Some observers have thought that Mercury is more easy to observe than Venus, and that, speaking generally, its surface, if we could only get to see it constantly under favourable circumstances, might be considered to resemble in most respects that of Mars. Mercury revolves round the Sun at a mean distance of 36 millions of miles. Owing, however, to the fact that the eccentricity of its orbit (or its departure from the circular form) is greater than that of any of the other major planets, it may approach to within 28½ millions of miles or recede to more than 43 millions of miles. Its apparent diameter varies between 4½″ in superior conjunction to 13″ in inferior conjunction. The real diameter may be taken at about 3000 miles. CHAPTER IV. VENUS. The planet Venus has two things in common with Mercury. One is, that being an inferior planet, that is to say, a planet revolving round the Sun in an orbit within that of the Earth, it is never very far distant from the Sun, and therefore can never be seen on a distinctly dark sky. The second point alluded to arises out of the first; Venus exhibits from time to time a series of phases which are identical in character with those of Mercury, and therefore with those of the Moon. Venus differs, however, from Mercury in the very important point of size. Inasmuch as its diameter is considerably more than double the diameter of Mercury it has a surface more than six times as great, and therefore exhibits a far larger area of illumination than Mercury does. The result of this (coupled with another fact which will be stated presently) is that the planet may often be easily seen in broad daylight, and sometimes casts a sensible shadow at night. Under special circumstances, which recur every 8 years, this planet shines with very peculiar brilliancy. True, that only about ¼th of the whole disc is then illuminated, but that fraction transmits to us more light than phases of greater extent do, because these latter coincide with epochs when the planet is more remote from the Earth.