Again, the star Mercury, in the same way, mounts up in both directions[239] from his morning rising, and having followed the sun through a space of 15 degrees, he becomes almost stationary for four days. Presently he diminishes his altitude, and recedes from his evening setting to his morning rising. Mercury and the Moon are the only planets which descend for the same number of days that they ascend. Venus ascends for fifteen days and somewhat more; Saturn and Jupiter descend in twice that number of days, and Mars in four times. So great is the variety of nature! The reason of it is, however, evident; for those planets which are forced up by the vapour of the sun likewise descend with difficulty. CHAP. 15.—GENERAL LAWS[240] OF THE PLANETS. There are many other secrets of nature in these points, as well as the laws to which they are subject, which might be mentioned. For example, the planet Mars, whose course is the most difficult to observe[241], never becomes stationary when Jupiter is in the trine aspect, very rarely when he is 60 degrees from the sun, which number is one-sixth of the circuit of the heavens[242]; nor does he ever rise in the same sign with Jupiter, except in Cancer and Leo. The star Mercury seldom has his evening risings in Pisces, but very frequently in Virgo, and his morning risings in Libra; he has also his morning rising in Aquarius, very rarely in Leo. He never becomes retrograde either in Taurus or in Gemini, nor until the 25th degree of Cancer. The Moon makes her double conjunction with the sun in no other sign except Gemini, while Sagittarius is the only sign in which she has sometimes no conjunction at all. The old and the new moon are visible on the same day or night in no other sign except Aries, and indeed it has happened very seldom to any one to have witnessed it. Prom this circumstance it was that the tale of Lynceus’s quick-sightedness originated[243]. Saturn and Mars are invisible at most for 170 days; Jupiter for 36, or, at the least, for 10 days less than this; Venus for 69, or, at the least, for 52; Mercury for 13, or, at the most, for 18[244]. CHAP. 16. (18.)—THE REASON WHY THE STARS ARE OF DIFFERENT COLOURS.