Another curious and unexplained feature in connection with the planet is what is frequently termed the 'phosphorescence' of the dark side. This is an appearance precisely similar to that seen in the case of the moon, and known as 'the old moon in the young moon's arms.' The rest of the disc appears within the bright crescent, shining with a dull rusty light. In the case of Venus, however, an explanation is not so easily arrived at as in that of the moon, where, of course, earth-light accounts for the visibility of the dark portion. Had the planet been possessed of a satellite, the explanation might have lain there; but Venus has no moon, and therefore no moonlight to brighten her unilluminated portion; and our world is too far distant for earth-shine to afford an explanation. It has been suggested that electrical discharges similar to the aurora may be at the bottom of the mystery; but this seems a little far-fetched, as does also the attribution of the phenomenon to real phosphorescence of the oceans of Venus. Professor Newcomb cuts the Gordian knot by observing: 'It is more likely due to an optical illusion.... To whatever we might attribute the light, it ought to be seen far better after the end of twilight in the evening than during the daytime. The fact that it is not seen then seems to be conclusive against its reality.' But the appearance cannot be disposed of quite so easily as this, for it is not accurate to say that it is only seen in the daytime, and against Professor Newcomb's dictum may be set the judgment of the great majority of the observers who have made a special study of the planet. We may, however, safely assign to the limbo of exploded ideas that of the existence of a satellite of Venus. For long this object was one of the most persistent of astronomical ghosts, and refused to be laid. Observations of a companion to the planet, much smaller, and exhibiting a similar phase, were frequent during the eighteenth century; but no such object has presented itself to the far finer instruments of modern times, and it may be concluded that the moon of Venus has no real existence. Venus, like Mercury, transits the sun's disc, but at much longer intervals which render her transits among the rarest of astronomical events. Formerly they were also among the most important, as they were believed to furnish the most reliable means for determining the sun's distance; and most of the estimates of that quantity, up to within the last twenty-five years, were based on transit of Venus observations. Now, however, other methods, more reliable and more readily applicable, are coming into use, and the transit has lost somewhat of its former importance. The interest and beauty of the spectacle still remain; but it is a spectacle not likely to be seen by any reader of these pages, for the next transit of Venus will not take place until June, 2004.