The Moon is the nearest celestial body to the Earth, orbiting at an average distance of 384,400km. However, the lunar orbit is rather eccentric, causing it to come as close as 363,300km at perigee and as far as 405,510km at apogee. Interestingly, the Moon is the largest satellite when compared to its planet. The Moon is one quarter the size of the Earth.

However, the most important fact about the Moon is that it is the only other body in the universe upon which human beings have walked. This is indeed a singular distinction.

Due to tidal interactions between the Earth and Moon, it always keeps one face toward us, rotating on its axis once per revolution. As it revolves around the Earth, the Moon appears to go through phases.

The Facts

• Equatorial Radius (km): 1738
• Mass (kg): 7.35x10²²
• Mean Density (g/cm³): 3.34
• Period of Revolution Around the Earth¹ (days): 27.3217
• Period to Revolve Around the Earth with Relation to the Sun² (days): 29.5

The Formation of the Moon

The Moon is believed to have formed approximately 4.7 billion years ago when an object the size of Mars collided with the primordial Earth. The force of this collision released an enormous amount of debris into space, some of which settled into orbit around the Earth until it accreted into the Moon.

At about 4.6 billion years ago, the Moon began to cool and the first solid crust began to form. As the molten surface began to cool, huge asteroids and meteorites began their bombardment of the surface, a process which continued for millions of years.

At about the 4 billion year mark, the bombardment slowed down. Around the same time, the interior burst forth with great floods of lava, pooling in the bottom of some of the larger impact basins, thus creating the lunar maria which can be seen from Earth. These lunar 'seas' are found primarily on the near-side of the Moon, drawn by the same tidal influences that keep that face permanently pointed earthwards. The Moon's far side has a thicker crust of 100km, as opposed to 60km on the near side, which is also due to the tidal influence of the Earth.

After the formation of the maria, the impacts decreased to a rate which has remained more or less what we find today. Of course the far side is exposed to more impacts because the near side is shielded by the bulk of the Earth. Since seismic instruments were left on the lunar surface by the Apollo astronauts, scientists have recorded an average of 150 impacts per year, with the largest being a one-ton impact on the lunar far side in 1972.

Observing the Moon

For centuries, the Moon has been a favourite target for astronomers. Obviously, it is the nearest and largest object in the night sky. When using a small telescope or binoculars, the best details can be found near the lunar terminator - the line separating night from day on the Moon's surface. A large instrument should be fitted with a lunar filter because even the weak light of the Moon can cause eye damage if viewed for extended periods through a large light-gathering telescope.

There are three lunar features that are easy to spot with the naked eye, a pair of binoculars or a small telescope.

• Maria - These lunar 'seas' (as they were called by the ancient astronomers) are the dark patches visible on the Moon's surface. Visible in the exact center of the first quarter moon is the most famous of all lunar seas, Mare Tranquilitatis, the 'Sea of Tranquillity', which was the site of the first manned Apollo landing. Other watery terms used to describe lunar features include: Oceanus (Ocean); Lacus (Lake); Sinus (Bay); and Palus (Swamp).

• Craters - Named mostly after scientists, these objects are the result of meteoroid impacts. They can range in size from microscopic to more than 150 miles in diameter. Many craters have central peaks and ray systems extending away from the crater. The Copernicus and Kepler craters display excellent ray systems across the maria of the last quarter moon. Also visible at last quarter, the Tyco crater's ray system extends across much of the lunar surface. It is interesting to note that well into this century many astronomers believed that lunar craters were extinct volcanoes.

• Highlands - The light-coloured bits of the lunar surface, this is the oldest and most heavily cratered surface on the Moon. The mountains of the Moon are usually named after mountain ranges here on Earth. A small telescope can often reveal long shadows extending from these mountainous regions.

The Moon's Future

The Moon is receding from Earth about 1.6km every 28,000 years due to the complex gravitational interactions with the Earth. If this rate continues, the Moon from Earth will appear 15 percent smaller in about one billion years.

The month of February will be very unusual in 2.5 million years. There will be the rare event of a month (February) with no full moon. The last time this occurred was in 1866.

If undisturbed, the footprints on the Moon's surface left behind by the Apollo astronauts will remain visible for some 10 million years.

Lunar Influences

Ask any policeman or emergency room worker, and they'll tell you there is a definite relationship between human behaviour and the lunar cycle. Increases in crimes, traffic accidents and patient admissions to mental hospitals all increase during the full moon³.

There may be a connection between the Moon, love and sex. On average, the menstrual cycle for women is exactly the same duration as the lunar synodic month (29.5 days) and the human gestation period is exactly nine times the lunar synodic month (265.8 days). Furthermore, studies have found that there are more births during the full and new moon. Also, there are more male children born after the full moon and more female children are born after the new moon.

The h2g2 Tour of the Solar System

Take the h2g2 Shuttle for your whistle-stop tour of the Solar System.

• Next Stop (Inward Journey):
• Earth
• Explore Locally:
• Lunar Phases
• Lunar Eclipses
• The Moon and the Tides
• Total Solar Eclipses
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• A87842037