Photographed 8 June 2004 from Cheshire

Transits of Venus are rare. They usually occur in pairs, eight years apart. The previous pair took place in 1874 and 1882 (both in December), and after this one and 2012 (both June) the next will be in 2117 and 2125 (both in December again).

So why are they so far apart? And why do they seem to happen in June or December?

There are two things going on that affect the first question.

First, Venus’ orbit is slightly inclined (3.4°) to that of Earth, so that Venus must be near one of its orbital nodes (where it crosses Earth’s orbital plane) for a transit to be possible.

Secondly, the orbital periods of Earth and Venus are in resonance, 8 Earth orbits (years) equalling 13 of Venus. So when a transit does occur there will usually be another 8 years later. After a further 8 years, Venus has usually moved too far away from its node for a transit to occur.

As to the second question, currently Venus crosses the plane of Earth’s orbit in June and December so these are the only months in which a transit can occur. Because the orbital resonance is not exact, the transit dates are advancing slowly through the respective months, but June and December will continue to host Venus transits until after 4000AD.

As this transit was visible from Britain, I was able to stay at home in Cheshire and photograph it (on film) with my Meade 10″ SCT and Nikon F70 SLR using a Thousand Oaks Optical glass neutral-density filter. Thin cloud was present on the day which accounts for the slight glow around the edge of the Sun.