MERCURY : Part 1



In Roman mythology, Mercury or Mercurius (or the Greek Hermes) is the swift messenger or courier to Jupiter (Zeus) and the other gods. His many followers associated him as the important god of travel, trade and prosperity. Mercury’s mother was Maia, the daughter of Atlas, and from her union with Jupiter (Zeus), gave birth to him at dawn in a cave at Mount Cyllene in the legendary and fabulous Arcadia. During his younger days, he played the lyre at midday, which he invented from a tortoise shell - ans is now placed in the sky as the northern constellation of Lyra. The strings he obtained from the innards of a cow, one of the many he had stolen from Apollo. For his bold cunning in stealing cattle from under the very nose of Apollo, Mercury was suitably rewarded. He was soon elevated into god-hood, to be and taken to Mount Olympus along with the many other classical gods.

In some of these ancient mythologies, this daring theft also made Mercury associated with thieves and robbers. This story is antithetical on a number of levels, as Apollo is named after the opposing god of truth - whom Mercury had stole. (Apollo is also the celebrated for the arts, divination, as the bearer of plagues on humanity, and is the patron of archery.) These opposing views were also expressed by the Ancient Greeks of c.450 BC, who thought that the planet Mercury, like Venus, was two separate objects. They named them Apollo and Hermes, corresponding to the respective observed evening and morning apparitions of the planet. These separate appearances of the same planet remained in legend well after it was determined these two bodies were one of the same body. The story of his birth at dawn, which he shares gloriously with Eos (Aurora), for example, reflects the apperance of the Hermes (Mercury) in the early morning sky. His divine music from the lyre during the middle of the day reflects the heavenly music of the gods, whispering gently on the airs of the breezes as he takes his unknown and invisible errands to the gods. He only appears again during the evening twilight, where he travels to the region of Pieria near Macedonia in northern Greece to rest. Thus he appears to the in the west after the setting sun.

We often portray Mercury wearing golden winged shoes or sandals (talaria), a winged helmet or cap made from wool or leather (petassos), and a suitable purse or bag to hold his messages. He also carries the golden Caduceus, the winged staff with two snakes wrapped around it. In ancient times, this represented trade and commerce, but is now commonly epitomises medical organisations and hospitals, medicines and other places that heal the sick. He was formally venerated to symbolise the crops grown by the ancient rural farmers, and was celebrated by the Romans each year on 15th May, during the festival known as the Mercuralia. He was once considered the Roman protector of the important wheat trade and had a dedicated temple to worship him near the Circus Maximus in Rome from the beginning of the 5th Century B.C.

The first observational records we have of Mercury were made by the Mayan civilisation, who recorded the positions of the planet during the morning apparition of 733 BC. The first of Mercury evening sightings were given by the Mayans in 727 AD, where they calculated that the planet would appear in the exact some position once every 6 years 8.4 days or 2 200 days. However, the planet’s existence was known well before this. Ie. Around 2 200BC, the Sumerians, later followed by the Babylonians, named one of their gods after the planet.


Aptly named Mercury is the nearest planet to the Sun, and really does show quite rapid apparent movement cross the sky as it passes through the constellations of the zodiac. Orbiting the Sun once every eight-eight (88) days, Mercury’s mean solar distance spans about 0.39 Astronomical Units (AU) or 57 million kilometres. Any daytime observer on Mercury would see a sky full of stars because of the lack of any real atmosphere. Poised somewhere in the sky during daylight hours would be the huge 1.4o solar disk - being some three times the diameter as seen from Earth. The crater covered surface is bathed in intense solar radiation whose surface is consequently exceedingly hot - enough in fact to melt lead. If you could stand on the night time surface, then looking back towards Earth would find a very bright bluish ‘star’ that during opposition would shine around -4.8 magnitude, while Venus would exceed -7.7 magnitude!


Discoverer : Prehistoric
Satellites : None
Equatorial : 4 879.4 km.
Polar : 4 879.4 km
Period (P) : 87.969 days
Synodic Period : 115.88 days
Orbital Velocity : 47.87±11.1 kms-1
Eccentricity (e) : 0.2056
Inclination (i) : 7.0o
Mean Density : 5.43 g. cm-3
Mean Distance (a) : 57.9±11.9 x106 km
Sidereal Rotation : 59.7 days
Day Length : 175.4 days
Maximum Diameter : 13 arcsec
Minimum Diameter : 4.5 arcsec
Maximum Magnitude : -1.9

For terrestrial observers, Mercury’s biggest drawback is it always remains for long periods hidden behind the solar glare. Like nearby Venus, Mercury remains an inferior planet as it lies inside the Earth’s orbit. For this reason, it never exceeds more than 28o away from the sun, and cannot stray more than 1 hour 45 minutes in right ascension either east or west of the sun. Due to Mercury’s highly eccentric orbit, such greatest elongations are mostly less favourable, and can be as small as 18o. Only for a few weeks during the time of the greatest elongations, can Mercury can be seen with the naked-eye - either in the morning or evening twilight but always close to the horizon. This short observational period can be often less than thirty minutes in duration, and this is only under ideal conditions. A further disadvantage is the inclination of the ecliptic to the horizon. If the ecliptic is inclined sharply, then the elevation of the planet is poor, and therefore Mercury becomes easily lost in the coloured glare of the atmosphere seen after the sun has set. Those living in temperate climates will find Mercury at its best during the times of the equinoxes - namely late March and late September. In the southern hemisphere, the best times are more likely when Mercury is at southerly declinations in April and October. Northern observers will find the times better when the planet has its greatest elongations and lies at northern declinations. In places like northern Europe, seeing Mercury at all can only be achieved rarely, and this is only when the atmospheric conditions are very good or even perfect, and only in spring or autumn. It is for this reason that it is alleged that the great Nicolas Copernicus, the original founder of the heliocentric view of the Solar System, never saw Mercury with his naked-eye!

