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The Path of the Moon

The Moon like all other celestial objects rises in the east and sets in the west. Since the Moon is roughly in the same plane as most of the other objects in the Solar System, it moves along the ecliptic. The Moon always will be found close to the constellations of the zodiac. All the stars and even the relatively nearby planets are so distant from the Earth that their location is fixed within any night. If Jupiter rises from the eastern horizon between two stars, it will remain between these two stars for the entire evening. The Moon is so close to the Earth that its movement is detectable within a single night. The covering up of a star or a planet by the Moon is called an occultation and their timing is used by scientists to determine the exact orbit of the Moon and other objects in space. Within any one night, the Moon will move a distance relative to the stars equal to its own diameter. What about from night to night? If we can learn about the path of the Moon, particularly when it rises and set during various times of the lunar month, then we will know how too find the Moon when we wish to look at it and avoid it when we want to something else.

Let us start again at the new Moon. To simplify matters, imagine that it is now the month of March and both day and night are of almost equal lengths. During the new Moon, the Moon is in roughly the same direction from the Earth as the Sun. The Sun and the Moon both rise and set at about the same time. At this point the lunar month, the Sun, Moon and finally the Earth are all in more or less a straight line. Since the Moon is in the middle, the Sun lights up one side and we see the other. During this time, not only is the near side of the Moon dark, but also is only above the horizon during the daytime. One can gaze at any faint object in the sky, at any time at night during a new Moon.

Although the movement that dominates the Moon during a night is from east to west because of the Earth’s rotation, the Moon is revolving around the Earth in the opposite direction. Imagine the new Moon setting over the western horizon at sunset. During each successive night, the Moon at sunset will be slightly higher in the sky as it moves eastward from night to night. We are discussing two separate movements. The movement within a night of the Moon and all other celestial objects from east to west, and the movement of the Moon from west to east on each successive night. As the Moon climbs through the sky from night to night, it also sets later. When the Moon has reached its first quarter phase, at sunset it will be directly overhead and will set around midnight. Around the Moon’s first quarter you can observe the Moon during the early evening and the stars from midnight to early morning.

The Moon gradually moves eastward as it changes from the first quarter to a full Moon. The full Moon at sunset is rising over the western horizon. Once again the Sun, the Earth and the Moon are roughly in a straight line, but this time the Earth is in the middle. The Moon is lit up by the Sun which is at the opposite end of the sky; as the Sun rises, the Moon sets and as the Sun sets the Moon rises. The Moon will rise at sunset, be directly overhead at midnight and set close to sunrise. As I mentioned before, a full phase Moon is not the best time for observation and it will be difficult to see any but the brightest stars on that night. Use a full Moon night to take in a movie or catch up on your reading.

Now let us take a look at sunrise at which time a full Moon will be setting over the western horizon. Each following night it will be a little higher in the sky at sunrise. The Moon will set later in the morning each day, and also rise later at night each day. You may be able to see the Moon during the day as it nears the last quarter phase. Who cares as long as it is not overpowering with its reflected light the good stuff at night? At the last quarter, the Moon will rise at midnight and be directly overhead during sunset. We have returned to the same situation as during the first quarter except that the times are now reversed. We can use the evening up to midnight, during the last quarter to observe stars and other faint objects and after midnight for Moon watching.


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