Did you know that there are just two times of year that a transit of Mercury can happen? Mercury, the innermost world of the Solar System, can only cross the Earth-Sun line of sight within a few days of either 8 May or 10 November.
Mercury is so far away from us and so much smaller than the Sun that during a transit of the Sun it appears tiny against the solar disc, unlike the Moon during a solar eclipse.
Mercury Transit by Daniel, Ely, Cambs, UK. Equipment: Coronado PST, Televue 2.5x Powermate, ASI120-MMS Camera
However, transits of Mercury are visible from the entire day-lit hemisphere of Earth when they happen, whereas solar eclipses are viewable only from within a narrow track.
Mercury also travels through space faster than Earth, so when we see it moving across the Sun we are glimpsing its forward motion, rather than the changing perspective resulting from the Earth’s motion.
Mercury Transit Draws to a Close by Charles Thody, North Lincolnshire, UK. Equipment: Skywatcher Equinox 80, Canon 40D, 3x Barlow, Celestron Advanced AVX mount.
What does a Mercury transit look like?
When watching a transit of Mercury you should see the planet, in Sir William Herschel’s words of 1786, as “a very black, round spot”.
It may be noticeably darker than any sunspots, though small, dark objects on a bright background are hard to compare.
If you are watching during the start or the end of the transit, look carefully at the moment when the full disc of Mercury is touching the inside edge of the Sun’s disc (only three minutes after the beginning, or three minutes before the end of the transit).
You may experience the momentary impression of a teardrop joining Mercury to the sky beyond the Sun.
This ‘black drop effect’ is caused by optical defects and is more prominent when our atmosphere is turbulent.
Mercury Transit by Rob Little, Corbridge, UK. Equipment: Skywatcher 250, Canon DSLR EOS 30D
When do transits of Mercury happen?
There are 13 or 14 transits of Mercury over the course of a century. Transits of Mercury always occur in May and November.
What makes them so oddly regular? It’s to do with the eccentricity of Mercury’s orbit and the resonance it has with Earth’s orbit around the Sun.
Mercury has a more eccentric orbit than any other planet, being 0.31 AU from the Sun at the closest point in its orbit (perihelion) but 0.47 AU away at its furthest point (aphelion).
Image of Mercury taken by the MESSENGER spacecraft. Credit: NASA/JPL-Caltech NASA/JHUAPL
It rotates exactly three times for every two orbits round the Sun. This is described as a 3:2 spin-orbit coupling, and almost certainly results from the strong tidal forces experienced as a result of Mercury’s changing distance but general proximity to the Sun.
It rotates slowly, bringing opposite hemispheres to the sub-solar point at alternate perihelia.
This results in its day length (measured from sunrise to sunrise) being twice as long as its year, which lasts 88 Earth-days.
Mercury’s apparent size changes by up to 20% and that’s a function of it having the most eccentric orbit of all the planets of the Solar System.
Composite image of Mercury transiting across the sun on 9 May, 2016, as seen by HMI on NASA’s Solar Dynamics Obersvatory. Credit: NASA/SDO
When we see a transit of Mercury in May, it’s while Mercury is closer to Earth and so it has a 12 arcsecond disc.
In November, the transits occur when Mercury is closer to the Sun so the planet appears slightly smaller from our perspective, about 10 arcseconds.
At 88 days, Mercury’s orbit of the Sun is almost a quarter of that of Earth, which means it’s close to being in orbital resonance with Earth, hence the transits occur at the same times of year… for the time being.
The timing between them will drift and though they occur in May and November now, that will change in centuries to come. But they’ll always be six months apart.
Mercury passing in front of the Sun captured in 2006 by the Solar Optical Telescope. Credit: Hinode JAXA/NASA/PPARC
How to observe a Mercury transit
It is dangerous to look directly at the Sun, and for a transit of Mercury, there is absolutely no point in doing so!
Mercury’s apparent diameter as it crosses the Sun’s disc is too small to see without magnification, and so eclipse glasses designed for looking through with the naked eye will be no use, nor will pinhole-based gadgets.
Mercury will look smaller than a large sunspot, but will not be too hard to see with a little help.
If you have access to a solar telescope (your local astronomical society may have one) you will be well catered for.
Astronomers and visitors observe Mercury transit the Sun, Sofia, Bulgaria, 11 November 2019. Here, the transit-watchers are projecting an image of the transit onto a piece of white card. Photo by Petar Petrov/Impact Press Group/NurPhoto via Getty Images
Otherwise, watch the safety advice video on the Open University website revealing how to watch a transit of Mercury and get hold of a mirror and lens-based ‘solar scope’ that projects an image onto the inside of a box.
If you have binoculars or a small telescope but don’t have specialist solar filters to cover the objective lenses, you can still use them to project an image of the Sun onto a white card held about 0.5m beyond the eyepiece.
You can tell when your telescope is aimed directly at the Sun without looking through the eyepiece by manoeuvring it until the shadow of its tube is circular.
Who was first to observe a Mercury transit?
