Mariner 10 mosaic of Mercury

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Mercury elongations Elongations of Mercury

Mercury is the closest planet to the Sun and is named after the winged messenger of the gods because of its rapid speed across the sky. Greek astronomy placed the Earth at the centre of the universe, however, Heraclitus realised that Mercury was always so close to the Sun that it must orbit the Sun rather than the Earth. The Greeks also had two names for Mercury, Apollo when it appeared in the morning and Hermes when it appeared in the evening. Some sources indicate that they were aware that it was the same object but others believe they thought the two were different objects. Mercury orbits the Sun in 88 days. It is the fastest moving and also the most elusive of the naked eye planets. This is because it is never more than 2 1/4 hours away from the Sun so when you are trying to observe it, it is always twilight. It is best observed within a few days either side of its elongations (see above diagram). From the UK and other northern temperate latitudes, it is best observed as a 'morning star' in the Autumn (at western elongation) and as an 'evening star in the Spring (at eastern elongation). No surface detail is visible through amateur telescopes, however Mercury does exhibit phases. The majority of our detailed knowledge of Mercury comes from only one space probe, Mariner 10.

Thumbnail of the Mariner 10 space craft Mariner 10 space craft

This was launched on 3/11/1973 and reached Venus on 5/2/1974. The gravity of Venus was used to send Mariner 10 on to Mercury. Three passes were achieved (29/3/1974, 21/9/1974 and 16/3/1975) before the equipment began to fail. The last contact with the probe was on 24/3/1975).

Thumbnail of Schiaparelli Giovanni Schiaparelli

The Italian astronomer Schiaparelli made observations of Mercury in the 1880's. He thought that the rotation was directly synchronous (i.e. one rotation per orbit) and recorded various bright and dark features. He used 22 cm and 49 cm refracting telescopes for this work. In 1934, another well known planetary astronomer Antoniadi published a new map. This was made using the 83 cm refractor at Meudon in France. He named several of the features. He too thought the rotation was synchronous so when Mariner 10 sent back actual photos of the planet's surface, these maps were seen to be as inaccurate as Schiaparelli's maps. Sadly, such grandly named features of Mercury as 'solitudo Hermae trismegisti' (the wilderness of Hermes the thrice greatest) have been thrown aside. Mercury actually rotates 3 times on its axis for each two orbits of the Sun, a fact recognised by the Italian astronomer Guiseppe Colombo, so the observable features are constantly changing. The actual rotation period was not established until the mid 1960's when radar was used. The investigators looked at the differences between frequencies of radar pulses beamed at the edges of Mercury. Their results were not consistent with an 88 day rotation, but the data did fit a 58.6 day rotation period.

Thumbnail of Antoniadi's map Antoniadi's map


Mercury has a huge temperature range from about -170 degrees C at night to over 400 degrees C during the day. It is a small planet, only 40% larger than our Moon. Its high density (see next paragraph for more detail) means that it's gravity is much greater than that of the Moon, more than twice the value (0.377 as opposed to 0.165 - both values compared with Earth gravity). It is still considerably less than that of the Earth which means that it has a low escape velocity. It can therefore not hold much of an atmosphere. The atmosphere is at a pressure of about one 10 thousandth of a millibar! Its main constituent is helium, probably derived from the Solar wind. At first sight, when you compare the gravity of Mercury with that of Mars (0.377 vs. 0.379 - Earth remember = 1) it seems strange that the atmosphere of this planet is so much less than that of Mars (about 10 millibars I believe). The difference is the proximity of the Sun - the high levels of heat and radiation causes any atmosphere to be lost into space.


Mercury has a magnetic field This indicates that there is a probably a molten nickel-iron core. In order to account for the planet's high density, it has been worked out that the core must be about 3600 km in diameter, i.e. bigger than the entire Moon. The core contains about 80% of the mass of Mercury. Mercury has the distinction of having the largest core (relative to the overall size of the planet) in the Solar System. The surface value of the magnetic field is about 1% of that of the Earth and is just enough to deflect the solar wind away from the planet's surface. Nevertheless, surface radiation levels are very high owing to the proximity of the Sun. The magnetic poles have the same orientation as those on Earth. Why should this planet be so dense? When the planets were forming, the lighter elements were pushed into the depths of the Solar System by the power of the Solar wind so only the heaviest would remain to form Mercury.


The groups of features identified by Mariner 10 have been named in the following way:

Craters after people

Plains (planitia) after the names of Mercury in different languages

Valleys (valles) after radar installations

Scarps (dorsa) after famous ships of discovery

Ridges (rupae) after astronomers who have been active in observing Mercury

Caloris basin, Mercury Caloris basin 'weird terrain', Mercury Weird terrain

Mariner 10 mapped less than half of the surface since the same regions were in sunlight at each pass. Its surface is very heavily cratered and it actually looks very much like the Moon. It is coated with a layer of dust, creating a regolith of unknown depth. The craters have been named after people. The largest seen is named Beethoven and is 600 km across. The larger craters generally have flatter floors than smaller ones, as well as terraced walls. The distribution is not perfectly random, there are chains and groups of craters. Where one crater breaks into another, it is nearly always the smaller one that is the intruder. Some craters are ray craters, notably Degas and Kuiper. A crater called Chao Meng Fu has the distinction of marking the south pole. For longitude measurements, the centre of the crater Hun Kal (the words for twenty in the language of the ancient Mayan civilisation) has been declared as defining the line of the 20th meridian. The equivalent of the Greenwich meridian on Earth has thus been set exactly 20 degrees away from Hun Kal.

