There are many predictable events in astronomy. The time of sunrise, the monthly appearance of the full Moon and the return of Halley's comet once every 76 years or so are a few examples that spring to mind. One such occasion, that takes place this year on June 5/6, is among the rarest of all predictable events. In fact it is so uncommon that since it was first observed way back in 1639, it has only taken place a further five times since then. The event in question is none other than "the transit of Venus".
In astronomical language, a transit is an event that occurs when one celestial body appears to move across the face of another celestial body as viewed from a particular position. If the first celestial body covers all of most of the second celestial body, then it is an occultation rather than a transit. Position is the key; for a transit or occultation to occur there has to be a high degree of alignment between the observer and the other two celestial bodies. In this case, the Sun, Venus and the Earth are aligned and with Venus stuck in the middle it will appear as a small black disk crossing the Sun's disk when viewed from Earth.
Transits of Venus occur in pairs separated by 8 years with 105 years between cycles. We are now about to witness part two of the latest grouping. The first part and last transit of Venus occurred on June 8, 2004 and was one of the most widely observed astronomical events in history. It was visible in its entirety from Europe, India and most of Asia. After this months transit, the next chance to see one is not until December 2117 and if you miss that December 2125!
The transit begins at 22:09 UT on June 5 and will finish at 04:49 UT on June 6. It is visible from most parts of the World. The complete transit can be seen from the Pacific Ocean, Central and Eastern Australia, Japan, Eastern China, Eastern Asia, Alaska and the far North Polar Region. For most of Europe (except Portugal and parts of Spain), Eastern Africa, Middle East, India and Western Asia the transit is in progress at Sunrise. At the other end of the scale, observers in the United States, Mexico, the Caribbean and the Northern tip of South America witness the start of the transit but the Sun will set before the transit ends. Unfortunately for most of South America, Western Africa, Antarctica, Portugal and the western parts of Spain the transit is not visible at all.
Transit of Venus 2012 - pdf format
Safe observing of the transit is extremely important. Venus is so large at this time that it can be observed without magnification by people with good eyesight. Of course, NEVER LOOK AT THE SUN DIRECTLY, it is extremely dangerous and will cause permanent eye damage. The transit can be observed safely with proper certified special solar glasses or solar telescopes. But the safest method of all is projection; use a telescope or one half of a pair of binoculars to projection the solar disk onto a shielded white card. Importantly make sure that all lenses not used such as finderscopes and the other half of the binoculars are safely covered.
Once you have established your safe method of observing, enjoy the last chance this century to view this rare event.