The Planet Mercury




      Almost 5 billion years ago, the actual solar system was only a cloud made of gas (mostly hydrogen and helium) and very diffuses dust grains (carbon and silicate). When the hydrogen started burning, and the protostar was born.  Around the protostar the material that has not been incorporated in the star rotates around it, forming a disk. During this phase, that is called disk phase and that can last up to 100 millions years, the grains of dust grow in size very rapidly (this phenomenon being called accretion). After a relatively short period (100000 years) the grains will have grown into objects of some Km of diameters called planetesimals. These planetesimals have a composition that depends on the region where they have formed: if in the inner parts the temperatures are higher and gas sublimate, leaving rocky planetesimals, in the outer parts of the disk we can instead find ices. This is the birth of the terrestrial planets as Mercury and Earth.   

      At this point of the evolution, a period of heavy bombardment is thought to have take n place (about from 4 to 3.5 millions years ago). At this phase, planets were already formed and planetesimals had time to grow into very big objects (the size of the Moon) that can have very violent impacts with the planets. Most of the craters today seen on the Mercury and on satellites without atmosphere are due to that period.


    Some of the researches in last years are indicative of the fact that the eccentricity of the Mercury increases. If it is true, in a near pass of the planet to Venus, it is possible Mercury to be thrown of the Solar system by the gravity field. It could happen in a billion years, but the probability is 1/1000. Otherwise , Mercury’s destiny depends on the Sun. At the end of its life, the Sun will grow into a red giant. Then it will become a white dwarf, surrounded by a planetary nebula. Along the way, the Sun's core will contract and grow hotter, while its outer layers expand and cool. The Sun will grow to many times its current size and it will swallow the nearest planet Mercury. Then it will swallow also Venus and Earth. In other words, it will become a red giant - an old, bloated star that is rapidly nearing the end of its life.


  • Mercury’s orbit is highly eccentric (e= 0,206); at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million km. The perihelion of its orbit precesses around the Sun at a very slow rate. The incidence of the orbit to the ecliptic is 7º, 0 but the distance between the Sun and Mercury is 0,39AU (57 910 000), which is 2,5 times shorter than the distance between the Sun and the Earth is.

  • Mercury makes a tour on its orbit for 88 earth days with average speed of 54 km/s (almost 2 times bigger than Earth’s speed). But Mercury turns very slow on its axis. Its axis turning takes 59 days, which is 2/3 from its path round the Sun.

  • Mercury’s diameter is 4880 km (2,6 times shorter than the Earth’s diameter). Its visual diameter is 13". Its stellar quantity is between –1,2 m and 1,1 m.

  • Mercury’s mass is 3,30x10 23 kg (18 times less than the Earth’s mass) and its density is 5,43 gm/cm 3 , which means that Mercury is much denser than the Moon is (5,43 gm/cm 3 for the Moon). But for the Earth, Mercury is the densest major body in the Solar system. In fact, Earth’s density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth.

  • Temperature variations on Mercury are the most extreme in Solar system ranging from 90K (nightly) to 700K (daily). For instance, the temperature on Venus is slightly hotter but very stable.

  • Mercury actually has a very thin atmosphere consisting of atom blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. (These in contrast to the Earth and Venus, whose atmospheres are stable, Mercury’s atmosphere is constantly being replenished.)

  • Mercury is the only body in the Solar system known to have an orbital/rotational resonance with a ratio other than 1:1(though many have no resonance at all). This fact and the high eccentricity of mercury’s orbit would produce very strange effects for an observer on Mercury’s surface.

  • At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving 3 times faster across the sky. Observers at other points on Mercury’s surface would see different but equally bizarre motions.

  • From Mercury’s density we can conclude that Mercury’s dense iron core is relatively larger than Earth’s, probably comprises the majority of the planet. Mercury therefore has only a relatively thin silicate mantle and crust.

  • A large iron core whose radius is 1800 to 1900 km dominates Mercury’s interior. The silicate outer shell (analogous to Earth’s mantle and crust) is only 500 to 600 thick. At least some of the core is probably molten.

  • The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high. Some cut thru the rings of craters and other features and such a way as to indicate that they were formed by compressions. It is estimated that the surface area of Mercury shrank by about 0,1% (or a decrease of about 1 km in the planets radius).

  • One of the largest features of Mercury’s surface is the Caloris Basin; it is about 1300 km in diameter. It is thought to be similar to the large basins on the Moon. Like the lunar basins, it was probably caused by a very large impact early in the history of the Solar system. That impact was probably also responsible for the odd terrain on the exact opposite side of the planet.

  • In the addition to the heavily cratered terrain, Mercury also has region of relatively smooth planes. Some may be the result of ancient volcanic activity but some may be the result of deposition of ejecta from cratering impacts.

