From the earliest times the movements of the heavenly bodies have excited the interest of the inhabitants of the earth. The ancients believed that the earth was a flat disc floating in a great ocean, and they thought the sun was a god who drove his chariot across the sky every day. During the night he travelled back to the east across the great ocean, but his light was shut off by a range of high mountains which existed in the northern part of the earth. At night the majority of stars were noticed to move across the sky as if they were all fixed in some spherical shell which revolved. Several stars, however, seemed to move irregularly and were called planets or wanderers. Seven of these bodies were known—Moon, Mercury, Venus, Sun, Mars, Jupiter—and it is probably to these seven bodies that we owe our week of seven days.