Now in like manner, to transfer the process to the sky, let the two ends of the base be represented by two astronomical observatories, for example, Greenwich in the northern hemisphere and Cape Town in the southern. The base line is the chord or straight line through the earth connecting the two observatories, and we know the length of this line pretty accurately, because we know the size of the earth. The angles measured are somewhat different from those in the terrestrial example, but the process amounts to the same thing because the astronomers at the two observatories measure the angular distance of the center of the moon from the zenith, each using his own zenith at the same time; and the same science of trigonometry enables them to figure out the length of any side of the triangles involved. The side which belongs to both triangles is the distance from the center of the earth to the center of the moon, and the average of many hundred measures of this gives 238,800 miles, or about ten times the distance round the equator of the earth. We have said that the orbit in which the moon travels round the earth is practically a circle, but the earth's center is found not at the center of this orbit, but set to one side, or eccentrically, so that the distance spanning the centers of the two bodies is sometimes as small as 221,610 miles at perigee, and 252,970 miles at apogee. The moon's speed in this orbit averages rather more than half a mile every second of time--more accurately 3,350 feet a second, or 2,290 miles per hour. Once the moon's distance is known, its size or diameter is easy to ascertain. An angular measure is necessary, of course, that of the angle which the disk of the moon fills as seen from the earth. There are many types of astronomical instruments with which this angle can be measured, and its value is something more than half a degree (31'7"). The moon's actual diameter figures out from this 2,163 miles; and it would therefore require nearly fifty moons merged in one to make a ball the size of the earth. Still, no other planet has a satellite as large in proportion to its primary as the moon is in relation to the earth. But the materials that compose the moon have less than two-thirds the average density of those that make up the earth, so that eighty-one moons fused together would be necessary to equal the mass or weight of the earth. If we figure out the force of attraction of the moon for bodies on its surface, we find it equals about one-sixth that of the earth. Athletes could perform some astounding feats there--miracles of high jump and hammer-throw.