The 2012 Solar Transit of Venus

Over tonight and the early hours of tomorrow morning a true once in a lifetime event will occur. Venus will be seen to make a solar transit. “What is such an event you may be thinking” and why “is this one so special?”

The transit event is very similar to the much more common solar and lunar eclipses so we can start their.

A solar eclipse occurs when the Sun, moon and Earth line up exactly so allowing the moon to block a portion (or indeed the entirety) of the sun’s disk as seen from a region of the Earth’s surface. A total solar eclipse (one where the Sun is totally obscured) occurs roughly every 18 months. This comparatively short interval is helped by the proximity of the moon to the Earth and the large size of both the moon and the sun (the potential region of overlap between the two disks is hence quite large).

Diagrammatic Representation of a Solar Eclipse Credit: Peter Clark

Lunar eclipses occur when the position of the moon and earth are reversed and the Earth blocks out the sun as seen from the surface of the moon such events occur much more frequently with at least two occurring each year and they also last for a longer duration aided by the larger size of the Earth compared to the moon.

A Diagrammatic Representation of a Lunar Eclipse Credit: Peter Clark

Transit events occur because of the same principle as a solar eclipse, though in the case of transits the moon is replaced by the other planet. Transit events are much much rarer than standard eclipses (either lunar or solar).

The distance between the Earth and the planet involved is much, much larger compared to the distance between the Earth and moon. This means that as viewed from Earth the planet involved in the transit (either Mercury or Venus in the case of Earth) is much smaller than the apparent size of the Moon. Combined with the Sun remaining the same size in comparison there is a smaller possible region of overlap which in turn means that the chance of the overlap between the discs of the sun and and planet occurring is much smaller.

The occurrence of the transits relies on the exact line up of the orbits of the planets involved and the correct orbital inclination – so the planet that tracks in front of the sun actually passes across the disc of the sun rather than above or below it. As such line ups occur very rarely such transit events are are exceptionally rare. With transits of Venus visible from Earth occurring in pairs once every 121.5 or 105.5 years separated by 8 years in the pair.

What will the transit look like?
As the transit occurs the shadow of Venus will appear to track across the surface of the sun as a dark shadow over the course of several hours.

The transit has already begun and you can see the live NASA stream here:

Myself and a group of my friends will be attempting to observe the event ourselves in the early hours of the morning so stay tuned!

Update:

Well thanks to the good old British weather we saw absolutely nothing!

I hope your luck was better!