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FOR THOUSANDS OF YEARS, ASTRONOMERS had no telescopes. They had only their eyes to observe stars and planets - the "heavenly bodies". Ancient scientists came to know six planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Since they moved past the "fixed" stars of the night sky, the planets earned the name "wandering stars". Four thousand years ago, the Egyptians called Mars - which glows orange-red - Har Decher, the "Red One". Centuries later, Babylonians named it Nirgal, the "Star of Death". By the 5th century BCE, Romans had named the planet Mars, after their god of war. The 2nd-century astronomer Claudius Ptolemy believed that Mars, the Sun, Moon, and other planets all revolved around the Earth. Ptolemy's theory was "geocentric" - Earth-centred. This theory ruled the thinking of astronomers for more than 1,400 years.