A slight change of title this month, as I will concentrate on the transit of Mercury on May 9.
A transit is the term used when a celestial object passes in front of a bigger one from the relevant view point, and can therefore at least in theory be seen against the bigger object’s disc. In the solar system, the most commonly observed transits are those of Jupiter’s four Gallilean moons, which frequently pass in front of Jupiter from our viewpoint – though spotting them is another matter, even when you know they are there! (If the object passing in front is comparable in apparent size to the object it is passing in front of, that is called an eclipse – as when the Moon eclipses the Sun. If the object passing in front has much greater apparent size, that is called an occultation).
The other important type of transit is when one of the two inferior planets – so termed because their orbits are closer to the sun than ours – Mercury or Venus, transit over the Sun’s disc. Both of these planets pass close to the Sun in the sky as they whizz round in their shorter orbits, but normally they pass below or above the Sun as their orbits are not quite in the same plane as ours; we say they have inclined orbits. But if you think about inclined ellipses around the same focal point, there will be two points in the orbit when they will be in the same plane. When the earth and planet concerned are both at , or very close to, those points, they will be precisely lined up with the Sun, and thus we on Earth will see the inferior planet against the Sun’s disc.
Because this takes place at a fixed point in the earth’s orbit, that means such occultations only occur at particular times of year. In the case of Venus, this is early June or early December;for Mercury, early May or early November. Because Venus is much closer to us, the inclination of its orbit makes it much less likely that it will be precisely lined up with the Sun’s disc. The most recent occasion was 2012, and the next is 2117!
Transits of Mercury are much more frequent, with 14 this century, in a complicated pattern that sometimes has two separated by a 3 year interval. The May 9 transit is the first of such a pair; the second one is in November 2019. However, a May eclipse is generally a much better bet for us in the UK. Our day is much longer, so there is more of a chance that the Sun will be up for us when the transit takes place; and it will almost certainly be higher in the sky. As it happens, the transit this May is the last one of this type until 2049, so this really is the one to catch if you can.
But, how do you catch it? The standard advice about observing the Sun applies. You should never look directly at the sun with your naked eye, still less so with a normal telescope or binoculars as they will magnify its heat many fold and permanently damage your eyesight. (You may well damage the telescope, too). What you require is either a specialised solar telescope, or a specialised whole aperture solar filter, for safe viewing of the transit; you will know if you have one of these! In theory it is possible to project an image through an old telescope, but this is rather haphazard, and risky to both you and your telescope, so not advised unless you really know what you are doing. The other option you may have thought of was using eclipse glasses if you still have some in good condition. The trouble with this option is that Mercury will have an apparent diameter of just 12″ – too small to show up against the Sun’s disc without magnification. You should not try to use it in combination with a telescope or binoculars; it cannot be used safely in this way.
So, you need a friend with the right kit, and of course clear skies. But at least there is a fair amount of time available; the transit begins at 12:12, and finishes at 19:42, BST in both cases. If you do get a chance to see it, see how the tiny black dot compares with any sunspots on the Sun’s surface. It makes you realise how big the sunspots are, but also that they may be darker than the rest of the sun, but they are still much brighter than the black dot of Mercury.
Transits have always had a scientific importance, as well as offering a spectacle. In the 16th and 17 centuries, they were used to measure the size of the Solar System via a triangulation technique involving timing particular start and endpoints of the transit from widely separated points on the earth’s surface. This proved impractical with transits of Mercury, but worked well with Venus, and gave us the first good estimate of the distance from the Earth to the Sun.
Another more recent application is that detecting planetary transits of the discs of stars is one of the main methods for detecting planets in orbit around distant stars simply by detecting regular tiny reductions in the total light reaching us from that star. Observations of the total light received from the sun have been monitored in the same way to understand better how the detail of transit effects might show up in stellar light curves.