Astronomers are in for a rare treat next week, when the planet Mercury passes across the face of the Sun.

This rare celestial event only happens about 13 times per century, so you won't get another chance to see it until 2032 - 13 years from now.

The transit of Mercury happens because it is one of only two planets in our solar system that orbit the sun closer than Earth - the other one being Venus.


Occasionally, however, the orbits of Earth and Mercury line up in such a way that Mercury passes directly between the Earth and the Sun.

When this happens, Mercury is visible from Earth as a tiny dot - about 0.5 per cent of the diameter of the sun itself. 

'The sky will put on a stellar show on November 11, 2019, as Mercury crosses in front of the Sun,' said NASA.

'From our perspective on Earth, we can only ever see Mercury and Venus cross in front of, or transit, the Sun, so it's a rare event you won't want to miss!'

With the right safety equipment, viewers nearly everywhere on Earth will be able to see the tiny dark spot moving slowly across the disk of the Sun. 

The transit starts at 11:35 GMT (04:35 PST) on Monday, November 11, and will last for about 5.5 hours, so there will be plenty of time to catch the show.

Viewers in certain areas, such as the West Coast of the United States, will not be able to see it until the Sun is visible in the sky. 

At approximately 15:20 GMT (08.20 PST) , Mercury will be as close as it is going to get to the centre of the Sun.  

The transit will still be underway as the sun sets in the UK an hour later.

Unlike a transit of Venus, where the planet appears large enough to be seen with the naked eye, Mercury is so small that you'll need binoculars or a telescope with a Sun filter to see it.

However, looking at the Sun directly or through a telescope without proper protection can lead to serious and permanent vision damage, so make sure you use a solar filter.

How to watch the transit of Mercury 

Dedicated solar telescope

These telescopes have the filter built in and it is irremovable, making it a particularly safe option. 

Coronado telescopes, for example, allow to you see the sun in hydrogen-alpha, which means that it will appear orange/red. 

Ordinary telescope with filter

If you already own a telescope then you can buy filters that fit onto the front end. 

You should always make sure the filter is attached securely and hold it up to a light bulb before attaching, to check for any damage, such as pinholes, in order to protect your eyes.

Projection through refracting telescope

An alternative to a solar filter is to project an image of the Sun through your telescope onto a piece of white card. 

Add a cardboard shade collar to the telescope itself to mask the rest of the Sun's glare. Use apertures of less than four inches to prevent overheating.

Projection through your binoculars

You can use a similar projection method with binoculars. 

Attach them to a photographic tripod and cover one of the apertures with a lens cap. You'll find the card will need to be quite close to the binocular eyepiece.

How to film the transit

Given that transits of Mercury are reasonably rare many observers are keen to capture the moment as a keepsake. 

Luckily, photographic equipment for astronomy has become relatively cheap and widely available and so this is pretty easy to do, particularly if you are already viewing the transit through a filtered telescope.