Will you be able to see the "Transit of Venus"?
Transits of Venus are so rare because the planet's orbit is tilted just over three degrees from the plane of the solar system. This means that most of the time Venus passes above or below the sun's disk, as seen from Earth.
On average, we see four transits of Venus within 243 years. The events happen in pairs spaced eight years apart, and they alternate whether Venus crosses the top or the bottom of the solar disk, Williams' Pasachoff said. This year, for instance, the planet will transit the top of the sun.
Astronomers first used telescopes to observe a transit of Venus in 1639.
But it wasn't until 1769 that dozens of scientists scattered across the globe to make detailed measurements of the event, including the famous voyage of British lieutenant James Cook, who had astronomers collecting transit data from the island of Tahiti during his South Pacific expedition.
(Related: "Journals of Captain Cook Go Online.")
Observations from different locations on Earth allowed scientists to not only triangulate the true size of the sun but also to more accurately determine the distance between the sun and Earth.
Venus at Sunrise: Photograph by David Cortner, Galaxy Picture Library/Alamy
This Tuesday or Wednesday, depending on where you live, sky-watchers around the world will be able to see a cosmic spectacle known as a transit of Venus. The events are so rare that only six Venus transits have been observed since the invention of the telescope more than 400 years ago.
Evening Star: Photograph by H. Baesemann, Blickwinkel/Alamy
Venus hangs low in the evening twilight near a razor-thin crescent moon in an undated picture taken from Troms County in northern Norway.
Purple Haze: Image courtesy ESA/NASA
A crescent Venus shines in an ultraviolet snapshot taken by the NASA-ESA Hubble Space Telescope in 1995. As Venus circles the sun, it appears to go through phases that mimic those of our moon when seen through a telescope. (Related: "'Hot Jupiter' Planet's Phases Seen-A First.")