Favourable Month To Watch For Mercury

Press, Volume CVIII, Issue 31772, 31 August 1968, Page 19

Favourable Month To Watch For Mercury

E 1 Although the planet Mercury shines as brightly as some of the brightest stars, very few people ever notice this planet. The reason is that Mercury is only above the horizon during the evening or morning twilight, or when the sun is actually shining. Even when it is furthest from the sun, Mercury will set or rise \ within two hours of the sun. \

September is a particularly favourable time to watch for Mercury in the western sky, because on September 20 it will reach its greatest elongation east of the sun. It is then setting some two hours after the sun.

At 7 p.m. on September 20, it will be very close to the bright star. Spica, in the constellation Virgo, the Virgin. The bright planet, Venus, will also be close by and the three objects will make a brilliant sight in the evening sky. The time required for a planet to make one complete revolution around the Sun is called its sidereal period. For Mercury this period is 88 days. Since Mercury is very much closer to the Sun than the Earth, it moves a great deal faster to overcome the extra gravitational pull which the Sun exerts upon it Whereas the Earth moves at an aver- • age speed of 18j miles a I second, Mercury travels at 30 miles a second. As a result Mercury overtakes the Earth at fairly regular intervals, averaging 116 days. Moon-like Phases Mercury shows phases similar to those of the Moon. When furthest from us it is full and when it is nearest it shows its dark side to the Earth. When it is at greatest elongation Mercury shows through the telescope as a half moon. Because of its I closeness to the Sun and its | rapid motion the planet j appears to swing rapidly back I and forth on either side of the Sun.

On the accompanying figure if Mercury is at Ml and the Earth at El, then the planet is at inferior conjunction and is invisible to us because its dark side is turned towards us.

By the time Mercury has moved to M2, the Earth has only moved to E2 so that the planet is at its greatest western elongation, when it is best seen in the morning sky before sunrise. By the time the Earth reaches the position marked as E 3, Mercury is at M 3 and is again invisible because it is hidden ■by the glare of the Sun which then lies between us and the planet This is called superior conjunction. When the Earth is at E 4, Mercury has reached M 4 and is then at its greatest elongation east of the Sim and is visible in the evening sky after sunset as will be the case this

month. Shortly after this Mercury overtakes the Earth and again inferior conjunction takes place. The mean distance of Mercury from the Sun is 36 million miles. However, because it revolves in an eccentric orbit, it can approach as close as 28,600,000 miles to the Sun, or be as far away as 43,400,000 miles. Until recent years it was always thought that Mercury rotated on its axis in the same period as it takes to go once around the Sun, viz 88 days. In fact this value is still given in quite a number of books. This idea arose because in 1889 Schiaparelli detected dark markings on the surface, which appeared to remain stationary with reference to the terminator. Thus he concluded that the planet always turned the

same face to the Sun, much as the Moon keeps one side always turned to the Earth.

Radar Observations

In 1965 radar observations indicated that the correct period for the rotation of Mercury was 59 days. Theoretical studies had also indicated that such a period, which is two-thirds of the orbital period, should be correct.

French astronomers had secured a large number of drawings and photographs of Mercury, which had been made with large refracting telescopes at high altitude observatories under ideal conditions. Following the radar observations these results were studied in detail and it was confirmed that the period given by the radar observations agreed with the rotation period determined from this data. To check their results, the French astronomers next went back to a long series of classical visual observations made by the famous astronomer, E. M. Antonladi. From these observations they made of the various features and com-

pared them with the map made from their own observations. Again the period of almost 59 days held true. Dense Planet The equatorial diameter of Mercury is 3100 miles. It is difficult to determine its mass, because the planet has no satellites, but the generally accepted value is 1/19 that of the Earth’s. This value will probably be refined since the recent passage of the minor planet, Icarus, has provided a unique opportunity of determining the mass of Mercury. If the present value is correct then the density of Mercury is 5.6 compared with water. This makes it slightly denser than the Earth and it is thus the densest planet known. It is possible that Mercury has an extremely tenuous atmosphere. Its low mass and velocity of escape combined with its high temperature makes it probable that any atmosphere retained by Mercury would be extremely thin. Radio observations indicate that the dark side of Mercury may not be as cold as had been thought This would indicate that a small amount of atmosphere exists • through the medium of which heat is transported from the sunlit side to the dark side. Polarisation measurements also indicate that there is at least some atmosphere present It is certainly very thin because when the planet transits across the face of the Sun no atmosphere has been detected. By contrast with the dark side, ttie sunlit side of Mercury receives about seven times as much heat and light per unit area as the Earth. In the absence of clouds this would heat the surface to a very high temperature. It is probable that the point directly beneath the Sun attains a temperature of around 327 degress Centigrade—the melting point of lead.

the Sup and the Earth. At such times it transits across the surface of the Sun against which it is projected as a small, round dark spot. If the planet and the Earth went around the Sun in the same plane such transits would occur at every inferior conjunction. However the orbit of Mercury is inclined at an angle of seven degrees to the ecliptic. Thus it will only pass through a transit when inferior conjunction takes place on the line along which the planes of the two orbits intersect, which is known as the line of the nodes. The Earth crosses this line twice each year, on either May 8 or 9,’and again on November 10 or 11. A transit of Mercury can only occur when its inferior conjunction falls within a few days of either of these dates. There are an average of thirteen such transits each century. The last

took place on November 6, 1960, and the next ones occur on May 9, 1970, and on November 9, 1973. The early Egyptian and Greek astronomers failed to realise that it was the same body which alternatively appeared as a morning and evening star. The Egyptians gave the names of Set and Horus to what they thought were separate planets, whilst the Greeks called them Apollo and Mercury. Later when it was proved that it was the one and same planet which appeared alternatively on either side of the Sun, Mercury was called the winged messenger of the Gods because of its rapid motion. Whether this speed had anything to do with Mercury becoming the god of traders, doctors and thieves I don’t know. At least its speed and elusiveness are suggestive.

Under the best seeing conditions and with large instruments, bright and dark spots have been detected on the surface of Mercury, both visually and photographically. Such markings appear to be permanent because they do not vary in size and they also appear to rotate with the planet as it turns on its axis. It is generally thought that the surface of Mercury is very similar to that of the Moon and that the two bodies are physically very much alike. At irregular intervals Mercury passes directly between