Rare Mercury pass of Sun begins
For a few hours, astronomers in the Americas, East Asia and Oceania have a rare opportunity to see Mercury pass in a direct line across the Sun.
The closest planet to our star should be visible as a tiny black dot creeping over the solar face from 1912 GMT on Wednesday to 0010 GMT on Thursday.
The last Mercury transit was in 2003; the next will be on 9 May 2016.
The entire transit can be seen from the western US, south-east Australia, New Zealand, Canada and the South Pacific.
Part of the transit should be visible before sunset on Wednesday in the rest of the Americas, and after sunrise on Thursday in East Asia and the rest of Australia.
Mercury races around the Sun in only 88 days, but it is rarely in direct alignment between us and the Sun because its orbit is tilted with respect to the Earth’s. Transits occur roughly 13 times every century.
“It takes about five hours for Mercury to cross the Sun. As long as you’re in daylight for those five hours, you will be able to see it. But it is nighttime [in Europe] this time around,” said Peter Bond, of the UK’s Royal Astronomical Society.
Neither is the transit visible from Africa, the Middle East or Asia west of Burma.
Mercury is so tiny that transits could not be seen before the invention of the telescope. The first person to witness one was the French astronomer Pierre Gassendi, in 1631.
His discovery sparked the realisation that transits could be used to establish a way of measuring distances in the Solar System.
The method is that of simple geometry - observing an object from two points that are a known distance apart, which provides the base line for a triangle.
Anyone observing the transit should use safe solar filters on their telescope or project the image on to a screen to avoid damaging their eyesight.
An even better prospect may be to follow the event online. A number of astronomical institutions and agencies are streaming the transit on the web.
For many astronomers, both amateur and professional, this is simply an event to wonder at.
But researchers at the University of Hawaii’s Institute for Astronomy are using the opportunity to measure the amount of sodium in Mercury’s tenuous atmosphere, measure its altitude, and determine how it varies from Mercury’s pole to its equator.
Several spacecraft are also looking on, including Japan’s Hinode probe (the recently launched and renamed Solar-B spacecraft), the Nasa/Esa Solar and Heliospheric Observatory (Soho), and the Nasa Transition Region and Coronal Explorer (Trace).
Spacecraft have visited Mercury only once - the US Mariner 10 mission in 1974-75.
But this is all about to change.
The US Mercury Messenger probe is due to arrive at the planet in 2009.
The 1.2-tonne, $430m spacecraft carries seven scientific instruments that will gather information on the composition and structure of Mercury’s crust, its geological history, its polar regions, atmosphere and magnetic environment, as well as the make-up of its core.
Messenger will be followed by Europe’s BepiColombo mission. It is expected to rendezvous with the first planet in about 2015-16.
The entire transit should be visible from the western US, south-east Australia, New Zealand, Canada and the South Pacific
Some regions see just some of the transit - before sunset on Wednesday in the rest of the Americas, and after sunrise on Thursday in East Asia and the rest of Australia
The transit is not visible from Europe, Africa, the Middle East or Asia west of Burma. These regions are on the night side of Earth