Pierre Gassendi (1592 – 1655), French philosopher and mathematician. Credit: Photo 12/Universal Images Group via Getty Images
Solar transits of Mercury are more common than a transit of Venus. The first observation of a transit of Mercury was achieved on 7 November 1631 by the French Jesuit astronomer Pierre Gassendi (above), thanks to a successful prediction by Johannes Kepler.
There is no record of a transit of Venus having been observed until 1639, even though those can be seen by the naked eye (at the risk of damaging your eyesight).
Edmond Halley observed a transit of Mercury while at St Helena on 3 May 1676, and realised that if observers at known but distant locations were to record the same transit, the parallax would give the information required to deduce the scale of the Solar System.
Mercury Transit and AR2543 by Pete Williamson, Shropshire, UK. Equipment: 90mm Coronado, 2.7x Barlow, ZWOASI174MM.
That’s why James Cook was sent to observe the 3 June 1769 transit of Venus from Tahiti.
Less well known is that the expedition’s astronomer Charles Green, along with Cook himself, observed a transit of Mercury on 9 November of the same year, from the shores of Mercury Bay in New Zealand.
Green later fell ill, and died soon after putting out from Batavia (modern-day Jakarta) in 1771, but is credited with being the first to note that Mercury’s crisp outline during transit demonstrates that the planet must have little or no atmosphere.
Sir William Herschel caught only brief glimpses of the 4 May 1786 transit through cloud, but had better luck on 9 November 1802.
When is the next transit of Mercury?
Find out when the next Mercury transit is happening via our table below.
Pictures of the transit of Mercury
Below is a selection of images of recent Mercury transits. For more info on planetary astrophotography, read our guide on how to photograph the planets.
And don’t forget to send us your images or share them with us via Facebook, Twitter and Instagram.
Mercury Transit by Jaspal Chadha, London, UK. Equipment: Skywatcher Esprit 100ED, Canon D550 DSLR, DIY Solar Filter.
Mercury Transit, 9 May 2016 by Neil Hibbs, Holmes Chapel, UK. Equipment: 5″ Cassegrain, Bader filter, Canon SLR
Transit of Mercury by Sarah & Simon Fisher, Bromsgrove, Worcestershire, UK. Equipment: Canon 600D, Mak 127mm telescope, homemade Baader Solar film filter
Transit of Mercury, 9 May 2016 by Peter, London, UK. Equipment: Orion SkyQuest XT8i Classic Dobsonian Telescope, Canon Powershot SX700
Mercury Transit by Callum Pennington, St. Helens, Merseyside, UK. Equipment: Sky-Watcher BK 1309 EQ2 130mm, white light solar filter, super 25 wide angle eyepiece and a smartphone.
Mercury Hits a Prominence by Paul Mason, UK. Equipment: Coronado PST, Neximage Burst Colour.
Mercury Transit by Paul Mason, UK. Equipment: Meade LS 8, Canon EOS1100D, White Light Filter.
Mercury Transit by Paul Mason, Cannock, UK. Equipment: Meade LS8, Canon 1100D
Mercury Transit by Trevor Pitt & Mark Griffith, Wallingford, Oxfordshire, UK. Equipment: Skywatcher Adventurer Astro mount, Skywatcher Equinox 66mm refractor, ZWO ASI120MC Colour CMOS camera, Lunt Solar Wedge.
Mercury Transit with Contrail by Darren Buchan, Corby, UK. Equipment: Skywatcher 200p, EQ5 mount, Canon 1200d.
Mercury Transit by Stephen Jennette, Morecambe, UK. Equipment: Lunt L60 pressure-tuned solarscope, double-stacked etalon, Zwo ASI 174 mono camera.
2nd Contact by Paul Read, Northamptonshire, UK. Equipment: Canon 600D, 17mm Baader Hyperion, Baader Solar Continuum filter, Lunt Hershel Wedge, Skywatcher ED80 DS Pro, Skywatcher HEQ5 Pro.
Mercury Transit at Jaisalmer Wind Farm by Abhinav Singhai, Atish Aman, Delhi, India. Equipment: 2000mm focal length
Mercury Transit – A Couple of Minutes into Pass by Anthony Day, Ash, Surrey, UK. Equipment: LS50THa, DS
Mercury Transit by Anthony Day, Ash, Surrey, UK. Equipment: LS50Tha, DS Unit
Transit of Mercury by James Robertson, Croydon, UK. Equipment: 9×25 Nikon binoculars, Tripod, Canon camera.
Transit of Mercury by Matt Jarvis, UK. Equipment: Canon PowerShot SX60 HS, White Light Solar Filter
Transit of Mercury by Alessio Vaccaro, Italy. Equipment: Apo Refractor, Solar Filter, Unmodded Reflex.
Transit of Mercury 9 May 2016 by Jonathan Turner, UK. Equipment: Nikon D50, Skywatcher Evostar 120.
