In 1610, Italian astronomer Galileo Galilei (1564–1642) made a telescope and used it to observe the Moon. His own watercolour pictures (Figure 1) show the Moon’s phases. The phases of the Moon arise because, as the Moon orbits Earth, we see all, part, or none of its illuminated surface over the course of a lunar month (Figure 2). Seen through a telescope, the edge of the illuminated part is not smooth, revealing that the Moon’s surface is covered in mountains, valleys and craters. Observations such as these helped persuade people that ‘heavenly bodies’ were not the perfectly uniform spheres that they had once believed them to be. By measuring the lengths of the shadows cast by the Moon’s surface features, Galileo was able to estimate their height. He showed that the mountains were comparable to those on Earth.
In this issue of PHYSICS REVIEW you can read about telescopes and find out how their designs have been improved since Galileo’s time (see pages 8–11). On pages 2–5 you can learn more about radio telescopes and, using ideas about microwave radiation, discover whether a modern radio telescope could detect a mobile phone signal sent from the Moon — a situation unimaginable in Galileo’s time.
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