On November 11th, 2019, between 12:35 and 18:04 universal time (UTC), Mercury, the innermost planet, will pass in front of the Sun as seen from Earth: an astronomical spectacle called a “planetary transit”. Planetary transits visible from Earth are relatively rare events: only the inner planets Mercury and Venus can ever pass between the Sun and Earth, and they are only seen to cross its disc when the plane of the planet’s orbit intersects the plane of the Earth’s orbit (the ecliptic) close to the time when the planet is at inferior conjunction with the Sun. On most inferior conjunctions, the orbital planes do not align (or, in other words, are not close to a node crossing) and the planet “misses” the Sun, passing above or below it as seen from Earth.
Mercury’s orbit crosses the ecliptic around May 8 and November 11 at the present epoch, and so transits always occur within a few days of those dates. The most recent transit of Mercury was on May 9th, 2016 (when, despite being clouded out for most of the event, I managed to briefly observe and photograph it through thin clouds), and the next transit will not occur until November 13th, 2032, so if you miss this one, you’ll have a thirteen year wait until the next opportunity.
Transits of Venus are even more rare. They come in pairs, in June or December, separated by intervals longer than a present-day human lifespan. The last transits of Venus were on June 8th, 2004 and June 6th, 2012, and the next pair will not occur until December 11th, 2117 and December 8th, 2125. In 2004, I burned up around three months of computer time searching for all transits in the solar system for a quarter of a million years centred on the present. See my “Quarter Million Year Canon of Solar System Transits” for details, including the upcoming simultaneous transits of Venus and Mercury in July of the year 69,163, and the monster event of December 23,364 when a “triple transit” of the Earth, Moon, and Venus will be visible from Saturn while, simultaneously, Venus will transit the Sun as seen from the Earth and Moon.
A transit of Mercury or Venus is not like a solar eclipse. Because the inner planets are so far from the Earth, your location makes little difference in what you’ll see. Except for very rare “grazing transits”, where the planet just skims along the edge of the Sun’s limb, as long as you can see the Sun during the event, you can see the transit. And since transits last for hours (this one takes around five and a half hours), they are visible from a substantial part of the Earth’s surface. Here is a map of the visibility of the upcoming event, courtesy of Fred Espenak of EclipseWise.com.
The entire transit, from the ingress of Mercury to the Sun’s disc to egress, will be visible in South and Central America, the eastern United States, and a bit of west Africa. Observers in the rest of Africa, Europe, and the Near East will observe the start of the transit, which will still be in progress when the Sun sets. For observers in most of the rest of North America, with the exception of most of Alaska, the Sun will rise with the transit already in progress and the transit will end with the Sun in the sky. Only in the dark grey areas, including India, China, Australia, east Asia, and most of Russia, will the transit not be visible at all.
Observing the Transit
The disc of Mercury crossing the Sun will be very small, just ten arc-seconds in diameter, only 1/194th the Sun’s apparent diameter on transit day. This is far below the limit of human naked-eye visual acuity (30 to 60 arc-seconds, varying among individuals), so you’ll need optical assistance to see the transit. This means you’ll need a telescope, ideally one with at least a magnification power of fifty times, to see Mercury in transit. The good news is that, as is well known, the Sun is bright, so you don’t need a large aperture telescope designed for observing faint objects at night, nor do you need a fancy mounting and drive for tracking the motion of objects in the sky. The most modest of telescopes with an eyepiece of suitable power will do the job.
The bad news is that, as is well known, the Sun is bright, and if you just aim a telescope at the Sun and look through it, you’re likely to be instantly blinded. Never point any kind of optical instrument directly at the Sun without first equipping it with a safe solar filter which is securely affixed to the objective (Sun-pointing) end so it can’t fall off or be otherwise dislodged. Any telescope must be equipped with a full-aperture solar filter, which should be carefully inspected before use (by holding it up to the Sun and examining it visually for scratches and other damage which might let unfiltered sunlight through), then mounted on the telescope so it can’t come off (if a more elegant solution is not at hand, duct tape will get ’er done).
