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Captain Cook left England's Plymouth harbour in 1768, bound for Tahiti so he could watch Venus pass in front of the Sun. He navigated across uncharted seas, arriving in 1769 to fulfil his mandate to time the transit and so provide crucial data for calculating the size of the Universe.

Tomorrow, Mercury will transit the Sun; astronomers are dashing to Hawaii to take a good look and make a few measurements. But the information to come out of the transit is unlikely to have the same impact as Cook's data on Venus, if it has any impact at all.

At most, a transit proves that we understand celestial mechanics, says George Fraser, at Leicester University, UK, who will not be watching the transit although he admits that some scientists would quibble with the assertion.

Fraser is in charge of the Mercury Imaging X-ray Spectrometer on the European Space Agency's BepiColombo mission, which will set off to orbit Mercury in 2013 and study the planet. At a recent BepiColombo working group meeting the upcoming transit caused "no major concern".

Mercury mystery

Glenn Schneider of the University of Arizona at Tucson works on the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS). He is in Hawaii at the Mees Solar Observatory to watch the transit, which he hopes will open up some otherwise unexplored avenues of investigation.

Schneider will measure the sodium from Mercury's thin outer atmosphere. Sodium is a puzzle on Mercury because, according to some theories about the planet's birth, the element is too volatile to be there. Accurate sodium data will help decipher whether it comes from the planet or from meteor impacts.

But the exosphere has been studied in detail already, says Sean Solomon, a scientist on Nasa's Mercury Messenger mission. Solomon doesn't anticipate any scientific revelations tomorrow, and rather than watching the transit, he intends to wait for the high-resolution data promised by Messenger in the coming years.

Messenger started its circuitous journey to Mercury in 2004 and flew by Venus last week. It will start its Mercury flyby in January 2008, and will enter Mercury's orbit in 2011. Being there will make it much easier to see what's in the atmosphere, says Solomon.

Schneider's team might also try to detect Mercury's very weak magnetic field against that of the Sun's never yet measured from Earth. "But that will be much harder that the proverbial needle in the haystack," he admits.

Watch and learn

Transits might be better off left to amateur astronomers. Schneider says this transit provides a "wonderful learning experience for students and anyone interested in astronomy". Solomon agrees: "It's like when you look through a telescope and see Saturn's rings," he says. "It's better than looking at it in a text book."

If you do fancy getting a glimpse of Mercury for the last time until 9 May 2016, the transit is fully visible from the west coast of America, the Pacific and the East coast of Australia. Everywhere else will see parts of the transit.

Never stare at the Sun: use Sun filters. And don't try looking for Mercury with the naked eye. Strong binoculars or a small telescope are needed.

The transit will start at 8 November 19:12 UT (14:12 EST or 11:12 PST), and will last for just under five hours.

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