The Moon's phases and eclipses

The Moon is the Earth's nearest neighbour and only natural satellite. At a mean distance of 384,400 kilometres, and travelling at about 1 km/s, it takes the Moon about 27 days 7 hours to orbit the Earth once. Because the Earth moves in its orbit around the Sun at the same time, the Moon's cycle of phases (one new Moon to the next) lasts 29 days 12 hours. The Moon rotates on its axis once every 27 days 7 hours, the same time as it takes to orbit the Earth (synchronous rotation). This explains why we only ever see the same features of the Moon's near side. With a diameter of about 3,476km, the Moon is about 0.25 times the size of the Earth. While this makes the Moon about 400 times smaller than the Sun, the Moon is also about 400 times closer so both objects appear the same size in the sky. The Moon is barren and rocky and has no atmosphere. Its surface is heavily marked with craters, the largest measuring over 200km across, and plains or seas of old lava flows. Surface temperatures range from 130C in direct sunlight to -180C in darkness. Gravitational forces between the Moon and the Earth cause movement in the Earth's oceans called tides. Between 1969 and 1972, experiments were conducted on the Moon's surface and samples of Moon rock returned to Earth by astronauts during six successful Apollo missions. Most astronomers now believe the Moon to have formed at the same time as the Solar System itself, 4.6 billion years ago, when an object, perhaps as large as Mars, collided with the early Earth sending debris into space that later formed the Moon. The same collision may have knocked the Earth over resulting in the tilt of the Earth's axis of rotation.

The Moon's phases The Moon is almost perfectly spherical in shape. Over a period of one lunar month (29 days 12 hours), however, the Moon's shape appears to change in a regular and predictable way. Like the Earth, the parts of the Moon that face the Sun are lit by it and experience day (a lunar day) while the parts that face away from the Sun remain in darkness and experience night (a lunar night). The apparent shapes or phases of the Moon are caused by the location of the Moon in its orbit around the Earth and the different amounts of the Moons sunlit side that we can see from it. The line separating the lit part of the Moon from the dark is known as the terminator. From a new Moon, when the Moon is situated in orbit between the Earth and the Sun with its dark side in shadow facing towards us, the amount of lunar surface visible from the Earth increases through waxing crescent, first quarter and waxing gibbous phases until it becomes full. From full Moon, when the Moon is situated in orbit on the side of the Earth opposite the Sun with its lit side facing directly towards us, the amount of lunar surface visible from the Earth decreases through waning gibbous, last quarter and waning crescent phases until it disappears from view again.
Eclipses A total solar eclipse occurs when the Moon is exactly positioned between the Sun and the Earth and the Earth passes through the Moon's shadow. Solar eclipses can only happen during the day and at the time of a new Moon. Total solar eclipses last for only a few minutes. A total lunar eclipse occurs when the Earth is exactly positioned between the Sun and the Moon and the Moon passes through the Earth's shadow. Lunar eclipses can only happen at night and when the Moon is full. Total lunar eclipses last for about 2 hours. While new Moons and full Moons occur every 29 days 12 hours, eclipses only happen once or twice a year. This is because the plane of the Moon's orbit around the Earth is tilted at an angle of 5 relative to the plane of the Earth's orbit around the Sun. Partial solar and lunar eclipses can occur too.


The Universe
The Milky Way Galaxy
Constellations, Surveying the Solar System
The Sun
Planets and their moons
Asteroids, comets and meterorites, Exploring the Earth-Sun-Moon System
Day and night
Seasonal change
Further Reading
Self assessment(1)
Self assessment(2)
Self assessment(3)