the transit also tells us something about possible future observations. The next observable transit of Venus will not occur until December 2117, but this is not necessarily the next time we will see this event. Venus (and any other body in the Solar System) is always transiting the Sun from some point of view. It is possible that in the next century humans will come to explore the Solar System far more extensively than we have done to date. While it is unlikely that we will ever send a mission specifically to observe a transit of Venus, it is quite possible that the next humans to observe such a transit will not be on Earth, but rather in space visiting, or on route to, some other body in out Solar System.
See also NASA releases digitally remastered view of Copernicus Crater, New Cassini Images of Titan, Tethys and Methone, Images of Vesta, Martian Dust Devils and NASA releases new Cassini images of Saturn's moons Enceladus, Janus and Dione.
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