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.
Please wait while this video loads. If it doesn't load after a few seconds you may need to have Adobe Flash installed.
But catching it - in the UK at least - will be a true test of astronomical commitment. The journey of the planet across the sun will only be visible at sunrise - at 4.45 in the south, 4.30 am in Scotland, and the weather means the prospects for viewing in the south of Britain are poor.
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 happens 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.
Picture Gallery: Venus beauty
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.