When is the Transit Mercury and how can I watch it?

The sun high in the sky at Hunstanton beach. Picture: Matthew Usher

The sun high in the sky at Hunstanton beach. Picture: Matthew Usher - Credit: Matthew Usher

As the mercury in our thermometers creeps ever higher we have a rare opportunity to glimpse the planet Mercury falling across the face of the sun.

Mercury (AP Photo/NASA)

Mercury (AP Photo/NASA) - Credit: AP

With a diameter equalling the United States, Mercury is one of only two planets on an inner orbit and a visible transit across the sun, and will appear as only a tiny speck from the UK on Monday afternoon.

Astronomers have warned the event, which was last visible in 2006, is not as accessible as a solar eclipse.

A pair of eclipse glasses would not be able to magnify the sight sufficiently and a solar telescope would be needed for a full view.

'It is dangerous to look directly at the sun unless you have got the proper equipment and filters,' said Dave Balcombe, member of the Norwich Astronomical Society.

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'You will need specialist equipment to see it and the thing to emphasise is safety. You will need a solar telescope to catch a glimpse of Mercury because it is only 105th of the size of the sun and will appear only as a tiny dot across the face of the sun from our perspective.

'It is too small to see with the naked eye and as soon as you magnify the sun it becomes even more dangerous.'

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Starting at around 12.12pm, the transit will take around seven hours until it passes out of view just after 7pm.

'This is not a very regular occurrence, and that is because of the angles that the planets are relative to each other,' added Mr Balcombe. 'It is not every time Mercury passes between us and the sun that it can be seen, as often it passes slightly above or below.'

The last Mercury transits occurred in 2003 and 2006, and this year it will be visible from right across Europe.

'We can see Mercury in the sky in the early morning and evening but it rarely rises very high,' said Mr Balcombe.

'Only two planets; Mercury and Venus, that are inner planets inside our own orbit with the sun and therefore pass between us. 'The Venus transit is even more rare and happens in pairs, with the last transits in 2004 and 2012. That appears bigger in the sky than Mercury so is more impressive, but because it will not happen for another 121 years will not return in our lifetimes.'

The next transit of Mercury is expected to be November 2019, but will not be as easily visible from the UK.

-Astronomer Lawrence Harris explains the Mercury transit in more detail on p12 of EDP Weekend today.

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