Page:The Moon (Pickering).djvu/99

 CHAPTER IX

In this chapter we shall deal with such work as has been done within the last few months prior to the publication of this volume. In treating of Linné, however, it will be necessary to refer first to some earlier work. In astronomical investigations it is generally more convenient to express distances in angular rather than linear measure, and in what follows we shall use as our unit the second of arc, which at the centre of the Moon is a distance a trifle more than one mile in length.

In Chapter VII. it was shown that the white spot surrounding Linné is subject to fluctuations in size analogous to those of the polar caps of our Earth and of Mars. Soon after sunrise on Linné the spot appears of its maximum diameter. As the Sun rises higher and higher upon it, it rapidly diminishes in size, reaching its minimtmi dimensions about one day after noon. This would correspond to a colongitude of the sunrise terminator of 90°. The spot then immediately begins to increase in size, nearly reaching its original dimensions shortly before stinset. In 1898 the maximum diameter was 4″.0, the minimum 2″.1. In 1899 the minimum reached 2″.0, since which time it has gradually increased tmtil it is now about 2″.8 in diameter. Compare the size of Linné on plates 7A, 5B, 5C and 5E.

Assuming that the spot was due to a deposit of hoarfrost, which evaporated under the intense solar radiation—Linné is in latitude +28°—it occurred to the writer that a crucial test of the truth of this hypothesis could be made at the time of a lunar eclipse. At such a time the spot is at about its minimum area, and during the interval of the withdrawal of the Sun's rays it was thought that it might sufficiently increase in size to render the change visible from the Earth. Accordingly an effort was made to measure its dimensions before and after the total eclipse of December 27, 1898. Unfortunately, the sky was so hazy on that occasion that nothing could be done in Cambridge. Fearing bad weather, the writer had asked Mr. Douglass, of the Lowell Observatory, also to make some measures. He had more favourable skies and found a marked increase of size. This increase lasted, 67