The moon having always one half illuminated by the sun, while the other half is dark, presents different "phases" to us on the earth, according to the position in which we view it; thus, in fig. 12, E represents the central position of the earth, and S the direction from which the light of the sun comes, the outer circle of moons represents the various real positions of that luminary with respect to the earth and sun, and the inner circle shows its appearance when in these various positions. When the moon is between the earth and sun, as at _a_, it is said to be in "conjunction," and as the darkened side is towards the earth, of course it is not visible, this is "new moon" (a corruption of "no moon"); when at _b b_, but a small part of the illuminated half is seen, and it is then said to be "crescentic" (first and last quarter); at _c c_, half of the bright side is seen, the moon is said to be at "quadrature," and the appearance is that of a "half-moon;" at _d d_, the greater part of the bright side is seen, and it is called "gibbous" and appears as a "three-quarter-moon," and at _e_ the whole of the illuminated side is seen; it is then "full-moon," and is said to be in "opposition." [Illustration: FIG. 12.] Beyond the earth the planet Mars (fig. 13) moves in its orbit round the sun; it is the smallest, but one, of the larger planets, having a diameter of but 4085 miles, and being only about one-seventh the magnitude of the earth; it makes its revolution in 687 days, at a distance of 144,780,000 miles from the sun. [Illustration: FIG. 13.]