5 and 6 June 2012 will be the last time Venus will cross the Sun relative to our planet for 105 years, until December 2117, marking a historic event for astronomers around the globe. This is the second and last transit of the Sun performed by Venus that will be visible to anyone on Earth.
A solar transit is when an object passes between the Sun and the Earth. This usually applies only to transits of Mercury and Venus, but satellites and spacecraft also transit the Sun, albeit to a much lesser degree. In the case of Venus a transit will usually only last for a matter of hours.
The reason a visible transit of Venus from Earth is so rare is because the two planets orbit in slightly different planes. Venus’ orbit of the Sun is at a slight inclined compared to that of Earth’s, so even though Venus passes between the Earth and the Sun every 1.6 years (due to its faster orbit) it is usually a bit above or below the Sun and thus obscured by the Sun’s glare. These solar transits mark the time when Venus and Earth are aligned in the same plane.
Transits of Venus occur in pairs, with two transits able to be observed eight years apart when the first one occurs. The transit on 5 and 6 June 2012 followed the previous transit on 8 June 2004, while the transit in December 2117 will be followed by one in December 2125. This pairing of transits is due to the Earth and Venus orbiting in the same plane for a small length of time before Venus moves back into an inclined orbit. The last pair of transits occurred in December of 1874 and 1882.
While a fascinating sight to behold, transits of Venus also have two major uses for scientists. First, they can be used to more accurately measure the distance from the Sun to the Earth using an observational method known as parallax. Secondly, they can help in the discovery and observation of planets outside the Solar System. Most exoplanets are found using the transit method, which involves measuring the dip in brightness of a star as a planet transits it relative to Earth. By measuring the amount of light blocked by Venus as it transits the Sun, better estimates can be made of the types of planets being observed elsewhere in the Milky Way.