  • A reanalysis of the Mariner -10 data provides some preliminary evidence of recent volcanism of Mercury. But more data will be needed for confirmation.

  • Amazingly, radar observation of Mercury’s north pole (a region not mapped by Mariner-10) show evidence of water ice in the protected shadows of some craters.

  • Mercury has a small magnetic field whose strength is about 1% of Earth’s.

  • Mercury has no known satellites.

  • Mercury is often visible with binoculars or even the unaided eye, but it is always very near the Sun and difficult to see in the twilight sky.

Researches of Mercury

            From time immemorial observing the night sky has gone along with the development of the humanity. People from the earliest civilizations have watched the sunrises and the sunsets, the Moon and the stars and they have been trying to measure the time by themselves. Astronomical observations were done in Egypt, Central America, England (Stonehenge) in 4000 BC. The first astronomical records in Egypt, Babylon and China exist from about 3000 BC. Even then it was noticed that there are 5 objects looking like stars on the night sky, which moves in sophisticated way not resembling the motion of the other stars. These were the well–known nowadays planets visible with the naked eye (Mercury, Venus, Mars, Jupiter and Saturn). The nearest planet to the Sun–Mercury focused attention of the ancient people with its fast apparent motion. Therefore, it was connected with the Greek god Chermes – the herald of the gods, the protector of the travelers, robbers and orators. Hermes conformed to Mercury in the Roman mythology. In ancient Babylon the influence of the planet Mercury was connected to the God of the wisdom–Nabu. In the mythology of the ancient Egypt it existed in an idol shown as two animal images–ibis and baboon. That was god Tot which was identified with Hermes by the Greeks. These are the first evidences for the large interest in the planet Mercury honored as an idol.

Scientist contributed to the researches of Mercury.

  •             Pier Gassendi (1592 – 1655) – was a French philosopher , astronomer, mathematician and technician. On 7 th November 1631 he observes transit of a planet (Mercury) in front of the Sun’s disk, which is calculated previously from Kepler in 1629.

  •             Christian Huygens (1629 – 1695) – was Netherlandish physician, technician, mathematician and astronomer. In 1659 he made the first measures of the angular diameters of the planets.

  •             James Gregory (1638 – 1675) – a Scottish mathematician and astronomer. He developed method for calculating of the Sun’s parallax from the observations of the passage of Mercury and Venus in front of the Sun’s disk.

  •             Edmond Halley (1656 – 1742) – was an English astronomer, member of London’s Royal Association. In 1677 he observed transit of Mercury and define the decision Earth–Sun and Mercury–Sun.

  •              U rben Jan Jac Le Verrier (1811–1877) was a French astronomer, member of the Paris academy of science. He made continued researches about the motion of Mercury and in 1859 he concluded that the velocity of the moving of the perihelion of the planet has a value of 38” per century.

  •             Giovanni Sciaparelli (1835–1910) was an Italian astronomer. At the end of 19 th century he published reports in “Scientific American” magazine. It was mentioned there that there were many spots and lines identical with the mars’ channels. In the report it was specified that these are thin and long lines stretched in different directions.

  •             Herman Vogel (1841–1907) was a German astronomer, member of the Berlin academy of science. He made a lot of spectral observations of all planets from Mercury to Neptune.

  •             Bernard Lio (1897– 1 9 52) was a French astronomer, member of the Paris academy of science. The first time he did polarimetric measures of the planets is the period 1921–1929. His purpose was to receive physical characteristics of the surface layers and the atmospheres of the planets by comparison between the linear polarization of the reflected and the scattered sunlight with the polarization made by earth’s patterns. This showed the surface of the Moon, Mars and Mercury are closed by their polarizating properties to the earth's volcanic types.

  •             Albert Einstein (1879–1955) was a German physician– theorist. He was an inventor of the General Theory of the relativity explaining the anomalistic movement of Mercury.

  •             Gerald Clemance (1908–1974) was an American astronomer, member of the National academy of science. He made detailed analysis of Mercury and Mars motions. Based on a research about the movement of Mercury’s perihelion he confirms the importance of applying Einstein’s theory of relativity in the theoretical astronomy.

  •             Ralph Dies and Gordon Pettingill – were American astronomers who carried out radio-locating observations of Mercury by the radio telescope Aresibo in 1965 and fixed a period of rotation of Mercury – 59 d.

  •             Richard Baum – director of the sections for Mercury and Venus to the Britain Astronomy Association. In 1990 he wrote about multiple registered changes of the brightness of Mercury and about the growing and disappearing ice caps during the rotation of the planet. This statement was rejected by NASA with the argument that Mariner–10 did not register any ice, but in 1991 by radar scanning, radio astronomers identified large bright spot sized 300x600 km on the north pole of Mercury, reflecting the radio signal analogical to the reflection of the ice.

