When the telescope came into use, a nearly endless series of discoveries followed. In one of the unpublished manuscripts of Thomas Harriot, mention is made of the fact that he saw Jupiter’s four large moons before Galileo mentioned his discovery in 1610. Harriot also discovered sunspots. Galileo kept his discovery of sunspots secret, out of fear for his life! How could he dare say that the immaculate Sun had blemishes? In 1665 Christiaan Huygens discovered the large moon of Saturn, Titan. Giovanni D. Cassini (1625–1712) discovered a gap in Saturn’s rings. He also discovered four of Saturn’s moons: Iapetus (1671), Rhea (1672), Dione and Tethys (1684). In addition, he made very careful measurements of the periods of Jupiter’s large moons. This enabled Olaus C. Rømer (1644–1710) to make the first determination of the velocity of light (Figure 5.1). Rømer noticed that the times at which the four moons disappeared behind Jupiter became systematically later as the Earth moved away from its position of opposition, E1 (Figure 5.2), to the other side of the Sun, E2. As the Earth came back towards opposition, the times became earlier.
KeywordsZenith Angle Fundamental Measurement Astronomical Unit Minor Planet Prime Meridian
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