Fig. 42.—Galileo's method of estimating the height of lunar mountain.
AB'BC is the illuminated half of the moon. SA is a solar ray just catching the peak of the mountain M. Then by geometry, as MN is to MA, so is MA to MB'; whence the height of the mountain, MN, can be determined. The earth and spectator are supposed to be somewhere in the direction BA produced, i.e. towards the top of the page.
Galileo made a very good estimate of the height of lunar mountains, of which many are five miles high and some as much as seven. He did this simply by measuring from the half-moon's straight edge the distance at which their peaks caught the rising or setting sun. The above simple diagram shows that as this distance is to the diameter of the moon, so is the height of the sun-tipped mountain to the aforesaid distance.
Wherever Galileo turned his telescope new stars appeared. The Milky Way, which had so puzzled the ancients, was found to be composed of stars. Stars that appeared single to the eye were some of them found to be double; and at intervals were found hazy nebulous wisps, some of which seemed to be star clusters, while others seemed only a fleecy cloud.
Fig. 43.—Some clusters and nebulæ.