Through the Telescope

by James Baikie

If the long period be accepted, it follows that Mercury must always turn the same face to the sun—that one of his hemispheres must always be scorching under intense heat, and the other held in the grasp of an unrelenting cold of which we can have no conception. 'The effects of these arrangements upon climate,' says Miss Agnes Clerke, 'must be exceedingly peculiar.... Except in a few favoured localities, the existence of liquid water must be impossible in either hemisphere. Mercurian oceans, could they ever have been formed, should long ago have been boiled off from the hot side, and condensed in "thick-ribbed ice" on the cold side.'

From what has been said it will be apparent that Mercury is scarcely so interesting a telescopic object as some of the other planets. Small instruments are practically ruled out of the field by the diminutive size of the disc which has to be dealt with, and the average observer is apt to be somewhat lacking in the patience without which satisfactory observations of an object so elusive cannot be secured. At the same time there is a certain amount of satisfaction and interest in the mere detection of the little sparkling dot of light in the Western sky after the sun has set, or in the Eastern before it has risen; and the revelation of the planet's phase, should the telescope prove competent to accomplish it, gives better demonstration than any diagram can convey of the interior position of this little world. It is consoling to think that even great telescopes have made very little indeed of the surface of Mercury. Schiaparelli detected a number of brownish stripes and streaks, which seemed to him sufficiently permanent to be made the groundwork of a chart, and Lowell has made a remarkable series of observations which reveal a globe seamed and scarred with long narrow markings; but many observers question the reality of these features altogether.

It is perhaps just within the range of possibility that, even with a small instrument, there may be detected that blunting of the South horn of the crescent planet which has been noticed by several reliable observers. But caution should be exercised in concluding that such a phenomenon has been seen, or that, if seen, it has been more than an optical illusion. Those who have viewed Mercury under ordinary conditions of observation will be well aware how extremely difficult it is to affirm positively that any markings on the surface or any deformations of the outline of the disc are real and actual facts, and not due to the atmospheric tremors which affect the little image.

Interesting, though of somewhat rare occurrence, are the transits of Mercury, when the planet comes between us and the sun, and passes as a black circular dot across the bright solar surface. The first occasion on which such a phenomenon was observed was November 7, 1631. The occurrence of this transit was predicted by Kepler four years in advance; and the transit itself was duly observed by Gassendi, though five hours later than Kepler's predicted time. It gives some idea of the uncertainty which attended astronomical calculations in those early days to learn that Gassendi considered it necessary to begin his observations two days in advance of the time fixed by Kepler. If, however, the time of a transit can now be predicted with almost absolute accuracy, it need not be forgotten that this result is largely due to the labours of men who, like Kepler, by patient effort and with most imperfect means, laid the foundations of the most accurate of all sciences.

The next transit of Mercury available for observation will take place on November 14, 1907. It may be noted that during transits certain curious appearances have been observed. The planet, for example, instead of appearing as a black circular dot, has been seen surrounded with a luminous halo, and marked by a bright spot upon its dark surface. No satisfactory explanation of these appearances has been offered, and they are now regarded as being of the nature of optical illusions, caused by defects in the instruments employed, or by fatigue of the eye. It might, however, be worth the while of any who have the opportunity of observing the transit of 1907 to take notice whether these features do or do not present themselves. For their convenience it may be noted that the transit will begin about eleven o'clock on the forenoon of November 14, and end about 12.45.

CHAPTER VI

VENUS

Next in order to Mercury, proceeding outwards from the sun, comes the planet Venus, the twin-sister, so to speak, of the earth, and familiar more or less to everybody as the Morning and Evening Star. The diameter of Venus, according to Barnard's measures with the 36-inch telescope of the Lick Observatory, is 7,826 miles; she is therefore a little smaller than our own world. Her distance from the sun is a trifle more than 67,000,000 miles, and her orbit, in strong contrast with that of Mercury, departs very slightly from the circular. Her density is a little less than that of the earth.

There is no doubt that, to the unaided eye, Venus is by far the most beautiful of all the planets, and that none of the fixed stars can for a moment vie with her in brilliancy. In this respect she is handicapped by her position as an inferior planet, for she never travels further away from the sun than 48°, and, even under the most favourable circumstances, cannot be seen for much more than four hours after sunset. Thus we never have the opportunity of seeing her, as Mars and Jupiter can be seen, high in the South at midnight, and far above the mists of the horizon. Were it possible to see her under such conditions, she would indeed be a most glorious object. Even as it is, with all the disadvantages of a comparatively low position and a denser stratum of atmosphere, her brilliancy is extremely striking, having been estimated, when at its greatest, at about nine times that of Sirius, which is the brightest of all the fixed stars, and five times that of Jupiter when the giant planet is seen to the best advantage. It is, in fact, so great that, when approaching its maximum, the shadows cast by the planet's light are readily seen, more especially if the object casting the shadow have a sharply defined edge, and the shadow be received upon a white surface—of snow, for instance. This extreme brilliance points to the fact that the surface of Venus reflects a very large proportion of the sunlight which falls upon it—a proportion estimated as being at least 65 per cent., or not very much less than that reflected by newly fallen snow. Such reflective power at once suggests an atmosphere very dense and heavily cloud-laden; and other observations point in the same direction. So that in the very first two planets of the system we are at once confronted with that diversity in details which coexists throughout with a broad general likeness as to figure, shape of orbit, and other matters. Mercury's reflective power is very small, that of Venus is exceedingly great; Mercury's atmosphere seems to be very attenuated, that of Venus, to all appearance, is much denser than that of our own earth.