The Moon in Orbit

The Moon orbits the Earth every 27.3 days. Like the Earth the Moon is also spinning but with a period of 27.3 days. The result of this is that the same side of the Moon is always facing the Earth. Until man made probes were sent to orbit the Moon we had never seen the far side.

The Moon was possibly part of the Earth once or at least formed from the same rotating cloud of debris.

The diagram below (not to scale) shows that the plane of orbit of the Moon is about 5 degrees to the ecliptical plane which is why solar eclipses do not occur once a month but are very rare. Nevertheless the Moon in the sky stays close to the ecliptic and passes through the constellations of the zodiac.

Phases of the Moon

Of course the Moon does not appear the same in the sky throughout the course of a lunar month. As the Moon rotates around the Earth different parts of the Moon are in sunlight and others are in darkness. At any time we may see all, some or none of the half of the Moon which is in sunlight.

        
pic - NASA

Remember      -      Waxing = getting bigger     Waning = getting smaller

Find out what each of the phases above is called.

Although it takes 27.3 days for the Moon to orbit the Earth the lunar cycle above actually takes 29.5 days. This is because the Earth has moved relative to the Sun so an extra couple of days are needed for the Moon to get into the position it was in when the cycle started.

Refraction

Sometimes, when the Moon is close to the horizon, the Moon appears higher in the sky than it should be. This happens because light rays from the Moon are bent, or refracted when they enter the Earth's atmosphere, so they appear to come from a place higher in the sky.

Rays from the bottom half of the Moon are actually refracted more as they pass through slightly more atmosphere. This can cause the Moon to have a squashed appearance.