THE PLANET MERCURY

Evening Star, Issue 20140, 3 April 1929, Page 11

THE PLANET MERCURY

NEAREST TO THE SUN

VARIATION IN TEMPERATURE HOT ENOUGH TO MELT LEAD. Tho planet Mercury was well-known to the ancients, since, it the observer were in not too high a latitude, it could lie seen with the naked eye from time to time. The Greeks had two names for it—Apollo when it appeared as morning star, and Mercury when it appeared as evening star—but by the time of Plato it was recognised that the two were really the same planet, and the name Mercury was permanently given to it. The conditions for seeing it from middle and northern Europe are somewhat unfavourable, because of its proximity to the sun and its comparatively small altitude above the horizon, so that it is generally considered to he a difficult object to see with the naked eye; indeed, it is said that Copernicus never saw it. Nearer the equator the conditions for seeing the planet are more lavourable, while in the southern hemisphere, owing to the shape and position of the planet’s orbit, it is not at all unusual to be able to sec Mercury with ease. Of all the planets Mercury is the nearest to tho sun,' its Incan distance being 35,950.900 miles; hut owing to the pronounced eccentricity ot the orbit the distance of Mercury from the sun may he as small as 28,550.009 miles, and may be as great as 43,350,000 miles. Following in this very largo variatioii/in the distance, there is a ve.v large variation in the speed with which-Mercury moves in its orbit. When it is closest to the sun it moves at the rate of thirty-six miles a second while when it is farthest away its speed is reduced to twenty-four miles a second.

Owing to the variation of its distance from the sun the amount of tight and heat which Mercury receives per unit area also varies greatly. On the average it receive? about seven times as much heat as the same area on the earth, but the amount received when Mercury is closest to the sun is more than twice as great as that received when it is furthest from it. The variation in the distance of Mercury from the earth is very large, for at closest they may be only 50,000,000 miles apart, while at other times the distance between them mav be as great as 136,000,000 miles. THE SIZE OF MERCURY.

The diameter of Mercury is about 3,100 miles, so that it is the smallest of the planets, 'its volume being only about G per cent, of that of the earth. Owing to the large variation in its distance from ns the apparent diameter varies greatly—from ssec to about l.'lsec of arc. Its mass also is less than that of any other planet, but it is very difficult to determine it accurately, for the effect of the attraction of Mercury in disturbing the motion of other planets or of a comet which passes near it is extremely small, and no satellite has been discovered from the motion of which an accurate value of the mass of Mercury might bo computed. The latest value is one twenty-fourth that of iho earth, but this may be as much as 25 per cent, in error. From this it would follow that gravity at the surface would bo one-quarter that at the earth’s surface—substances would weigh just one-fourth as much there as here.

From what has been said, several interesting consequences follow. First of all, its variations in brightness are considerable, although on account of always appearing close to the sun these changes are not as noticeable as they would otherwise be. At brightest it is nearly as bright as Sirius, the brightest of the fixed stars, but, even during the time when it can be conveniently observed, it may be as iaint as Aldebaran, the bright star in the Hyades. These changes in brightness arise in part from the changes in phase’of the planet, but even when the phase is the same, the brightness of Mercury may at one time be two and a-half times the brightness of another. This is. due to the largo variation of its distance from the "sun arising from the pronounced eccentricity of the orbit. The researches of Mueller at the Potsdam Astropbysical Observatory show that the variation in brightness with change nf phase follows almost the same law as for the moon. So, then, it seems permissible to conclude that thc_ condition of the surface of Mercury is similar to that of the moon. This would mean that the surface nf Mercury is rough, and that the planet possesses no dense atmosphere. Of the . -Might that falls enry, the planet reflects only t per cent., the rest being absorbed, and serving to heat the surlace. This is very nearly the same as the proportion that is reflected by the moon’s surface, and is comparable with the reflection from rather dark-coloured rocks. Contrast this with the 60 per cent, that is reflected by clouds in our own atmosphere, the 59 per cent, that is reflected by Venus, or the 41 per cent, that is reflected by Jupiter. This also indicates that the light is reflected from a solid surface rather than a cloudy atmosphere. THE ROTATION OF MERCURY. About the beginning of the iiineteenth century, Schroeter, in Lilionthal, thought he observed, when Mercury was a narrow crescent, that the edge of the crescent was ragged. This appearance ho ascribed to the shadow of a high mountain, and from the reouiTcncc ot the appearance concluded that the planet rotated about its axis once in 21 hours smin. No weight can, however, be given to < this result, for the best observers with lar superior instruments have never been able to detect any such appearance. It can only be assumed that his observations were illusory. Unfortunately, such errors die hard, and a period of about 2f hours still appears in many lists. Schiaparelli, of Milan, whoso name is familiar as the discoverer_ of the socalled canals of Mars, studied Mercury assiduously during the years 1881. to 1889 with a large, refractor ot IDin aperture. He found markings, indeed, but he described them as extremely faint shadows, which could only be recognised with much difficulty. He fonml it best to observe in broad daylight when Mercury was near the meridian. From his observations he concluded that the planet rotates about an axis almost perpendicular to its orbit, the period of rotation being 87.97 days, just equal to its period of revolution round the sun. This value of the period of rotation is now fairly generally accepted as correct. So then there is a large region permanently exposed to sunlight, whose temperature must be very high, another region on the opposite side of the planet which never receives any sunlight, and whose temperature must h" extremely low. Between these there is a third region, where the sun alternately rises above and sinks below the horizon, and in which the variations of temperature must be very great. It is possible to measure the temperature ot the sunlit side of Mercury, and tie result obtained recently hv Rcttit and Nicholson at the Mount Wilson Observatory for this tempera-

ture is about 660 deg. Fahrenheit. 4be conditions are not inviting—on one side of the planet, the sun blazing out of a cloudless sky, the temperature high enough to melt lead, and violent winds always blowing; on the other side a temperature not far from absolute zero —nearly nOOdeg. below . freezing. Separating the two will he a zone where the changes of temperature will be far greater than any of which we have experience, the interval between hot spells being about, three months.