The Moon orbits the Earth about every 27.3 days with respect to the stars (this is called a sidereal month). However, during that time, the Earth and the Moon have moved as a system about 1/12 of the way in their yearly orbit around the Sun. So, if the Moon at a certain point in its orbit is directly between the Earth and the Sun, 27.3 days later it has not quite returned to that point directly between the Earth and the Sun. The Moon must orbit the Earth a bit further to get back to the same place with respect to the line between the Earth and the Sun. The Moon reaches this point in a couple of days, making the synodic month (also called a lunar month) of the Moon equal to about 29.5 days. (The synodic period is the interval between two successive conjunctions of two celestial bodies; in this case conjunctions of the Moon and Sun as observed from the Earth.) It is the time between two successive full moons (or new moons) and it is the synodic months that are taken into account in lunar calendars.
As the Moon orbits the Earth, it goes through its various phases, from new moon, to first quarter, to full moon to last quarter and back to new moon. Refer to figure 7-1. Like the Earth, one half of the Moon is always illuminated by the Sun and the phase of the Moon that we see at any time is as a result of the fraction of the illuminated half that we can see. In between the new moon and the full moon, the Moon is a crescent (less than half-illuminated), a half-moon (half the face we see illuminated) or is gibbous (more than half-illuminated). Gibbous comes from the Latin word that means a hump, or bulging. The first half moon after the new moon is called the first quarter, since we are one-quarter of the way through the monthly cycle of phases. The half moon after the Moon is full is called the last quarter (also known as third quarter). While the phases are changing from new to full, with more of the illuminated side of the Moon becoming visible, we say the Moon is waxing. Between full and new phases the Moon is waning.
The angle between the Earth, the Moon and the Sun determines which phase will appear, so the time of night the Moon rises depends on its phase. A full moon rises when the Sun sets and so will stay in the sky most of the night. A new moon rises when the Sun rises and sets when the Sun sets. The Moon rises about 50 minutes later each night. A first-quarter moon thus rises at about midday and sets around midnight and so is at its highest point for the day at sunset. Around third quarter, it rises late at night. You can often see the Moon in the daytime, especially if you firstly figure out approximately where to look for it. Since the Moon's orbit is elliptical, the Moon's speed around the Earth varies, making the period between its phases vary slightly.
Solar eclipses, when the Moon casts a shadow on the Earth, can only happen at new moon. Lunar eclipses, when the Earth casts a shadow on the Moon, can only happen at full moon.
The Moon is often the most prominent object in the night-time sky. The Moon is a rocky body but unlike the Earth, it has no atmosphere and likely no life or liquid water.
As it orbits the Earth, the Moon keeps essentially the same face toward the Earth, because a slight bulge of matter exists in the side of the Moon that faces the Earth, allowing Earth's gravity to capture the Moon's rotation. As a result, the Moon turns once on its axis with respect to the stars during each of its orbits around the Earth.
The Moon is a little more than one-quarter the diameter of the Earth. This makes it the largest substantial satellite (moon) of the planets in the solar system when compared to its parent planet. Three moons of Jupiter and one of Saturn are physically larger than our moon.
Even with the naked eye, we can see that the surface of the Moon is varied in structure. The fact that the Moon has large flat areas and craters was discovered by Galileo when he firstly turned his small telescope to look at the Moon in 1609. The large flat areas are called maria (pronounced "MAR ee uh"; singular, mare, pronounced "MA ray"). Maria means "seas," though there is no water in these lunar seas.
The view of the Moon through even a small telescope can be breathtaking. See figure 7-2. You can see that the maria are very flat and that there are other regions, the highlands that are covered with craters. There are relatively few craters on the maria. These "seas" have, instead, been made flat by volcanic material – lava that flowed from beneath the lunar surface more than 3 billion years ago. This lava covered whatever craters existed at that time, so the craters now visible in the maria were formed by the impact of interplanetary rocks (meteorites) that have hit the Moon since then.
When observing the Moon with binoculars or with a telescope, you will find it most interesting to observe the terminator, the line separating light from dark (day from night, on the Moon). At the terminator, we are seeing the region where the Sun's rays are hitting the Moon at the most oblique angle and therefore shadows are longest there. This increases the contrast of the surface features. The height of lunar mountains can be calculated by measuring the lengths of their shadows.
- Diameter is 3 476 km (about a quarter the diameter of the Earth).
- Perigee (minimum distance from Earth) is about 356 400 km; Apogee (maximum distance from the Earth) is about 406 700 km. The average distance from the Earth is about 384 400 km.
- Mass is 7.348 x 1022 kg (about one eightieth of the mass of the Earth).
- Inclination of the Moon's orbit to the Ecliptic (plane of the Earth's orbit) is 5.145º.