The rate at which the planets revolve round the sun is not equable, that is, their progress is not through equal distances at equal times, but a line drawn from the planet to the sun would always pass over equal spaces or "areas" in equal times, for example, in fig. 5, if the area of the angle included in A B C be equal to that included in A D E, then a planet would pass from B to C and from D to E in equal times. Mercury is the smallest of the greater planets, and the nearest to the sun. Its diameter is about 3000 miles, and it is about one-fifteenth part the volume of the earth; its distance from the sun is 36,770,000 miles, and it performs its revolution in eighty-eight days. [Illustration: FIG. 5.] This planet is but seldom seen with the naked eye; for, being so (comparatively) near to the sun, it sets before dark, and does not rise till the grey of morning makes it scarcely visible. Nothing notable has been discovered on its surface. Venus is the second in rotation from the sun, and revolves round it, at a distance of 68,750,000 miles, in 224-1/2 days. The diameter of Venus is about 7760 miles, it is therefore nearly the volume of the earth. This is the brightest planet seen in the heavens, for although much smaller than many others, its comparative nearness to the sun and earth causes it to appear larger and brighter to the eye. Venus is the evening and morning star; for when to the west of the sun, it rises before it, and is then called "the morning star" (or, formerly, "Lucifer"), but when it is to the east of the sun, it sets after twilight is gone, and is called "the evening star," or "Hesperus." These two planets, Mercury and Venus, have "phases" (like the moon), or certain positions in which the whole of the side illuminated by the sun is seen from the earth, and other positions in which it is seen sideways; the planet is then said to be in "quadrature," as may be seen at Q Q, fig. 6. Mercury and Venus, being between the sun and the earth, are called "inferior" planets, while those whose orbits are outside that of the earth are called "superior" planets. When either of the inferior planets are between the earth and the sun, they are said to be in "inferior conjunction" (I C, fig. 6), and when on the opposite side, or behind the sun, so that a straight line from it to the earth would have to pass through the sun, then it is called being in "superior conjunction" (S C); at mid-distance, either east or west, it is said to be in "eastern" or "western quadrature." [Illustration: FIG. 6. PLANETS SEEN IN QUADRATURE.]