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Elusive Mercury - By Nick Kollerstrom

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Fact File:

Diameter: 4850 km

Distance to Sun: 0.4 AU

Solar Orbit: 88 days

No of Moons: 2


Imagine living on a planet where all the stars go round three times, every day. Mercury has a very long day, longer than any other planet in the solar system. This tiny planet revolves hundreds of times more slowly than Jupiter or Saturn, the giants. Mercury can appear to dart around the Sun 'like a demented bee' in Bernard Eccles' fine phrase - he was alluding to one of Joachim Schultz's diagrams (its motion during 1961 is here shown) (1). At dusk or dawn, we only get to observe the tips of these erratic curves (or, many of us never see it at all!), but if one could see it when close to the Sun then this is what its path would look like to us. Three times each year at morning and evening we get a chance to see Mercury, as you can see from the diagram, over one year. The Sun stands at the centre.



In his 1950s sci-fi classic I, Robot, Isaac Asimov had a bunch of robots going to Mercury. His robots saw the 'terminator', the line between day and night, fixed in the ground. Everyone then believed that one side of Mercury faced ever sunwards, i.e. that it had a 'phase-locked' rotation in synch with its orbit. After all, the enormous pull of the Sun on tiny Mercury while it was still molten, long ago, ensured this, didn't it? Astronomers observing Mercury during the brief periods when they could see it, noted that the same side seemed to be pointing Earthwards. Ha, Mercury had them well foxed.



More recently we learned that Mercury had what I call its rock‑and‑roll sunrise, whereby the Sun rises over Mercury, stops, sinks down again below the horizon, and then again rises and moves against the sky. The 'terminator', the line separating day and night, reverses direction owing to the fierce accelerations to which Mercury is subject. So Mercury did have a day, after all, but it was rather blending and blurring together with its year.


Mercury moved to an elegant music of its own, never suspected by astronomers, waltzing round the Sun to an elegant 1-2-3 rhythm. For one of its days, it has two axial rotations and three years (i.e. rotations round the Sun):


Mercury's day is 176 days (the interval between sunrises)

It spins in space once per 58.65 days

It goes round the Sun once in 87.97 days (i.e., its year)


So, while Mercury apparently buzzes all over the place (Figure 1), in fact it has a beautiful rhythm to its motions. It isn't phase-locked facing sunwards, as Luna is towards Earth - nor on the other hand does it have an independent rotation, but something curiously in-between. The same part of Mercury faces sunward every alternate perihelion (from peri- near and helios- Sun, Greek) . An observer on Mercury could have a problem measuring time, because he would see the Sun go round once in the sky, while the stars went round thrice!  These are subtle and elusive matters. The Venus day, for comparison, is phase-locked into a pleasantly rhythmical relationship with the Earth, not the Sun.


Earth's period of spinning in space, i.e. its axial rotation, is more or less the same as its day, so we don't normally have to bother about the difference. They differ by only a few minutes. But for Mercury and Venus they are totally different. Astronomers had erroneously believed for nearly a century that Mercury had no day of its own distinct from its 'year', mainly it seems because they noted that the same surface markings returned to the sunlit side every two orbital revolutions.  Thus, astronomers could have been fooled, because looking at Mercury after two of its orbital periods they would see the same markings on the sunlit side and would find no disagreement with the 88 day period ...Thus, half the telescopic observations that led to the 88-day interval were actually correct, but many of the conflicting observations were ignored or missed (2). What new tricks, we can only wonder, will Mercury have up its sleeve?



This diagram by Mr Keith Critchlow shows the retrograde loops of Mercury that appear in a geocentric perspective - although, of course, we never get to see them (3). We can see how there are roughly three synodic cycles of Mercury in a year, when it comes nearest to Earth. These loops are called 'inferior conjunctions' with the Sun. Transits of Mercury may then take place (the alignment is Sun-Mercury-Earth).


For the early astronomers, the errors of Mercury's position were huge. Copernicus may never have seen it, reporting no observations of his own for it, and remarked: 'This star tormented me, with its many twistings and toilings, in trying to explore its motions' (4). Kepler at the start of his career confessed: 'Certainly this is the one planet which most of all disgraces the reputation of astrologers, and confounds the whole theory of things on high' (5). In Galileo's time, the tables of Mercury available could err by ten degrees. In the horoscopes he prepared, it was often out by two or three degrees (6).


Then, in the 19th century, a new problem arose, as the plane of Mercury's orbit was found to keep shifting from where it should have been. This was called, 'precession' and it was an anomaly which defied the principles of Newton's gravity theory.  No-one could account for it until Einstein's rather elusive and inscrutable relativity theory came along.


For some pleasant harmonies concerning Mercury's motion, see 'Mercury's Three Halos' and 'Earth and Mercury'

Mercury goes retrograde for three weeks at a time (shown by the loops in the above Keith Critchlow diagram) and for 2006, these periods are:

2 - 25 March,    4 - 28 July,  and    28 Oct - 18 November

Have you ever seen Mercury? Try to spot it at dusk, during Feb 15 - 2 March 2006, and then May 31 to 20 June.


(1) - Schultz, p.144, re-drawn by Martineau, 2001 p.6.

(2) - Lang & Whitney, 1991, p.63

(3) - Critchlow, 1979, p.166

(4) - Copernicus, Revolutions, Book V, Ch 30.

(5) - Kepler, Mysteriunm cosmographicum Trans by A Duncan, NY 1981 p191

(6) -