Sen— Around the world, and in space, astronomers and explorers are preparing for a rare astronomical event - the Transit of Venus.
A transit occurs when a smaller body passes in front of a larger one - in this case, the planet Venus passing in front of the Sun.
Venus orbits the Sun every 224 Earth days at a distance of 108 million kilometres, whilst Earth completes an orbit every 365 days at a disance of 150 million kilometres. The Sun, Venus and Earth line up every 584 days. However, as the orbits of both Venus and Earth are tilted with respect to one another, Venus normally appears to pass above or below the Sun. A transit therefore only occurs when the Sun, Venus and Earth are almost exactly in line.
This transit will be the last this century. Venus transits occur in pairs as follows. First, two transits take place in December, 8 years apart. There follows a wait of 121 years 6 months, after which two June transits occur, again 8 years apart. After 105 years 6 months, the pattern repeats.
The next visible transit will be in December 2117, followed 8 years later by a second December transit.
Observing the transit from Earth
The transit will take just under 7 hours and will start at 22.04 UTC on June 5. This is when 'first contact' occurs - when Venus first encroaches onto the disk of the Sun. It will take about 20 minutes from first contact until Venus is fully silhouetted ('second contact'). Venus will move across the northern part of the Sun with mid-transit at about 01:30 UTC on 6 June. Venus begins to leave the Sun ('third contact') at about 04:37 UTC, and the transit will be over ('fourth contact') at 04:55 UTC. Timings differ by a few seconds for different latitudes, but the transit will be visible from any place where the Sun is up (clouds permitting).
Summary of timings:
Venus first encroaches on the disk of the Sun (first contact): 22.04 UTC
Venus in full silhouette (second contact): starts about 22.25 UT
Venus begins to leave the disk of the Sun (third contact): 04.37 UTC
The end of the transit (fourth contact): 04.55 UTC
Whilst many astronomers will be hoping to observe the transit, please remember never look directly at the Sun, with or without a telescope or pair of binoculars, without using a safe solar filter. To do so is very dangerous and is likely to result in permanent blindness.
Observing the transit in space
Whilst astronomers will be observing the transit from Earth, six astronauts aboard the International Space Station will be observing the transit from orbit. This will be the first time in history that astronauts have been able to observe a Venus transit because during the 2004 transit the crew did not have solar filters aboard. This time, astronaut Don Pettit remembered to pack his camera's solar filter before joining the ISS crew in December 2011 and will be hoping to capture some great images as Venus makes its way across the disk of the Sun.
As well as six humans observing the Venus transit from space, several missions will be recording the event.
Orbiting Venus is the European Space Agency's Venus Express which will use the extra sunlight to make observations of the planet's atmosphere. Recordings on Earth will be used to compare the data captured by the orbiting spacecraft.
As well as Venus Express, Japan's solar satellite Hinode will be recording the event as will the Hubble space telescope. Hubble will use the Moon as a giant mirror to capture diffuse, reflected sunlight: a small fraction of that light will have passed through the atmosphere of Venus en-route to the Moon.
“The most spectacular images and movies should come from Hinode’s Solar Optical Telescope, which has by far the highest resolution of any solar instrument in space,” says Bernhard Fleck, ESA’s Hinode and SOHO project scientist.
December 1631 and 1639
The Transit of Venus was first predicted by German astronomer Johannes Kepler. NASA's Kepler mission, which uses the transit method to detect planets around distant stars, is named after him. Kepler predicted the December 1631 transit but died in 1630 before he could observe it. A young English astronomer, Jeremiah Horrocks, was the first to record observations of the transit in December 1639. Horrocks observed part of the transit from his home at Much Hoole, near Preston in England. His friend William Crabtree also saw it from Manchester, having been alerted by Horrocks. As far as is known, they were the only people to witness the transit.
June 1761 and 1769
By the 18th Century the transits had become big news events and expeditions were sent around the planet to observe them. And so the transits of 1761 and 1769 were observed from many places around the world. James Cook's expedition to Tahiti in 1769 is one of the most famous.
December 1874 and 1882
The Royal Astronomical Society archives contain illustrations and observations of the December 1874 and 1882 transits. The 1882 transit was mentioned on the front pages of most newspapers. Thousands of ordinary people saw it, and Professor Sir Robert Stawell Ball described his own feelings on watching the transit in his 1885 book, The Story of Astronomy:
"To have seen even a part of a transit of Venus is an event to remember for a lifetime...It was then easy to sympathize with the supreme joy of Horrocks, when, in 1639, he for the first time witnessed this spectacle. The intrinsic interest of the phenomenon, its rarity, the fulfilment of the prediction, the noble problem which the transit of Venus helps us to solve, are all present to our thoughts when we look at this pleasing picture, a repetition of which will not occur again until the flowers are blooming in the June of A.D. 2004."
June 2004 and 2012
After 1882 the next transit was in June 2004 which was widely photographed and observed from Earth, though not from space as the astroanuts aboard the space station did not have solar filters and were not able to witness the event.
For an animation of the 2004 Venus transit by astro photographer Thierry Legault click here.
The next transits will be in December 2117 and 2125.