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