Mercury Transit by Jack Reeves, Birmingham, UK. Equipment: Celestron astromaster 90 AZ mount, smart phone
Mercury Transit by Konstantinos Tranganidas, Perth. Equipment: Panasonic Lumix DMC-FZ72, x1.7 teleconverter, Velbon UT-63D Tripod, Baader Solar Filter (OD 3.8), Multicoat UV filter
Mercury Transit by Graeme Vick, UK. Equipment: Canon EOS DSLR
Mercury Transit by Andrew Relf, Basildon, UK. Equipment: Skywatcher 8″ reflector , Gp MonoCam
Transit of Mercury by Tom Howard, Crawley, Sussex, UK. Equipment: Nikon D7000 DSLR, Nikon TC-20E teleconverter, Meade 5000 127mm refractor, Skywatcher AZ-EQ6 mount.
Mercury Transit by Jamie Welton, Bradford, West Yorkshire, UK. Equipment: Skywatcher Skymax 127, Baader AstroSolar Filter
Mercury Transit by Nico Altena, Netherland. Equipment: Lunt 60MM Solar telescope, DMK 41 Camera
Mercury Transit by Nico Altena, Netherland, Equipment: WO 98mm, CA-K B1800 Module, ZWO ASI 174MM.
Mercury Transit by Nico Altena, Netherland. Equipment: WO 98MM, CA-K B1800 Module, ZWO ASI 174MM.
Mercury Transit by Nico Altena, Netherland. Equipment: WO 98MM, CA-K B1800 Module, 2X Barlow lens, ZWO ASI 174MM.
Beginning of Mercury Transit, 9 May 2016 by Nico Altena, Netherland. Equipment: 60 MM Lunt Solar telescope, DMK 41 Camera
Mercury Rising by Peter, London, UK. Equipment: Orion 8″, Solar filter. Canon camera.
Mercury Transit as seen from Esfahan Iran by Kamran Janamian, Esfahan, Iran. Equipment: Skywatcher BKP 200, HEQ5, Modified Canon 600D, Astronomik uv/IR cut, Baader Solar filter ND 3.8
Mercury in Transit by Phil Benson, SE Essex, UK. Equipment: Lunt LS152 Ha Solar Scope, PG Flea3 camera.
Mercury Transit by John Chumack, Presque Isle State Park, Pennsylvania, USA.
Mercury Transit by Steve MacDonald, Bolton, UK. Equipment: Skywatcher 80ED Pro, Baader safety filter, Astro GP-Cam.
Mercury Transit and Swallow by David de Cuevas, Treize Vents, France. Equipment: Canon 700D, Canon EF 70/200mm
Mercure transit and swallow – Crop by David de Cuevas, Treize Vents, France. Equipment: Canon 700D, Canon EF 70/200mm
Mercury Transit by Austin Guerin, Tipperary, Ireland. Equipment: Williams Optics FLT 110mm, Seymour solar filter, Nikon D5200, Sky-Watcher AZ-EQ5GT
Mercury Transit by John Short, Whitburn, Tyne and Wear, UK. Equipment: Celestron 8SE, Baader Solar Filter, Barlow 2x, UHC filter, Sony A7RII.
Mercury Transit Close Up by Chris Higgins, Ripon, N Yorks, UK. Equipment: LUNT 60 Ha solar scope, ZWO ASI 174 MM camera, Televue 2.5x Powermate.
Mercury Transit Over Ha Disk by Stuart Green, Preston, Lancashire, UK. Equipment: 150mm solar telescope, double stacked hydrogen alpha etalons, Basler acA1920-155um, Sony IMX174 sensor.
Transit Beginning by Trevor Smith, Ryhill, West Yorkshire, UK. Equipment: Skywatcher 70mm scope, solar filter, Canon 70d, tripod.
Mercury and Bird Transit The Sun by Necati Demiral, Euromus, Turkey. Equipment: Televue 102iis, ASI 120MM, Skywatcher NEQ6
Mercury Transit by Stuart Wilson, Falkirk, UK. Equipment: Solarscope SV50, Celestron Skyris 236m camera.
Transit of Mercury by Hayley Smith, Aylesbury, UK. Equipment: Skywatcher ED80, EQ5 mount, Baader film, Canon 600D
Mercury and Sunspots by Hayley Smith, Aylesbury, UK. Equipment: Skywatcher ED80, EQ5 mount, Baader film, Canon 600D.
Mercury Transit by Alan Rhodes, South Oxfordshire, UK. Equipment: Nikon D3100, Celestron 8SE SCT, Celestron GoTo Alt-Az mount.
Mercury Transit near Sunspot #2543 by John Chumack, Lake Erie, USA. Equipment: Lunt 60mm/50F HA scope, QHY5IIL CCD camera, 2x barlow.
Transit of Mercury by Sonny Carlino, West Yorkshire, UK. Equipment: Canon EOS 1100D, 12″ Skywatcher Flexitube Dobsonian, AZ SynScan.
Mercury Transit of the Sun by David Tolliday, Macclesfield, UK. Equipment: Canon 7D MkII, 500mm lens, Bembo tripod, Wimberley head.
Mercury Transit by Karl Lee, Southport, UK. Equipment: Meade ETX90, Iphone 6s, Solar Filter, Blue Filter