Planet Venus Makes Rare Transit Across Sun

Planet Venus is putting on a show for skywatchers by moving across the face of the Sun as viewed from Earth. The transit is a very rare astronomical event that will not be seen again for another 105 years.

Observers in north and central America, and the northern-most parts of South America saw the transit begin just before local sunset. The far northwest of America, the Arctic, the western Pacific, and east Asia will witness the entire passage. The UK and Europe, the Middle East, and eastern African must wait for local sunrise to see the closing stages of the transit.

Venus appears as a small black dot moving slowly but surely across the solar disc. Many citizens keen to get a view of the transit themselves have been attending special events at universities and observatories where equipment for safe viewing has been set up. For others, internet streams have provided an easy way to follow Venus’s slow trek. Scientists have been observing the transit to test ideas that will help them probe Earth-like planets elsewhere in the galaxy, and to learn more about Venus itself and its complex atmosphere.

Venus transits occur four times in approximately 243 years; more precisely, they appear in pairs of events separated by about eight years and these pairs are separated by about 105 or 121 years. The reason for the long intervals lies in the fact that the orbits of Venus and Earth do not lie in the same plane and a transit can only occur if both planets and the Sun are situated exactly on one line.

This has happened only seven times in the telescopic age: in 1631, 1639, 1761, 1769, 1874, 1882 and 2004. Once the latest transit has passed, the next pair will not occur until 2117 and 2125.

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Source: BBC News

Image: National Geographic

Asteroid Crash May Have Caused Mercury’s Strange Spin

A collision with an asteroid might have set the planet Mercury whirling oddly in its orbit, a new study suggests.

Scientists had long assumed that Mercury was tidally locked with the sun — the planet’s tiny size and proximity to the sun suggested the star’s gravitational pull would quickly force Mercury into such a state. However, radar observations of Mercury surprisingly revealed that the planet led a far stranger life, rotating three times on its axis for every two orbits it completes around the sun. Now, researchers suggest that Mercury was once tidally locked, initially spinning in the opposite direction to its orbit.

“Mercury once had a spin rate synchronous with the sun, like the moon with the Earth,” study co-author Alexandre Correia, a planetary scientist at the University of Aveiro in Portugal, told

Computer models suggest that a giant impact from an asteroid then knocked it into its current strange configuration. Evidence of this collision might include Caloris Basin, Mercury’s largest impact crater, which matches the predicted size, age and location of the impact, the researchers said. “It is the perfect candidate,” Correia said.

Such an impact might also explain certain hollows seen on Mercury’s surface. If the planet was tidally locked, one side would have been extremely bright and hot while the other extremely dark and cold. Substantial deposits of ice might have accumulated on the dark half, some of which might have been buried under matter ejected from impacts. When Mercury’s spin later changed and daylight began falling on the once dark side, this buried ice might have vaporized, leaving behind hollows, the researchers explained.

The results of the study were published online today (Dec. 11) in the journal Nature Geoscience.