Astronomical Notes

Press, Volume CXII, Issue 33062, 1 November 1972, Page 21

Astronomical Notes

by

F. M. BATESON

Daylight occultation of Mercury

There will be a daylight occultation of Mercury on November 8. Disappearance, behind the Moon, will occur between 4.45 p.m. and 5.10 p.m., the precise time being dependent on the exact geographical location of the observer. Reappearance occurs just over an hour later.

A telescope will be necessary to see this occultation because it takes place well before sunset. In addition the Moon will be only two days old and therefore a very slender crescent hard to see against the sunlit sky.

lhese circumstances will rob this occultation of the general interest that is attracted when a bright planet is occulted. Such

events, when they take place at the dark limb of the Moon, are startling as the bright planet is suddenly blotted out.

For the first fortnight of November Mercury • continues to be favourably placed, reaching its greatest elongation east of the sun on November 5. It can be found in the western sky in the constellation Scorpius, where for a night on either side of November 8 it passes close to the bright star Antares. It moves rapidly across the sky and the changes in its position, relative to the | stars, can be noticed from ;one night to the next. It sets close to 9.15 p.m. on November 1 and 15, after which it moves rapidly to inferior conjunction on November 26. Mercury is only visible to the naked eye when it is near

its maximum elongation from the Sun. This is never more than 28 degrees so that it is only visible within two hours of sunset or sunrise. Thus it is always low in the sky when it can be visible to the unaided eye. This is why it is often termed the “elusive” planet. Its low altitude requires a clear horizon because haze or mist will hide it, in spite of its shining like a first magnitude star. Maps of Mercury The first maps of Mercury were made by G. V. Schiaparelli, who recorded a number of dark, well-defined streaks. He concluded, from his observations extending over many years, that Mercury rotated on its axis in the same period of 88 days in which it revolved round the Sun. Lated, the great American observer, E. E. Barnard sketched vague surface details, which he likened to lunar maria. His maps were not in very good agreement with those of Schiaparelli, but Barnard also considered that the rotation period was 88 days. This period was confirmed by the French observers, E. M. Antoniadi in 1940, and A. Dollfus in 1950. Radar observations have proved that the correct rotation period is 58.7 days. This is two-thirds of the revolution period. Originally Mercury probably had a much faster rotation. Solar tidal friction lengthened this until it reached its present period in resonance with the period of revolution.

A parallel case is the Moon on which the Earth’s tidal forces acted to slow its rotation period until it was equal to that of its revolution. In the same manner the solar tidal forces slowed Mercury’s rotation until it became “locked on” at twothirds of its period of revolution. . To understand how the highly skilled visual observers determined a period of 88 days for Mercury’s rotation it is necessary to remember that all their observations were made from the northern hemisphere. There not all elongations of the planet are favourable. Every 116 days Mercury will be at its greatest elongation east of the Sun. Western elongations occur roughly half-way between the eastern elongations. The result is that in a year there are six elongations; three eastern and three western. However only one elongation from each set is favourable for observation for northern observers. Observers tended to confine their observations to the favourable eastern elongation, which occurs in the northern spring, and the best Western elongation in the northern autumn. These are separated by 176 days from each other. This is exactly twice the 88 day period. It also happens to be exactly three times the rotation period of 58.7 days. Any observations made at 176 day intervals would not reveal whether Mercury rotated in 88 or 58.7 days. A careful study of the maps from the ob- : servers mentioned has produced a composite chart which agrees with the result of the radar measurements. . No atmosphere Occasionally faint, white clouds have been suspected to obscure the denser dark markings on the surface. A I very slight excess in the polarisation of light from the cusps has also been reported. These observations suggested that there might be some residual atmosphere. It is unlikely because the velocity of escape is 4.3 kilometres a second and temperatures are high. Therefore only the heaviest of gases could be expected to remain on the planet’s surface. If any residual atmosphere remained solar storms would tend to carry it away.

All other evidence agrees that Mercury has no appreciable atmosphere. For instance, the horns of the crescent phase do not extend beyond the geometric limits. This indicates that there is no scattering or refraction effect such as an atmosphere would cause. A hydrogenhelium atmosphere would

escape in less than a year; one of oxygen-nitrogen in a few thousand years. Only traces of heavier gas molecules could be held for any length of time and these could be removed by storms on the Sun.

The temperature at the sub-solar point on Mercury is 260 degrees C. or even hotter, whilst on the night side it falls below minus 100 degrees C. The mean density of Mercury is equal to that of the Earth, being 5.5 times that of water. Its smaller mass means that the density of its basic materials is heavier than the Earth’s. This implies that Mercury has a sizable iron core. Resembles moon Radar mapping suggests that Mercury resembles the Moon in having a rough, irregular surface broken by; many craters. It also resem-; bles the Moon in size, being 4870 kilometres in diameter compared to the Moon’s diameter of 3476 kilometres. Both bodies have no appreciable atmosphere. They also reflect light in the same manner both as to colour and to intensity at various angles of reflection. Only space probes will be able to determine whether the resemblances of Mercury to the lunajr surface are real .

Leverrier found, a century ago, that Mercury did not move according to the gravitational theory. It appeared as if it was being pulled out of its orbit by an unseen planet. This lead to a search for a planet revolving closer to the Sun than Mercurv. This supposed planet was called “Vulcan.” The search was fruitless.

However, the direction of Mercury’s perihelion continued to advance at the rate of 43 seconds more than predicted on the basis of planetary perturbations. Then came Einstein. He proved that Newton’s law of gravitation, whilst true to a high degree in the outer parts of the Solar System, breaks down at small distances from the Sun. There gravitational effects do not obey the simple law so that calculated positions have to undergo small corrections. The main effect of these corrections is a secular shift of perihelion. One of the tests of ESnstein’s relativity theory was that the perihelion of Mercury would advance of 42.9 seconds per century. This is as close to the observed value as to be within the errors of measurement. Transits irregular Mercury, because its orbit lies between us and the Sun, occasionally passes directly between the Earth and the Sun. It is then said to transit, being visible as a small black spot against the bright solar disc. If Mercury and the Earth revolved in the same plane a transit of Mercury would occur at every inferior conjunction, averaging three times a year. However, the plane of Mercury’s orbit is inclined at an angle of 7 degrees to the ecliptic. As a result a transit will occur only when conjunction takes place on the line along which the two planes intersect, known as the line of nodes. The Earth crosses this line twice a year, on May 8 or 9 and again on November 10 or 11. If Mercury is at inferior conjunction within a few days of one of these dates a transit will take place. Such transit occur at irregular intervals, which may be three, seven, 10 or 13 years apart. The last transit was on May 9, 1970, and the next will be on November 9. 1973, when Mercury will pass centrally across the solar disc.