I use an Orion glass solar filter which I’ve had since the 1990s and used to observe and photograph four total solar eclipses, one transit of Venus, two transits of Mercury, and many sunspots. Orion filters come in sizes which fit many popular telescopes, and, if the diameter is compatible, work well on telephoto photographic lenses. These filters are expensive, but will last a lifetime and are ideal for sunspot and partial solar eclipse observation. Mylar solar filters are also safe, but must be treated carefully to avoid damage, and only purchased from reliable vendors specialising in products for amateur astronomers and photographers: don’t put your vision at risk by using some cheap-o “bargain” you found on eBay: a filter that produces a comfortably viewable image but passes infrared (which you can’t see) can cook your retina without your knowing it’s happening.
If you’re using a telescope with a finder scope, be sure the Sun-pointing end of the finder is covered by a securely-fastened lens cap or aluminium foil: you don’t want your hair set on fire (or, in my case, singe the scalp) by sunlight from the finder while you’re looking through the main eyepiece.
Photographing the Transit
Any photographic set-up which works for sunspots will serve for photographing a transit of Mercury. You’ll need a long focal length lens (350 mm or more equivalent for a 24×36 mm camera) and a full aperture solar filter which fits it. As it happens, the solar filter I use on my telescope also fits two Nikon camera lenses I use for solar photography, so I simply switch back and forth (being extremely careful not to point either at the Sun except with the filter in place). In addition to filters designed for telescopes, purpose-built screw-in photographic solar filters are available from vendors such as B&H. In many cases, your camera’s automatic focus and exposure will do a fine job when shooting the Sun through a filter: in the case of a transit, it’s the total black of the planet’s disc against the brilliant Sun, so the contrast couldn’t be greater. For perfection, switch to manual and adjust until you’re happy with the results.
With small auto-everything cameras or even cameras built into mobile phones, you can often get reasonably good results simply by holding the camera’s lens up to the eyepiece of the telescope and shooting through it. Experiment some time before transit day: if you can get pictures which show the edge of the Sun sharply defined and see any sunspots which happen to be present (check the Solar Dynamics Observatory visual data images to see what’s there), it will work fine for the transit.
I’m posting this article about the upcoming transit two months in advance to give people interested in observing it plenty of time to obtain and test whatever gear they need for the event. I will be trying to visually observe and photograph the transit on the afternoon of November 11th, although my anticipation is tempered by realism: the weather here in November is typically foul, and I have been clouded out for the Leonid meteor shower, which occurs a week later in November, for twenty-five consecutive years.
A transit of Mercury is a rare and memorable celestial spectacle, well worth taking the trouble to observe. Although it can be brighter than Saturn ever gets, many people have never seen Mercury: it never wanders father than 27°45′ from the Sun and is often lost in the glare of dawn and sunset. What better time to spot Mercury than when you need only look at the Sun to find it?
Observations of transits, particularly those of Venus, were once of great importance to astronomers, since they provided the only means to determine the absolute scale of the solar system. By simultaneously observing a transit from widely-spaced locations on the Earth and noting the difference in the apparent location of the planet’s disc on that of the Sun and the time of transit events, it was possible to compute the absolute distance to the planet and Sun as a fraction of the diameter of the Earth, which was well known. Today, there is little professional astronomers can learn from transits, but they remain a rare, yet easily observed, phenomenon eagerly awaited by amateurs.
For those afflicted with cloud cover on transit day or in areas from which it won’t be visible, there are likely to be live Webcasts of the transit from various locations. I will add links to these as they are announced.
Here is a picture of the May 9th, 2016 transit of Mercury taken through thin clouds. Mercury is the dark dot around to 10 o’clock position from the centre of the Sun; a sunspot group, which is noticeably less dark than Mercury, appears to the right of the centre of the Sun.
This photo was taken with a 500 mm f/8 catadioptric “mirror lens” with a full-aperture solar filter on the front. Exposure was 1/1250 second at ISO 400.
Here is an amateur video of the 2016 transit which gives an idea what you’ll see with a telescope.
Thierry Legault managed to catch simultaneous transits of Mercury and the International Space Station as well as an airplane crossing the Sun. The first was the result of careful planning, the second persistence and luck.