Seen in relation to the plane of Earth's orbit, Venus's orbit has a slight tilt – about 3.4 degrees. Depending on where the two planets are in their respective orbits, that leaves Venus trailing the sun toward the horizon at nightfall or rising ahead of the sun before dawn. But on Tuesday the two planets' orbits are aligned in such a way that Venus will pass directly between Earth and the sun during Venus's inferior conjunction, or closest approach to Earth.
Venus's transits occur in pairs, separated by eight years – Tuesday's transit is paired with one in 2004. The pairs repeat their appearance twice in a 243-year cycle, with one return time spanning 105.5 years, and the other 121.5 years. So, if you miss this one, tell your children to tell their children to take heart. We're in a part of the cycle with the shortest return time – in December 2117.
Astronomers are working hard not to miss it.
A chance to study other Earths
Tuesday's transit represents "a great opportunity" to test approaches for studying atmospheres of Earth-scale planets orbiting sun-like stars, says Jean-Michel Desert, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and a member of the team conducting the experiment.
The team, led by Alfred Vidal-Madjar at the Institute for Astrophysics in Paris, will use the Hubble Space Telescope to watch the transit via the light reflected off the moon, since the orbiting observatory would be permanently blinded if it tried observing the sun directly.