Astronomers around the world are getting their telescopes ready for a very rare celestial event that will begin shortly after 2200 h Coordinated Universal Time (UTC) today. That is when the planet Venus will begin its passage across the face of the Sun as seen from Earth – an event that will not happen again until December 2117. Such "transits of Venus" always occur in pairs eight years apart – the last being in 2004 – with the gap between successive pairs alternating between 105.5 and 121.5 years.

It will take a little less than seven hours for Venus to cross the disk of the Sun, with the entire transit being visible at latitudes above the Arctic Circle as well as in much of the Pacific Ocean, Australia, and eastern and central Asia. People in many other parts of the world, meanwhile, will have the chance to observe at least some of the event. In Western Europe, the final hour or so of the transit will be visible after sunrise on 6 June, whereas observers in eastern North America will be treated to the first two or three hours of the celestial phenomenon before the Sun sets this evening.

Measuring the heavens

In addition to the novelty of rarity, transits of Venus have also played an important role in the development of astronomy. In particular, the transits of 1761 and 1769 famously allowed astronomers to estimate the absolute distance from the Earth to Venus, which could then be used to work out the distance to the Sun and hence to all other known planets in our solar system. "With this technique, the measurement [the astronomers] came up with was only 3% off the current measurement of the distance between the Earth and the Sun," says astronomer Zoe Leinhardt of the University of Bristol in the UK, who explains the method in the above video.

Another notable feature of the 1761 transit, which took place on 26 May of that year, was that it led to the Russian polymath Mikhail Lomonosov controversially claiming to be the first person to observe the atmosphere of Venus. The atmosphere is visible when the planet first touches the Sun ("ingress") and then when it finally leaves ("egress"), appearing as a glowing arc around the portion of the planet that does not cover the Sun. However, no-one else observing on the day of the 1761 transit noted such an arc, which has led some astronomers to claim that Lomonosov was merely seeing an artefact of the relatively primitive telescope he was using.

One astronomer who is convinced that Lomonosov did not see Venus's atmosphere is Jay Pasachoff of Williams College in Massachusetts, US, who is a leading expert on the transit of Venus. Comparing Lomonosov's reports with the observations taken from space during the 2004 transit that Pasachoff and Glenn Schneider studied, Pasachoff and William Sheehan have concluded that Lomonosov saw only artefacts. Writing in an article for Physics World, Pasachoff concludes that Lomonosov thought he had discovered Venus's atmosphere simply because the Russian believed that all planets had atmospheres. "In the end, he had the right result, but without a proper train of measurement and reasoning," says Pasachoff.

Black-drop effect

Pasachoff is one of many astronomers who will be studying the event today. "We want to get the most complete set of data possible, so that the astronomers of 2117 will think that their forebears way back in 2012 did a fine job even with their relatively primitive instruments," says Pasachoff, who is observing the transit at the Mees Solar Observatory on Haleakala in Hawaii. Apart from focusing on the atmosphere of Venus, he and his colleagues are also studying the mysterious "black-drop effect" – a strange, dark band linking Venus's silhouette with the sky outside the Sun that appears for about a minute starting just as Venus first enters the solar disc.

For many astronomers, the transit is also a unique opportunity to evaluate, in their own backyard, techniques that are used to study the atmospheres of exoplanets – planets that orbit stars other than the Sun. Astronomers have already made very preliminary studies of the atmospheres of several exoplanets that transit their own stars. As this takes place, some of the starlight passes through the atmosphere of the exoplanet, leading to light at specific wavelengths being absorbed by molecules in the atmosphere.

But although astronomers can get an idea of the chemical composition of the atmosphere by studying the absorption lines using a spectrograph, these exoplanet systems are so far away that the measurements are very difficult to make. Astronomers are therefore hoping that data from the 2012 transit of Venus will help them develop techniques for studying exoplanets. "By looking up close at transits in our solar system, we may be able to see subtle effects that can help exoplanet hunters when viewing distant suns," says Pasachoff.

Safety first

Anyone wishing to observe the transit should, of course, remember not to look directly at the Sun. The phenomenon can best be seen using a pin-hole camera or through special glasses; NASA has produced a guide to safe viewing.

To find out when the transit will be visible where you live, Astronomers Without Borders have created an invaluable website.

And if you have any great photos of the transit, please add them to our photo challenge on Flickr, while if you have any stories about what you were doing during the 2012 transit, please comment below.