But although the great men of Europe used the telescope on many occasions both in peace and in war, Galileo put it to a greater use. He conceived the idea of turning it to the heavens and examining the celestial bodies. In doing so he obtained a most surprising series of revelations. He first of all turned it on the moon, and saw at once that the markings on the surface of the moon were really mountains and level plains. He could watch the shadows of the mountains grow and diminish with the varying direction of the sun’s rays, and he could even prove that some of the mountains were higher than others. He then turned his telescope on the great planet Jupiter, and noticed to his great surprise that Jupiter had near it four smaller companions which circled round it in a few days. In fact, he found that Jupiter has four moons where the earth has only one. This discovery was entirely opposed to the ancient idea, then universally held, that there were only seven planets or moving stars, namely: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. On pointing the telescope to another great planet, Saturn, Galileo found that another surprise awaited him. It looked as if the planet had two handles, like a jug. This strange appearance was really owing to the fact that Saturn is surrounded by a fiat ring, which being slightly tilted gave the appearance of the two handles. This, of course, was not known at that time to Galileo, but he certainly was the first to observe that Saturn had a very unusual appearance. [Illustration: Fig. 27.--The Planet Saturn and its Rings.] Galileo next turned his telescope on the sun at a time when it was near the horizon, and not strong enough to blind him. He observed that the sun was not evenly bright all over its surface, but that there were spots on it. These spots came and went, and could be seen to appear at one edge of the sun, to move round, and to disappear at the other edge about a fortnight later. This gave Galileo the idea that the sun revolves on its axis. After that, Galileo turned his attention to the planet Venus, and found, contrary to all accepted opinions, that this glorious planet showed phases like the moon, and was sometimes seen in the shape of a crescent. Such appearances were not presented by Jupiter or Saturn, and it struck Galileo that the probable explanation of this must be that Venus revolves round the sun at a lesser distance than the earth, and the other planets at a greater distance. [Illustration: Fig. 28.--Photograph of the Sun on two successive days, showing Sun-Spots and the Rotation of the Sun.]