The motion of the Moon

Since the Moon orbits the Earth every 27.3 days (sidereal period), it is always changing its position with respect to the fixed stars, which serve as a backdrop. It is also changing its appearance, going through a full cycle of phases in the course of one month. The motion of the Earth around the Sun causes the phases to cycle in 29.5 days. (This number actually varies by up to 13 hours since the Earth doesn't move at a constant rate about the Sun.) The Moon rotates with the same period, so we always see the same face.

The Moon does not give off any light of its own. It shines by reflected sunlight. Therefore, the appearance of the Moon depends on the relative positions of the Earth, Sun and Moon. This is shown in Fig. 22.10. At any given time half of the Moon is illuminated. The changes in appearance are because different amounts of the illuminated side face the Earth. Let us follow it through one cycle. We start when the Moon is between the Earth and the Sun. Only the dark side of the Moon faces the Earth, and we see nothing. This is called the new moon. As the Moon moves over a little, a small piece of the illuminated side faces us and we see a crescent. Since the crescent is growing, we call it the waxing crescent. This appears to the east of the Sun (with the crescent side towards the Sun). This means that it is visible in the western sky at sunset. One-quarter of the way through the cycle, half of the visible side faces us, and we call it a first quarter. By that time, the Moon will have moved a quarter of the way across the sky,

Sunlight

Last Quarter

Last Quarter

First Quarter

Lunar phases.The numbered images on the circle show the actual position and illumination of the Moon. The outer images show that appearance of the Moon, as seen from Earth. (1) New moon; (2) waxing crescent; (3) first quarter; (4) gibbous; (5) full moon; (6) gibbous; (7) last quarter ; (8) waning crescent.

First Quarter

Fig 22.10.

Lunar phases.The numbered images on the circle show the actual position and illumination of the Moon. The outer images show that appearance of the Moon, as seen from Earth. (1) New moon; (2) waxing crescent; (3) first quarter; (4) gibbous; (5) full moon; (6) gibbous; (7) last quarter ; (8) waning crescent.

and will appear high up at sunset. For the next quarter of a cycle, the visible part grows, and is called a waxing gibbous. Halfway through the cycle, we see the full illuminated side, and we call it a full moon. The full moon rises roughly as the Sun sets.

The second half of the cycle goes back through similar phases. For the third quarter of the cycle, the illuminated side becomes smaller, and we call it waning gibbous. Three-quarters of the way through the cycle we again see half of the illuminated face, and it is called the last quarter. For the last quarter, the half the we see is on the opposite side from that at the first quarter. The last quarter moon will be high in the sky at sunrise. Through the last quarter of the cycle, we see a waning crescent, getting smaller, and getting closer to the Sun. Finally, we return to the new moon.

The Moon's axis of rotation is inclined by 1.5° with respect to the plane of its orbit. The inclination contributes to an effect, known as libration, which allows us to see more than 50%

Rotational Motion of Moon

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