During its flight Mariner-10 (1973 - 1974) made many photos. On them can be seen that the surface of the planet is covered with the same sort rubble layers and with the same craters like those on the Moon. The detailed view shows that there are differences on the surface of the two objects. The scattered craters on Mercury's surface are more than these on the Moon and they are often grouped or arranged in à line. One, indirect as it is, but eloquent confirmation for subsisting of iron nucleus in the planet, contains in the finding made by Mariner-10 that Mercury has dipole magnetic field whose axis is inclining to 12 degrees to the axis of the rotation. Another important finding made on board the spaceship was that Mercury is enveloped by thin atmosphere containing mostly helium with little neon and argon. Actually, this thin layer is supplied by the helium radiated from the Sun, which is attracted from the Mercury's magnetic field. Unfortunately, it has been the only mission to Mercury until now.

            Despite the many researches and the apparent vicinity to the Earth, the planet Mercury is steel not explored in details. Its dynamic changing nature has not yet been explored, and many of the theories have neither been improved, nor denied yet. The future space drills, which are planned to fly near the planet in the period of 2004– 2013 will have to answer the many questions, which have arisen, concerning Mercury. Such a spacecraft will be Messenger. It will be launched in March 2004 and will enter Mercury orbit in September 2009. The mission will take two flybys and then go into orbit around the planet. Clad with miniaturized instruments, the probe aims to unravel the mysteries of Mercury's high density, composition and structure of its crust, and whether its surface is shaped by volcanism. Other areas of study include: tectonic history, the thin atmosphere and miniature magnetosphere, and the polar caps.


The Perihelionof Mercury

            The astronomers from 17 th and 18 th century founded that their observations were not successfully describe Mercury’s motion. Even in the beginning of 19 th century, when it seems to be possible the creation of better theory, the motion of this planet has diverted from the predicted motion. In 1859 Urben Jan Jac Le Verrier (1811 – 1877) noted that Mercury do not move exact on the orbit. To resolve the problem, some astronomers supposed that there exist unknown planet between Mercury and Sun. They call it Volcano and started its searching, but it was unsuccessful. Others suggested zodiacal light . But this hypothesis was also denied. According to the classic theory (Newton, Kopernik) an orbit of a planet have to be a perfect ellipse and the Sun must be in one of its focuses. However, in the Solar system there are also other planets except Mercury. These planets also attract Mercury, although it is barely, which leads to insignificant diversions to its orbit from the perfect ellipse. This diversion is called perturbation of Mercury’s orbit.  Using the Newton’s gravitation law, the astronomers could calculate the exact value of this perturbation. But the observed velocity of the orbital rotation turned out noticeably higher than the predicted. The position where Mercury is closest to the Sun is called perihelion. Observing from the Earth, this position takes certain place on the sky. Mercury’s orbit moves slowly, that is why the position of the perihelion is also changing. This effect is so slight that the perihelion turns to only 1° 33’ 37” for a hundred years. Newton’s theory can explain only a turning of 1° 32’ 37” for a century. There is an additional movement of 42” for a century which cannot be explained with the effects of the Newton’s classic theory.

            In 1916 Albert Einstein proposed absolutely new theory about the gravity, named General Theory of Relativity. According to this theory the gravitational field of the subject is impressed in space time. As stronger the gravitation field is, the bigger is the deformation of space time. The fragments and the light rays travel through the shortest lines in such a curved space time called geodesic. After Einstein succeeded to find the equation of the gravity field in the general theory of the relativity he, of course, decided to apply his new theory into practical example. Einstein began with the equations of the gravity field in empty space. Solving these equations he found how the space time around the Sun is curved. Knowing the geometry of the space time Einstein started to solve the geodesic equation. In this way he wanted to find out how the planets move in such a curved space time.  And it was not ellipse. In the General Theory of Relativity, the orbit of each planet around the Sun is not just an ellipse, but slowly turning ellipse. Such an orbit turns on its own, even without influences of the other planets. The revolving ellipse is the shortest line in the curved space time near the Sun. Evaluating the velocity of the predicted by Einstein turnings of the elliptic orbits of the planets in the Solar system, he found out that such an effect should really be discovered for Mercury. Putting the distance Mercury–Sun in the equation, he got for the velocity of the precession the exact 43” for a century – exactly this quantity which have not been explained by Newton’s theory. The explanation of the turning of the perihelion of Mercury became a great success for the General Theory of Relativity.

The Resonance motion of Mercury

            Mercury is the closest planet to the Sun and that is why the influence of the Sun's tides should be most obvious. The calculations show that the Sun's tides are able to delay Mercury's rotating a few times for a period of a billon years.

