Mercury pass delights skygazers

Mercury seen as a tiny dot (circled) by the Soho spacecraft

Astronomers in the Americas, East Asia and Oceania have been enjoying a rare opportunity to see Mercury pass in a direct line across the Sun.

The closest planet to our star appeared as a tiny black dot creeping over the solar face between 1912 GMT on Wednesday to 0010 GMT on Thursday.

Some observatories held special viewing parties for the public.

The entire transit was visible from the western US, south-east Australia, New Zealand, Canada and the South Pacific.

Parts of the transit were visible before sunset on Wednesday in the rest of the Americas, and after sunrise on Thursday in East Asia and the rest of Australia.

This time, skygazers in Europe, Africa, the Middle East and Asia west of Burma missed out.

Click here to see a global viewing map

The previous Mercury transit was in 2003; the next will be on 9 May 2016.

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.

Mercury's disc appears only 1/194 the size of the Sun's

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.

Filtering out

In Los Angeles, about 30 enthusiasts set up telescopes on the lawn of the recently restored Griffith Observatory, one of several around the world to host viewing parties.

Others followed the transit on the web, with a number of astronomical institutions and agencies streaming video.

THE PLANET MERCURY Closest planet to the Sun Diameter: 4,800km Mercurian year: 88 days Equator surface temp: 450C Partially mapped by Mariner Has global magnetic field Composition is iron-rich May be ice in polar craters

For many astronomers, both amateur and professional, it was simply an event to wonder at.

But researchers at the University of Hawaii's Institute for Astronomy used 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 also looked 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 was visible from the western US, south-east Australia, New Zealand, Canada and the South Pacific

Some regions saw 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 was not visible from Europe, Africa, the Middle East or Asia west of Burma. These regions were on the night side of Earth

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