Mercury is the nearest planet to the Sun, with an orbit that lies inside that of the Earth. Therefore the possibility arises that Mercury can come between us and the Sun. This alignment, termed an "Inferior Conjunction", occurs on average every 116 days, but it is a rare event for Mercury to come precisely along the line of sight between the Earth and Sun.
Mercury's orbit is inclined at 7 degrees compared to that of the Earth so on most conjunctions the planet passes above or below the sun and is not seen. However, should a conjunction occur when the Earth lies at the intersection of the orbits, which happens around 7th May and 9th November each year, the alignment is so good that Mercury actually crosses the face of the Sun - this is referred to as a "transit". On average such an event occurs around 13 times a century.
This diagram (courtesy of NASA Liftoff Interactive Solar system) shows the position of the Earth, Mercury and Sun on 10 May 2003, 3 days after the event. North is upwards.
From this vantage point, above and behind the Earth, the orbit of Mercury can be seen to be tilted with respect to that of the Earth. The intersection point lies in our direction.
The Sun and planets are exaggerated in size so a transit will occur a lot less often than the picture might suggest.
Between the Earth and Mercury lies the orbit of Venus and outside the Earth is the orbit of Mars - both planets themselves lie off shot. The green lines are the orbits of selected asteroids and comets and can be ignored for now.
The last transit happened in November 1999, when Mercury just clipped the Sun, but the last time one was visible from the UK was in November 1973.
Although of less scientific interest today, in the past transits of Mercury have been used to help our understanding of the cosmos:-
Attention now is more turned to looking for transits across distant stars in the hunt for planets and possible life in star systems beyond our own. Although such planets are too far away to be seen directly with current telescopes, the miniscule drop in light caused by the shadow of a planet passing in front of a star is certainly measurable.
A good account and explanation of transits can be found at Fred Espenak's NASA / Goddard Space Flight Center Transits page.
I had never seen a transit before and Mercury is hard enough to see at the best of times (see here for an evening appearance on 7 April 2003) so I reckoned that a little effort was warranted to get a view of it. The transit was due to start at 6:13am BST (05:13 UT) so I hauled myself out of bed at a suitably early hour and set up my small Helios telescope in a field near the A25 between Oxted and Westerham to get a clear view to the north eastern horizon.
A layer of high cloud conspired to hide first contact and the first few minutes of the transit, but within 10 minutes the Sun appeared and remained on show for the rest of the event, a little over 5 hours. A few pictures before 7am show thin cloud across the Sun, but not enough to spoil the view - after this time the clouds were too thin to notice or had melted away completely.
This is a nice portrait of the Sun, showing Mercury silhouetted against the glowing ball of the Sun. It was taken at mid event (8:52am BST) when Mercury was at its furthest from the edge of the Sun...
The arrows show the direction of travel and mark the entry and exit points. The numbers refer to sunspot designations.
The blinding light from the Sun was reduced by a factor of 100,000 to a safe level using an Orion Neutral Density 5 white-light solar filter - do not try this at home without proper equipment!
7 May 2003 07:53UT. Minolta Dimage 7 through 80mm f/5 Helios StarTravel refractor using 25mm eyepiece; 1/256 @ effective f/4.8, ISO200
Mercury is only 12 arc seconds across, so is tiny compared to the Sun at approximately 1800 seconds (half a degree) across. Without a telescope and a suitable filter (or projecting it onto a screen) the event is completely invisible to the naked eye.
A time lapse sequence of shots at roughly 70 minute intervals shows the passage of Mercury across the face of the Sun...
North is at the top so Mercury's movement roughly corresponds to the orientation of the solar system diagram at the top of the page.
7 May 2003 07:53UT. Minolta Dimage 7 through 80mm f/5 Helios refractor using 25mm eyepiece; 1/256 @ effective f/4.8, ISO200
The following sequence tracks Mercury across the Sun at 10 minute intervals, except for a gap around at 8am BST when two shots are missing owing to battery / camera trouble! Please be patient if file is still downloading - 0.8MB. Thin cirrus cloud is evident on some of the earlier shots but by 06:30UT it clears to leave an interrupted view of the Sun for the rest of the transit.
29 shots covering an interval of 5 hrs 10 mins. Each frame lasts two seconds so this animation runs at about 300x faster than the real event. Even though to the eye Mercury appeared to creep slowly across the Sun, it was nevertheless travelling through space at about 200,000 km per hour.
Towards the middle of the transit a prominent sunspot makes its appearance to the lower left of the frames. Including the grey halo "penumbra" that surrounds it, the spot is about three times the size of Mercury, itself nearly 5000km across.
7 May 2003 05:21 to 10:29UT. Minolta Dimage 7 through 80mm f/5 Helios refractor using 25mm eyepiece; 1/181 @ effective f/9.8, ISO200. Frames reduced to 30% of original size to keep file to a manageable size.
I missed the arrival of Mercury onto the Sun because of cloud, but no such problem with the departure. The sequence that follows, heavily cropped but shown at original size, show the third and fourth contacts. Third contact is when the planet's leading edge reaches the edge of the Sun (forecast at 10:28:25 UT for London), and fourth contact is when the last trace of the planet leaves the Sun (forecast for 10:32:50)...
Mercury leaves the Sun...
The final hour of the transit while Mercury approaches the edge of the Sun. The initial frames are at 10 minute intervals, then the frames from 10:26 are at 30 second intervals to cover the departure in detail.
7 May 2003 09:30 to 10:32:30UT. Minolta Dimage 7 through 80mm f/5 Helios refractor using 25mm eyepiece; 1/181 @ effective f/9.8, ISO200.
As Mercury travels west it seems to connect with the edge of the Sun with a dark neck, the so-called "Black Drop" effect. This first appears in the 10:27:30 frame and is also seen at 10:28:00. This happens when the remaining strip of light between the Mercury and the edge of the Sun becomes too narrow to resolve in the optical setup, and makes it difficult to estimate the true moment of 3rd contact.
These close-ups give some clue to the real speed of Mercury: at a little under 5000km in diameter and travelling at 200,000 km per hour it covers its own diameter in about 1.5 minutes.
Fourth contact similarly appears early because 10:32:00 is the last frame to show any remaining trace of Mercury. By 10:32:30 the trailing edge of the planet is lost in the slightly blurred edge of the Sun, or has left it completely.
Visually in the telescope the planet appears completely black against the bright photosphere of the Sun - the brownish colour seen here is probably due to leakage of light in from surrounding pixels in the camera's image recording CCD.
The next transit of Mercury visible from the UK will be on 9 May 2016 - a long wait so I was glad to catch this one and am well pleased with the pictures obtained. For other pictures please try Space Weather Mercury Transit Gallery or the British Astronomical Association.
Rather sooner will be a transit of Venus on Tue 8 June 2004, also visible from the UK and the first since 1882. The timings are coincidentally similar to the event just happened (05:19 - 11:23 UT) so another early rise required to catch the start. However, this will be a naked eye event (with a suitable filter), so hang onto eclipse glasses a while longer! For full details see Fred Espenak's NASA / Goddard Space Flight Center Transits of Venus page.