Why don't we see a lunar eclipse every month during a full moon?

Why are there eclipses?

During your lifetime you can expect to see about thirty-six total eclipses of the Moon and perhaps forty-eight partial eclipses. Eclipses of the Sun are much rarer. Like any solid objects bathed in light, the Earth and the Moon, both lit by the Sun, cast shadows into space. The Sun’s light comes from a wide disc and not a single point. This gives the Earth a shadow with a soft and lighter edge known as the penumbra. When the Sun, Earth and Moon are in a direct line, the Moon moves into the darkest part of the Earth’s shadow. It loses much of the Sun’s light, and becomes totally eclipsed, turning a coppery red. The red light is weak sunlight, bent by the Earth’s atmosphere, a kind of sunset glow all round the edges of the Earth, some of which reaches the Moon. If you were on the Moon during an eclipse, that is indeed what you would see the Earth outlined by a trim of fiery red.

If the Moon passes into only the penumbra, the lighter part of the Earth’s shadow, it will grow dimmer, as you can see with the naked eye. This is a partial eclipse. If at that time you were standing on the Moon, you would see part of the Sun blacked out. If you had a thermometer, you would soon notice that outside your spacesuit the temperature had dropped sharply, because the Moon’s surface quickly loses its heat.

Eclipses of the Moon occur only when the Moon is full that is, when the Sun and the Moon are on opposite sides of the Earth. Because the Earth is so much larger than the Moon, it casts a large shadow, which varies according to the Moon’s distance from the Earth. For this reason, the Moon may take as long as 105 minutes to move through the Earth’s shadow, making eclipses of the Moon much longer than those of the Sun. Also, they can be seen by everybody facing the Moon at the time.
Eclipses of the Sun are caused by the Moon passing between the Earth and the Sun. All goes dark, because the Earth is in the Moon’s shadow. For about seven minutes, the Sun may be seen only as a vague circle, glowing dimly at its perimeter. The eclipse is visible only in a region some 275 km (170 miles) wide. Close by, people may see only a partial eclipse.

You may wonder why an eclipse of the Moon doesn’t occur every twenty-nine and a half days, whenever the Moon is full. The reason is that, for an eclipse to take place, the Moon’s path in the sky must be close to what astronomers call the ecliptic the Sun’s path in the sky relative to the stars. But the Moon’s orbit is tilted a little above or a little below.

Only at two points, or nodes, do the Moon’s path and the ecliptic coincide, putting the Moon into Earth’s shadow.

Because eclipses are comparatively rare, people in centuries past have had good reason to study them. Eclipses were recorded as early as 750 bc. In ancient times, they were seen as messages from the gods, so Babylonian and Greek astronomers paid them particular attention. Predicting an eclipse was like delivering an advance message from a supreme being, and ancient astronomers discovered how to do so with considerable accuracy. Some people believe that the great stones of Stonehenge were arranged so that ancients could predict when lunar eclipses would occur.