Roughly 13–14 times each century, Mercury passes directly in front of the Sun, casting a small black silhouette in front of the solar disk for a few hours. Such a transit will occur on 9 May 2016, lasting from 04:12 until 11:42 PDT.
Between those times, the transit will be visible from anywhere on Earth, providing the Sun is above the horizon, including from Africa, the Americas, Asia and Europe.
The simulation to the right shows the path that Mercury will take across the Sun's disk, drawn to scale. The Sun is more than 280 times larger than Mercury, and so the latter appears as a very small black speck in comparison to the Sun's disk.
By default the simulation is drawn with respect the local vertical in Seattle (Zenith up), matching how the Sun would appear when viewed through some suitably filtered telescope or a solar projection unit. Viewing the Sun through any optical instrument is incredibly dangerous, however, and tips for doing so safely are given below.
The compass within the simulation shows the direction of celestial north relative to the local vertical. You can orientate the simulation with celestial north orientated uppermost by selecting the option North up. Because Mercury is so far away, its path across the Sun is almost identical regardless of your location on Earth, though the orientation of your local vertical will differ.
Selecting the option Diagram of Mercury's path will produce a static display of the Mercury's path through the course of the transit.
The lower panel of the simulation shows the Sun's position relative to the local horizon in Seattle.
Mercury orbits the Sun once every 88 days, completing just over four revolutions around the Sun for every one that the Earth completes. This means that it regularly overtakes the Earth in its orbit, and at such times the two planets lie side-by-side in their respective orbits.
This alignment is called inferior solar conjunction and occurs once every 116 days – a period called Mercury's synodic period. It takes place as Mercury moves from the evening sky into the morning sky. Even though Mercury invariably passes very close to the Sun at these times, it rarely passes exactly in front of the Sun's disk.
Mercury orbits the Sun in a slightly different plane from the Earth: its orbit is tipped up at an angle of 7° relative to Earth's orbit. The diagram below show this inclination of Mercury's orbit (not drawn to scale), with a grid representing the plane of the ecliptic – the plane in which the Earth orbits in Sun.
Mercury spends much of its time significantly above or below the plane of the Earth's orbit, and at such times it cannot pass directly in front of the Sun. If it passes inferior conjunction at such a time, it will appear to pass to the side of the Sun.
Mercury can only pass in front of the Sun if it passes inferior conjunction while it is also very close to the Earth–Sun plane. It crosses through this plane twice on each orbit around the Sun, roughly once every 44 days, at the points on the left and right sides of the diagram above. These points are called the nodes of Mercury's orbit.
The nodes of Mercury's orbit lie alongside the points along the Earth's orbit that we pass on around May 8 and November 10 each year. Consequently, transits of Mercury can only ever occur within a few days of these two possible dates.
Due to the precession of the equinoxes, these two dates advance through the year at a rate of roughly one day every 100 years.
This requirement for Mercury to pass one of the nodes of its orbit at the same time that is also passes inferior conjunction is met on average only once every 7–8 years, though transits do not occur at regular intervals.
Each time, Mercury can be seen to pass in front of the Sun from any location where the Sun is above the horizon at the time.
Observing the transit
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.
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
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
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.
Do not leave the telescope unattended at any time. If you need to take a break, move the telescope away from the Sun. This will give the eyepiece a chance to cool off as well as ensuring no one injures themselves. It is best to leave the telescope pointing to the east of the Sun, because then you can be sure that the Sun won't drift back into the field of view. In the northern hemisphere, this means pointing the telescope to the left of the Sun.
These hazards aside, this is much the safest way to observe the Sun with a telescope. If you're careful, the worst that can happen is that you may lose an eyepiece if it shatters under the heat. The risks can be minimised by ensuring the aperture of the telescope is no larger than 60mm. If your telescope is larger than this, fix a piece of cardboard over the front and cut a 60mm disk out of the middle. This will reduce the amount of light entering the telescope.
Filtering the Sun's light
It is also possible to buy special filters which you can fit over the front of your telescope, and which allow you to look directly through the telescope at the Sun. It is essential that the filter fixes very securely to your telescope, that it is undamaged, and that it is designed for safe use with your telescope. Only buy from reputable suppliers you trust, and thoroughly inspect your filters for damage every time you use them. This is the only safe way to view the transit with a Schmidt-Cassegrain telescope.
Some filters are sold which are designed to be fitted to the eyepiece end of the telescope, rather than over the front. We recommend that these should never be used under any circumstances. They are often of suspect quality, and can crack under the Sun's heat, exposing you to the Sun's full light.
If you make a filter yourself, the only safe option is to buy a sheet of Mylar or Baader Astro Solar Film from a reputable optical supplier, and ensure that it is fixed very securely in place. If there is the slightest scratch or pinhole, you should discard the filter and make or buy a new one.
The dos and don'ts
- DON'T ever look at the Sun without proper eye protection.
- DON'T view the Sun through sunglasses of any type (single or multiple pairs), or filters made from photographic film, or any combination of photographic filters, crossed polarisers or gelatin filters, CDs, CD-ROMs, or smoked glass. None of these are safe.
- DO view the Sun ONLY through special filters made specifically for safe solar viewing. Ensure they are DESIGNED to be fitted SECURELY to the kind of instrument you have. These include, e.g. aluminised Mylar filters, or black polymer filters, identified as suitable for direct viewing of the Sun, bearing the CE mark AND a statement that it conforms to European Community Directive 89/686/EEC. Always read and follow the manufacturer's instructions carefully.
- DON'T fit any filter to a telescope without FIRST checking it thoroughly for damage. If it is scuffed, scratched, has pinholes in it, or you have any other doubts about it at all, DON'T use it.
- To repeat that again: if you are not certain that a filter is approved and safe, or you have any other doubts, DON'T USE IT.
For more information about safely observing the Sun, we recommend Lee Macdonald's excellent book How to Observe the Sun Safely , published by Springer in 2012.
The position of Mercury at the moment of closest approach to the centre of the Sun's disk will be:
The coordinates above are given in J2000.0.
|The sky on 09 May 2016|
3 days old
All times shown in PDT.
Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.
The circumstances of this event were computed using the DE405 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).
This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.
|09 May 2016||– Transit of Mercury|
|09 May 2016||– Mercury at inferior solar conjunction|
|19 May 2016||– Mercury at aphelion|
|05 Jun 2016||– Mercury at greatest elongation west|