Brahms, Mercury The crater Brahms Degas, Mercury The ray crater Degas

Between the most heavily cratered areas, you find the inter-crater plains. These have few large structures but many craterlets in the 5 - 10 km diameter range. Although they are clearly ancient features, there is barely any cratering between the main craters on the plain. In addition to the inter-crater plains, there are the smooth plains. These resemble the smooth plains of the Moon but they have no signs of obvious volcanic action such as lava flows. It is uncertain as to whether or not they are of volcanic action.

smooth plain, Mercury Smooth plain

The inter-crater plains are unique to Mercury, as are the lobate scarps. These are cliffs that are 20 to 500 km long and up to 3 km in height. They cut through many features and displace others. The largest basin on Mercury is the Caloris basin, this is about 1500 km across and is surrounded by mountains that rise to between 2-3 km above the base level of the basin. Only half of this structure has been seen because that is all that was in sunlight when Mariner 10 visited in 1974/5. Roll on the next mission to Mercury! Directly opposite to the Caloris basin on the globe of Mercury is the 'hilly and lineated' terrain. This is also referred to as 'weird terrain'. It consists of hills, depressions and valleys that have destroyed older features. It seems logical to assume that it is linked to the formation of the Caloris basin. It is presumed that the age of the features is similar to those on the Moon i.e. about 4000 million years. It has been claimed, rather implausibly, that there could be ice in some of the craters at the poles.


The orbit of Mercury is very eccentric, varying between 0.308 and 0.467 AU (46 to 70 million km). This causes the elongation distance to vary considerably, from just under 18 degrees if it is at perihelion to a maximum of almost 28 degrees if the elongation is when Mercury is at aphelion. Famously, the behaviour of the orbit of Mercury has created the search for a new planet and has provided evidence to support Einstein's General Theory of Relativity. The long axis of the orbit of Mercury precesses (slowly changes its orientation in space) by about 10 minutes of arc per century. In 1845, French astronomer Urbain Le Verrier was investigating the gravitational effects that the planets had on each other. He could account for almost all of this precession when he took into account the gravitational effects of the Earth and Venus. His conclusion was that there was another planet between Mercury and the Sun. He correctly deduced that this planet should transit the Sun every so often, appearing as a small black spot against the disc of the Sun, and would orbit in 23 days. On searching records of Solar observations, he found many unexplained black spots and worked out the date of the next transit as March 22nd, 1877. He even named the planet 'Vulcan'. The date for the transit of Vulcan dawned, but nothing was seen. We now realise that the black spots on the observations that Le Verrier used were simply poor observations. Nevertheless, Le Verrier discovered Neptune in 1846 using exactly the same methods. The explanation arrived when Einstein used General Relativity to calculate the expected value of precession, it agreed with the observed value very well indeed. Le Verrier was therefore partially correct in realising that the high value of precession depended on gravitational disturbances but used the wrong body and a too simple theory of gravitation. Newton's theories only really hold true when the gravitational forces are comparable to those on Earth. Further evidence to support Einstein's theory has come from radar studies of the planet.

transit of Mercury Transit of Mercury

If its orbit were in the plane of the Solar system, Mercury would transit the Sun once each synodic period. The plane of Mercury's orbit is actually inclined at 7 degrees to the plane of the Solar System so it generally passes above or below the Sun, as seen from Earth. A transit will occur when Mercury is at or near one of its nodes at inferior conjunction. The nodes correspond to the position of Earth on November 9th and May 7th so if inferior conjunction occurs at this time, a transit will be seen. The November transits are more common and occur every 7, 13 and 46 years whilst May transits occur at 13 and 46 year intervals. The next transit will occur on May 7th 2003, followed by one on Nov 8th 2006 and then again on May 9th 2016.


Diameter 4878 km
Sidereal period (day length) 58.65 Earth days
Inclination of axis 0 degrees
Density (water = 1) 5.43
Escape Velocity 4.25 km per sec
Surface gravity (Earth = 1) 0.377

Resources I used (some are getting a bit old now!):

Philip's Atlas of the Universe - Patrick Moore. This is an absolutely superb book, one of the best 25 investments that I have ever made.

The Planets - Peter Francis

Norton's Star Atlas

Astronomy Now magazines

Hutchinson's Splendour of the Heavens

Astronomy - Fred Hoyle

The Amateur Astronomer - Patrick Moore

Other resources (that I did not use but look potentially good!):

The Planets - David McNab, James Younger

Teach Yourself the Planets - David A. Rothery

Collins Pocket Guides: Stars and Planets - Ian Ridpath, Wil Tirion (Illustrator)



Elongations diagram drawn using MS Word

Mariner 10 picture scanned from a picture cut from a calendar about 10 years ago!

Schiaparelli portrait from Hutchinson's Splendour of the Heavens

Schiaparelli's map from The Amateur Astronomer 7th edition (Patrick Moore)

Images of Mercury's surface from Mariner 10 (NASA)

Transit of Mercury taken in 1914 by the Royal Greenwich Observatory