Transit of Mercury

2019 Nov 11

A simulation of how the transit will appear from London.

Time:       Altitude: °      Azimuth: °

On 11 November 2019, Mercury will pass in front of the Sun, casting a small black silhouette in front of the solar disk from 12:35 until 18:03 GMT.

The transit will be visible from any location where the Sun is above the horizon between these times, including from Africa, the Americas, Europe and French Polynesia.

This will be the first transit of the planet since May 2016, and the last until November 2032.

Planetary alignment

Mercury passes between the Earth and Sun on average once every 116 days, each time it is at inferior conjunction, as it moves from the evening to the morning sky. It rarely passes directly in front of the Sun, however. Mercury’s orbit is inclined at 7° to the Earth-Sun plane, which means it usually passes to one side of the Sun.

Mercury only transits in front of the Sun’s disk when one of these inferior conjunctions coincides with the planet passing through the Earth-Sun plane, which it does on average once every 44 days. These two cycles coincide on average only once every 7-8 years. Each time, a transit can be seen from any location where the Sun is above the horizon at the time.

A small black dot

For a newcomer, perhaps the most striking thing about a transit of Mercury is how small the planet appears in comparison to the Sun. At inferior conjunction, Mercury measures a mere 12 arcseconds across, about 150 times smaller than the Sun.

This makes transits of Mercury significantly trickier to observe than other solar phenomena such as eclipses and even transits of Venus. Mercury’s silhouette is not visible without some form of magnification.

As a result, it is especially important to emphasise that the Sun is a dangerous object to observe. Viewing it through any optical instrument – even a pair of binoculars or the finderscope on the side of your telescope – can cause instant and permanent blindness.

Map of where the transit will be visible. Click here to expand.

Group observing

If you have any doubts about what equipment to use, by far the safest thing to do is to go along to a public observing event. Many astronomical societies are likely to be hosting observing events on the day, and they’ll be sure to welcome newcomers. You may meet some new people at the same time as seeing the transit.

Projecting an image of the Sun

Two example of low-cost cardboard solar projection boxes. These two are sold in the UK by Green Witch.

The safest way to view the transit yourself is to buy a purpose-built solar projection box.

These typically consist of a cardboard box with a small lens on one side. They project an enlarged image of the Sun onto a white cardboard sheet inside the box. Once the transit is over, they’re also great for observing sunspots. They are safe to use, quick to set up, and ideal for use with children and groups.

Using a telescope to project an image

BAA Solar Section Director Lyn Smith demonstrates how to project an image of the Sun onto a piece of white card using a small refractor.

If you have access to a telescope, you may be able to use it to observe the Sun. Remember, though, that you must never look through the telescope, or even the finderscope, when either is pointing anywhere close to the Sun. To ensure you don’t forget, it’s best to remove the finderscope or leave its lens covers on.

If you have a refractor or small Newtonian telescope, you can quite easily project an image of the Sun onto a piece of white card. You should not attempt to do this with a Schmidt-Cassegrain (SCT) or Maksutov telescope, as you may permanently damage your telescope. If you are unsure what kind of telescope you have, check first. Many telescopes sold to beginners are of the Schmidt-Cassegrain design.

Setting up the equipment

To project an image of the Sun, you simply need to fit a cheap low-power eyepiece into your telescope and hold a piece of card 30-40 centimetres away from it.

Avoid using expensive eyepieces when doing this, as there is always a risk that the eyepiece will crack under the Sun’s heat. Such damage will not be covered under any guarantee, and may cost you a great deal of money. You should also avoid eyepieces made from plastic, as they are liable to melt.

Aiming your telescope at the Sun can be a challenge at first. You must not use a finderscope, as looking at the Sun through this is just as dangerous as with any other optical instrument. Sighting the Sun along the edge of the tube must also be avoided, since your eyes can be damaged by looking directly at the Sun even without magnification.

The best way to aim the telescope is to look at the shadow of your telescope tube on the ground. When the telescope is pointing to one side of the Sun, the shadow will appear as an elongated oval. If you move the telescope into better alignment with the Sun, its shadow will get smaller and become more circular.

Once you have oriented the telescope to make the shadow as small and circular as possible, it should be pointing in almost the right direction. Try projecting an image onto a piece of card. You may need to scan the telescope from side to side until it comes into view. Using a low-power eyepiece will make this easier.

The key is to be patient and to practice!

Hazards to watch out for

When projected an image of the Sun, you must take extreme caution to ensure no one looks through the eyepiece, especially if you are observing with children. Even touching the eyepiece can be painful, as it will rapidly become extremely hot.

Also take care not to pass your hand close to the eyepiece, such that the Sun’s light is focused onto it. This can cause painful burns in an instant.

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