Thousands of years ago, most people believed that the earth was flat, and that the gods were responsible for the motion of the sun, the moon, and the stars. The ancient Babylonians described the universe as a closed chamber with the earth as its floor. Around the earth lay a moat of water, beyond which stood high mountains supporting the dome of the heavens. Early travelers used the stars in the night sky to help them navigate. They looked for recognizable patterns in the distribution of the stars. Throughout the ages, the shapes of some of these star patterns (or constellations) came to represent the mythical characters of campfire stories. Years were measured by the passing of the seasons. Months were measured by the phases of the moon. The seven days of the week were dedicated to the seven gods who were thought to control the sun, the moon, and the five visible planets. These bodies appeared to move across the sky along a narrow band known as the zodiac. As they passed through the constellations, they were believed to have a mysterious influence on people’s lives. Attempts were made by some ancient civilizations to keep records about the motion of the sun, the moon, and the stars. As early as 4000 BC, the Egyptians had begun to count the passing of the years using 365 days in a year. But over the centuries, their years fell out of step with the seasons, because the actual number of days is 365.242. The ancient Babylonians devised a calendar with 12 months in a year. Each month had 29 or 30 days in accordance with the lunar cycle. Every few years, a 13th month was added to keep the years in step with the seasons. Variations of the Babylonian calendar continued to be used across much of the ancient world until the year 46 BC, when Julius Caesar introduced a new calendar throughout the Roman Empire. Recognizing the need for a stable and predictable calendar, Caesar invited the help of astronomers from the city of Alexandria who recommended disregarding the lunar cycle and fixing the average length of a year to 365¼ days. The new calendar was proclaimed to have 12 months and 365 days in a year with one extra day every 4 years. The Julian calendar was so successful that it continued to be used throughout Europe for the next 1600 years.