In ancient times the Moon was sometimes worshipped as a goddess, and even when people ceased to worship it their superstitions about the Moon lingered on. It used to be thought that the old Moon had a bad influence, while the new Moon and the full Moon were considered lucky. People therefore chose the times of new and full Moon for beginning important activities. Even today some believe that the Moon influences the weather, though probably that is not true. But the Moon does affect the tides of the sea, whose rise and falls are caused mostly by the combined gravitational pull of the Moon and Sun.
The Moon in Space
The Moon is the Earth's only natural satellite. As the Earth moves around the Sun, so the Moon in turn revolves around the Earth. As it orbits the Earth, the Moon also turns on its axis. The time taken to complete one rotation is the same as that taken for one orbit-about 29¼ days. So the same side of the Moon always faces the Earth. Photographs taken by orbiting space satellites show that the hidden side is similar to the visible side.
The Moon has no light of its own and shines only because it reflects the Sun's light. As it turns on its axis only once in a journey around the Earth, each part of its surface has first about two weeks of darkness and then about two weeks of sunlight. When the Moon comes between the Earth and the Sun it is invisible because the face turned towards the Earth is in darkness and sunlight is falling on the hidden side. This is the time of "new Moon". A few days later, a thin crescent Moon is seen in the western sky, as the Moon advances along its orbit and the Sun begins to light up the side turned towards the Earth. Occasionally, during this crescent phase, the whole disk may be seen faintly lit by Earthshine (light reflected from the Earth). This appearance is sometimes called "the old Moon in the new Moon's arms". The crescent waxes, or grows larger, as sunlight advances across the Moon's disk. At "full Moon" the whole disk can be seen, after which the Moon wanes, or grows smaller, first becoming "gibbous" and eventually receding to a thin C-shaped crescent. The Moon takes a lunar month (29½ of our days) to go through these phases, or changes of shape.
Occasionally, the Moon passes between the Sun and the Earth in such a way that the Sun's light is partially or totally blotted out for a while. This is known as a solar eclipse. Similarly, when the Moon is on the opposite side of the Earth to the Sun, it sometimes moves into the Earth's shadow, and its disk is darkened for a time. This is called a lunar eclipse. One might think there should be eclipses every month, but there are not. The reason for this is that the Moon's orbit is at an angel of 5° with respect to the plane of the Earth's orbit around the Sun.
© 1988 Encyclopaedia Britannica, Inc.