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PHASES OF THE MOON It takes the Moon approximately 29 1/2 days to make one complete orbit of Earth. During these 29 1/2 days, we here on Earth see what we regard as the phases of the moon. When the Moon is in its New phase, the Moons orbit carries it directly between Earth and the Sun. See figure 6-8-1. The side of the Moon facing Earth is in darkness. There is no moonlight. Just the opposite occurs when the Moon enters its Full phase. Now Earth
Figure 6-8-1.-Earth-Moon-Sun relationship during New Moon phase. is between the Moon and the Sun. The side facing Earth is now totally illuminated, while the side facing away from Earth is in darkness. See figure 6-8-2. Starting with the New Moon, the Moon passes through the following phases as it orbits Earth: New Moon, 1st-quarter Moon, Half-Moon, 2d-quarter Moon, Full Moon, 3d-quarter Moon, Half-Moon, 4th-quarter Moon, and New Moon. Figure 6-8-3 shows the moon phases as they occurred between 5 January 1989 and 3 February 1989.The times of moonrise and moonset can be estimated by timing the passage of the Moon across the nighttime sky during a particular phase. Each phase has its own time relation-ship with regard to crossing the meridian of the nighttime sky. For example, during a Full Moon, the Moon reaches the meridian approximately 6 hours after rising. From the meridian, it takes 6 more hours to set. The reason the time of moonrise and moon-set can only be estimated using the phases of the Moon is the Moons orbit. It is NOT constant about Earth. Changes in the orbit cause changes in the times of moonrise and moonset.Learning Objective: Identify the two factors in the Moons orbit that cause daily changes in the time of moonrise and moonset. |
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