Once each 116 day synodic period, the planet experiences an inferior conjunction and enlarges to some 12±1 arc seconds across. At the times of the more distant superior conjunction, this size can shrink to 4.5±1.0 arc seconds. Over this whole cycle, the telescopic appearance of Mercury shows regular phases that slowly wax and wane like the Moon. Mercury

Mercury’s high orbital eccentricity means that both the true distance and elongation change considerably between successive apparitions. As the 7o orbital ecliptic inclination is also high, the planet does not undergo transits across the Sun’s disk with every orbit. Transits may occur once every seven, eleven or forty-six years but preferentially around either the 7th May or 9th November. (See Transit of Mercury) Previous recent transits occurred on 15th November 1999 and 07th May 2003, and the last being on the 09th November 2006. The next transit will again happen until 10th May 2016 then again on 12th November 2019.


Being so close to the sun, the midday temperatures may soar as high as 450oC, or plummet in the darkness of shadows or at night to -170oC. An average daytime temperature is more like 320;oC. Mercury’s axial rotation is about 59.7 days or about two-thirds the orbital period. An average length ‘day’ between successive sunrises is roughly 175.4 days.

As Mercury has no real atmosphere to speak of, so the surface still shows the ancient scars of many impact craters and several types of lunar-like features, which are presently believed to have formed during the earliest times of the Solar System. The proximity to the Sun suggests that Mercury has been heavily bombarded by more meteors, comets and asteroids, than any other planet in the Solar System.


Mariner 10 has been the only spacecraft to approach the planet (2006), during the three flybys in February and September 1974, and again in March 1975. Here we have gained most of our meagre knowledge about Mercury. Disappointingly, this spacecraft saw only 45% of the entire surface. Planetary astronomers also detected its magnetic field that hints of a large iron core around 70% of the planetary mass. In 1994, Earth-bound radar observations have also suggested that within some craters of Mercury are ancient ice sheets near the north and south poles that have presumably never seen sunlight.

The next interplanetary visit to Mercury will be by the NASA 1.1 tonne Messenger (MErcury Surface, Space ENvironment, Geochemistry, and Ranging) spacecraft that was launched on 03rd August 2004. The final orbit around Mercury is scheduled to be achieved in March 2011, as the roundabout trip has to use gravity assists to make the correct trajectory. By this time Messenger will have orbited the Sun fifteen times, approached the Earth once (August 2005), Venus twice (24th October 2006 and June 2007), and by-pass Mercury three times (January and October 2008 and September 2009).

This dedicated mapping mission will explore the surface of Mercury for a whole year, and with investigate the planet’s magnetic field, mineral deposits geology, and composition, the unexpected very thin atmosphere, and if the polar regions have craters with ice not exposed to the solar radiation. Like Earth, it is believed Mercury has a highly dense metallic core. Investigations will explore the effects of high temperature on the surface rocks, which may have been transformed into heavier and denser metallic materials.

Another important goal is to examine Mercury’s very thin gaseous atmosphere, which is likely made from the intense solar radiation striking the surface material. It seems to be composed of only five main elements, being oxygen, hydrogen, neon, sodium and potassium; all discovered by Mariner 10. The actual process of this atmosphere production is not well understood, and maybe explained in upcoming interplanetary missions.

Another twin mission to explore Mercury is planned by the European Space Agency (ESA). Named BepiColombo and Gaia, this ion-propelled mission is scheduled for launch in August 2013 and will not reach the innermost planet until September 2019. These two quite different specialised spacecraft, are presently known as the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO), and individually weigh 500kg and 250 kg, respectively. This advanced spacecraft will investigate for a year or two the nature of Mercury surface from the polar orbits of 400x1500 and 400x12 000 kilometres. Like NASA’s Messenger, these two craft have to have there instruments carefully protected from the powerful energies and high temperatures so close to the Sun.


Telescopically, Mercury appears like a tiny featureless rosy-red disk. Like Venus, observations are best made in the daylight hours on the local meridian. Finding Mercury can be made by offsetting from the Sun’s position or using setting circles on the telescope. General precautions should be taken if either method is attempted because there is the real risk of accidentally exposing the main telescope’s optics to the Sun’s rays. Amateurs sometimes can do half phase dichotomy observations, but this requires sizeable apertures and reasonably good seeing conditions. Some have claimed to see odd-looking greyish features from time to time, but these are certainly contrast or optical effects.


Last Update : 09th November 2006

Southern Astronomical Delights © (2006)


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