JEAN-LOUP CHARMET / SCIENCE PHOTO LIBRARY JEAN-LOUP CHARMET / SCIENCE PHOTO LIBRARY
Moon phases. Medieval manuscript page explaining why there are phases in the appearance of the Moon. The Sun (at centre right) is shown illumin- ating one side of the Moon as it moves in its orbit. Both the Sun and Moon are shown moving in circular orbits around a central point. In this Ptolemaic astronomical system (since superseded), the central point would be occupied by Earth (not shown here). This diagram shows that the Moon will appear full when the Moon and Sun are on opposite sides of the Earth, whilst it will appear darkened when they are on the same side of the Earth. Photographed from the 15th century book De Arte Matematicum.
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