            Actually, the daily rotation of Mercury happens very slowly: the planet makes a circle around its axis (stellar day) for 58,646 d and in the same time it turns around the Sun for 87,969 d. It means that Mercury rotates three times around its axis for the time it orbits two times around the Sun. This motion is called resonance in ratio 3/2. The explanation of this phenomenon is in the fact that Mercury's orbit has relatively high eccentricity (e=0.206), which takes to significant differences in the tidal forces. They are 3.5 times bigger in perihelion than in aphelion. This results in an increase of the velocity in the perihelion to 1.5 times in comparison with the average orbital velocity. Therefore, the velocity of the daily rotation gets close to the orbital which is also highest here. All this is the reason for this strange resonance 3/2.

Transits of Mercury

What is a Transit?

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.

What can we observe visual and photographic?

  • Fixing contacts . By the passage of Mercury on Sun’s disc, there are four contacts we can fix. That is because Mercury is not a spot object but it has the form of a disk so there are two couples of contacts with the first and the second cross of Mercury with the Sun’s limb. Measuring the first contact is most difficult, because we do not know the exact place on the Sun’s disk where Mercury will appear. At the second and third contacts there exist an effect known as black drop effect and because of it the moments of fixing these contacts are when the effect finishes. Using this data we can calculate practically the parallax of the Sun comparing our moments of the contacts with these of distant observers. This gives us the distance between the Sun and the Earth.  (for more information about our works and results about that click here)

  • Observing Mercury out of the Sun’s limb . It is possible Mercury to be seen out of the Sun’s disc as a silhouette on the background of inner crown or the chromo sphere before the first contact or after the last contact.

  • Bright and dark orioles around Mercury during the transit . There is no atmosphere on Mercury which could provoke such effect that is why these visual effects are due to the contrast between the Sun and Mercury's silhouette.

  • The black drop effect . It happens when the two limbs– these of Mercury and of Sun– are too close one to another. The reason for this is the atmosphere scintillations– this is the same effect like the temperature waves come out of hot pavement in summer. Because of the invisibleness of Mercury's atmosphere this effect cannot be result of it. This effect is due to the diffraction which always descant the limbs of mercury and Sun creating false connection between the two limbs when they are actually separated. The black drop effect also can be seen on photographs which can be used for detecting the photometrical characteristics of this phenomenon. 

How can we observe it visual and photographic?

            Photographing Mercury's transit is the same as photographing the Sun in normal conditions. But when we photograph the Sun we should be very careful because it can damage our vision and the shutters of the cameras. The expositions usually are between 1/125s and 1/1000s that is why it is not necessarily to have equatorial head. The main difficulty is the excesses of light which have to be removed. This can be achieved by the following ways:

  • using filters for objective or eye–lens
  • Decreasing the aperture of the instrument
  • Using Hershel's pentagonal prism serving to divert 98% of the sunlight out of the telescope

            Most appropriate way for the dilletants is using neutral filters passing 0, 1% placed on the objective of the telescope. These filters usually are very thin and do not lead to serious deformations of the image. Eye–lens filters can be used in combination with Hershel's prism. In this way using eye–lens with different extensions we can change the expositions which can help us get correctly expounded photographs. Using the decreasing of the aperture of the telescope shrinks its resolution. It is recommended for the camera to have expositional times to 1/1000s and frosted glass for viewing. The possible films for photographing the transit are color and black–and–white with sensitivity 50, 100 or 200 ASA. We have to know that the black–and–white films give us better contrast.

  • We should mark the moments of photographing with precision 0, 1s. We also should know the coordinates of the place of photographing with precision 0, 2’.

  • We should avoid the concrete, brick, asphalt, rocky soil because it radiates heats and they are a source of large local turbulence.

What science information can we get ?

            In the past, the transits of Mercury was used for finding of more exact data about the distance to the Sun and detecting the orbital element of Mercury. Nowadays this is mostly phenomenal event, because of its rarity. Scientific researches using transits could be made only for measuring of the exact Sun diameter as it is suggested that it varies.

The transit on 7 may 2003

Astronomy Observatory “Jordano Bruno”, Dimitrovgrad, Bulgaria

            As members in the out school course of astronomy our team observed the Transit of Mercury visual and photographical. Because of our age only our leader Dimitar Kokotanekov was seen a transit in 1986.

            We worked in 3 directions.

  1. Fixing contacts and making photographs  for measuring the parallax of the Sun at time planed in advance (with educational purpose). We have connected with people on our age from India and Russia who are fond of astronomy and we planned to change data, but because of organizational problems we are still in process.

  2. Making photographs of the moments to register the “black drop” effect and use it for exhibition.

  3. Watching the transit on screen attached on telescope, visual observations and amateur photographical observations with educational purpose.


  • Moments of the contacts

  • Photos

  • Measuring the distance to the Sun (parallax)

Used Literature:

      Internet:                                                                                                                                         Books:


Working Group: Bilyana Hristova     

Vladimir Palankov

Iliya Valev     

Leader: Dimitar Kokotanekov


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Last modified: 28 October 2003