Luna

Luna, Planet Earth's moon is its only natural satellite.

Luna is usually the brightest object in the night sky but gives off no light of its own, reflecting light from the Sol, our sun. Like Earth and the rest of the Solar system, Luna is about 4.6 million years old.

Luna is much smaller than Earth. Luna's average radius (distance from its centre to its surface) is 1,737.4 km), about 27% of the radius of Earth. Luna is also much less massive than Earth. Luna has a mass (amount of matter) of 7.35 x 10¹⁹ tonnes. Earth is about 81 times that massive. Luna's density (mass divided by volume) is about 3.34 g/cm³, roughly 60% of Earth's density.

Because Luna has less mass than Earth, the force due to gravity at the lunar surface is only about 1/6 of that on Earth. Thus, a person standing on Luna would feel as if his or her weight had decreased by 5/6. And if that person dropped a stone, the stone would fall to the surface much more slowly than the same stone would fall to Earth.

Despite Luna's relatively weak gravitational force, it is close enough to Earth to produce tides in Earth's waters. The average distance from the centre of Earth to the centre of Luna is 384,467 km. That distance is growing, but extremely slowly. Luna is moving away from Earth at a speed of about 3.8 cm per year.

The temperature at the lunar equator ranges from extremely low to extremely high: from about –173°C at night to +127°C in the daytime. In some deep craters near the lunar poles, the temperature is always near –240°C.

Luna has no substantial atmosphere, but small amounts of certain gases are present above the lunar surface. People sometimes refer to those gases as the lunar atmosphere. This "atmosphere" can also be called an exosphere, defined as a tenuous (low-density) zone of particles surrounding an airless body. Mercury and some asteroids also have an exosphere.

Luna has no life of any kind. Compared with Earth, it has changed little over millions of years. On Luna, the sky is black (even during the day), and the stars are always visible.

References

Moon, World Book at NASA, 30 November 2007