Information...

As both Mercury and Venus lie within Earth's orbit around the Sun, it is possible to occasonally view them as they pass in front of the Sun - a planetary transit.

As Mercury, orbits the Sun about every 88 days, transits occur relatively frequently. i.e. 13-14 occur each century, with next due on 13th Nov. 1932. Venus on the other hand, with it's longer orbital period of about 225 days, only transits the Sun infrequently, with 243 year gaps between two transists which occur 8 years apart. In this case the next pair of transits won't take place until 10-11 Dec. 2117 and 8 Dec. 2125.

Historically, transits have allowed scientists to calculate the distance from the Earth to the Sun, by observing the transit from different locations on Earth. By measuring the time taken for the transistions to take place, and then using the latitude & longitude from the obervation points, the distance to the Sun can be calculated.

In more recent years, transits have also proved useful in developing techniques to study expolanets around more distant stars.

It is also possible to get transits of both Mercury and Venus at the same time, but not very often! Such an event last occurred in year 373,173 BC and the next is due in year 69,163 AD...

For much more detailed info. see the Venus Transit and Mercury Transit entries in Wikipedia.