As already indicated, Venus presents few opportunities for useful observation to the amateur. The best time for observing, as in the case of Mercury, is in broad daylight; and for this, unless in exceptional circumstances, graduated circles and a fairly powerful telescope are required. Practically the most that can be done by the possessor of a small instrument is to convince himself of the reality of the phases, and of the non-existence of a satellite of any size, and to enjoy the exquisite and varying beauty of the spectacle which the planet presents. Should his telescope be one of the small instruments which show hard and definite markings on the surface, he may also consider that he has learned a useful lesson as to the possibility of optical illusion, and, incidentally, that he may be well advised to procure a better glass when the opportunity of doing so presents itself. The 'phosphorescence' of the dark side may be looked for, and it may be noted whether it is not seen after dark, or whether it persists and grows stronger. Generally speaking, observations should be made as early in the evening as the planet can be seen in order that the light of the sky may diminish as much as possible the glare which is so evident when Venus is viewed against a dark background. CHAPTER VII THE MOON Our attention is next engaged by the body which is our nearest neighbour in space and our most faithful attendant and useful servant. The moon is an orb of 2,163 miles in diameter, which revolves round our earth in a slightly elliptical orbit, at a mean distance of about 240,000 miles. The face which she turns to us is a trifle greater in area than the Russian Empire, while her total surface is almost exactly equal to the areas of North and South America, islands excluded. Her volume is about 2/99 of that of the earth; her materials are, however, much less dense than those of which our world is composed, so that it would take about eighty-one moons to balance the earth. One result of these relations is that the force of gravity at the lunar surface is only about one-sixth of that at the surface of the earth, so that a twelve-stone man, if transported to the moon, would weigh only two stone, and would be capable of gigantic feats in the way of leaping and lifting weights. The fact of the diminished force of gravity is of importance in the consideration of the question of lunar surfacing. [Illustration: FIG. 21.--THE TIDES. A, Spring Tide (New Moon); B, Neap Tide.]