On Wednesday morning a dark dot crossing the disc of the sun will mark the passing of an astronomical wonder that will not be repeated for more than a century: the transit of Venus.
The phenomenon occurs when Venus, our immediate neighbour in the solar system, passes directly between the earth and the sun. Its silhouette is seen as a black dot about 1/32nd the diameter of the sun. The full transit takes about six hours, but only the last half hour or so will be visible from the UK.
Transits of Venus happen in pairs eight years apart. The last transit in this pair was in June 2004 but the next won't roll around again until December 2117.
The transit was of huge importance in the history of astronomy. The first transit observed, was in 1639 by English astronomer Jeremiah Horrocks. Horrocks's measurements allowed astronomers to estimate the size and distance of the planet Venus and the distance between the sun and the earth. This measurement, called an "astronomical unit", is now the fundamental yardstick for studies of our solar system and the universe beyond.
Astronomer Jeremiah Horrocks's measurements allowed astronomers to estimate the distance between the sun and the earth.
By the 18th century it was realised that the best measure of an astronomical unit could be made by watching the transit of Venus from different places on the earth and then using trigonometry to calculate the distance.
Exploiting such "parallax" measurements became an international cause of the Enlightenment age. For the 1769 transit, astronomers were dispatched across continents to get as many views as possible. Captain Cook's first voyage east made observations from Tahiti at a place still known as "point Venus".
Those missions used the transit to calculate the distance from the earth to the sun of between 93 million to 97 million miles. And they weren't far off. The internationally accepted distance today is 92.9 million miles.
Once in a lifetime
But as a once-in-a-lifetime astronomical experience, watching the Venus still draws a crowd. Events are planned across east Asia and Australasia where the entire transit will be visible. Most of the US will see the event at sunset this evening. A range of public viewing events have been organised by local astronomy groups across the UK (see the Royal Astronomical Society website for more detail).
It's quite possible we'll be sitting around talking about what we could be seeing while eating bacon sandwiches.
Grey Lipley, Flamsteed Astronomy Society
Grey Lipley, of the Flamsteed Astronomy Society, told Channel 4 News his group, like many others, hopes to offer an enhanced view of the transit at their event in London tomorrow morning. Specialised solar telescopes can allow all of the sun's surface detail to be observed, as well as the transit. "We should see Venus as it passes across the outer layers of the sun - giving us an extra 30 minutes of observations," he said.
But he had to admit cloudy weather could well put a dampener on things. "I've already checked that the café will be open," he said. "It's quite possible we'll be sitting around talking about what we could be seeing while eating bacon sandwiches."
Because looking at the sun directly can cause permanent blindness, experts advise those planning to watch the transit try projecting the image of the sun onto a wall or sheet through binoculars or a telescope. Eclipse viewing glasses or goggles can be used, but only for brief glimpses. For more information, go to the Royal Astronomical Society website.
What is the transit of Venus?
A rare astronomical event that happens when Venus travels across the face of the sun and appears as a small black dot on its surface.
When does it happen?
Transits occur in pairs eight years apart. There are two in December that repeat every 121.5 years, and two in June that repeat every 105.5 years.
The last transit of Venus of the 21st century occurs on Tuesday and Wednesday (5 and 6 June 2012) depending on where you are viewing from. The transit starts at 11.04pm BST (6.04pm ET) on Tuesday, when it will be visible from the US. The final hour of the transit will be visible from the UK just before 5am BST on Wednesday (12am ET US), clear skies permitting. The transit will not happen again until December 2117.
How long does the transit last?
Venus takes nearly seven hours to cross the face of the sun, but the event is divided into four "contacts" that mark different phases of the transit. Venus makes first contact when it encroaches onto the disc of the sun. Twenty minutes later, on second contact, the planet will be fully silhouetted. On third contact, at 5.37am BST (12.37am ET), Venus will begin to leave the sun, and the transit will be over on fourth contact at 5.55am BST (12.55am ET).
Where can I see it?
The whole transit is visible from Alaska, parts of northern Canada, and from New Zealand, much of Australia, Asia and Russia. In the US, the transit will be in progress as the sun sets on 5 June. In East Africa, Europe and Scandinavia, the transit will be under way as the sun rises on 6 June. Much of South America and western Africa will not see the event.
How can I watch it safely?
Never look directly at the sun, it will damage your eyes. You can use eclipse viewing glasses that carry a CE mark and are not damaged or worn, but only for a few minutes at a time. Venus is large enough to see with the naked eye and will appear as a spot about 1/32 the width of the sun. It is not safe to look at the sun through regular sunglasses. For a better view, use a small telescope or a pair of binoculars to project an image of the sun onto a screen. Never look at the sun directly through either binoculars or a telescope.
Can I watch online?
Nasa will broadcast a live webcast of the transit from the Mauna Kea Observatories in Hawaii.
Stonehenge Tour Guide