 The Earth's Moon

 Mass 7.349 × 1022 kg Mass (Earth = 1) 1.2298 ×10-2 Equatorial Diameter 3,474.8 km Equatorial Diameter (Earth = 1) 0.27241 Mean Density 3.34 × 103 kg m-3 Rotational Period 27.32166 days Average Length of Lunar Day 29.53059 days Mean Distance from Earth 384,400 km Orbital Period 27.32166 days Orbital Eccentricity 0.0549 Orbital Inclination 5.1454° Mean Orbital Velocity 1.03 km s-1 Axial Tilt 1.5424° Surface Gravity 1.62 m s-2 Escape Velocity 2.38 km s-1 Albedo 12 % Apparent Magnitude (Full Moon) -12.74 Solar Irradiance 1367.6 W m-2 Mean Surface Temperature (Daytime) 107°C Mean Surface Temperature (Night) -153°C Maximum Surface Temperature 123°C Minimum Surface Temperature -233°C

Mass

Mass is the amount of matter that an object contains. On the Earth mass can be measured by weight. The Moon's mass is 81 times less than the Earth's. All satellites have less mass than their planets.

Equatorial Diameter

The Moon's diameter is a quarter that of the Earth.

Density

Density tells how concentrated the matter in a planet is. The Moon is about half the density of the Earth.

Rotational Period

This is how long the Moon takes to rotate once on its axis. As for many satellites, the period of rotation is the same as the orbital period. One side of the Moon always faces the Earth. We can never see the far side. This is due to tidal effects from the Earth slowing down the rotation of the Moon in the past.

Average Length of Lunar Day

Each point on the Moon experiences day and night. On the Moon, the Sun is visible for 2 weeks and below the horizon for 2 weeks. There is no permanent "dark side of the Moon".

Mean Distance From Earth

The Moon travels around the Earth in an elliptical orbit. Its closest distance is 363,300 km; its farthest distance is 405,500 km. The figure given is the average distance.

Orbital Period

This is how long the Moon takes to revolve around the Earth. The period of revolution is identical to the rotational period. This period is called the Sidereal Period.

Orbital Eccentricity

The orbits of most objects in the Solar System are ellipses. This curve resembles a flattened circle. The eccentricity describes how much the ellipse differs from a circle. An orbit with an eccentricity of 0 is a circle. An orbit with an eccentricity of 1 would be an open curve called a parabola. The Moon's orbit has a low eccentricity, close to zero.

Orbital Inclination

The planets and satellites generally revolve around the Sun in almost the same plane. This figure shows how many degrees the Moon's orbit differs from the Earth's orbit. It is this difference that makes eclipses rare. If the orbital inclination were 0° (or close to it), eclipses would occur every month.

Mean Orbital Velocity

This is the Moon's average velocity in orbit. The Moon changes its velocity as it travels in its elliptical orbit. It moves faster when it is closer to the Earth in accordance with Kepler's Second Law.

Axial Tilt

The Moon rotates on its axis. This is the angle that the axis is tilted relative to the plane of the Moon's orbit. The value is far smaller than the Earth's.

Surface Gravity

On the Earth, the acceleration due to gravity is 9.806 65 meters per second per second. The Moon's surface gravity is a sixth of the Earth's.

Escape Velocity

This is the speed that an object must attain in order to escape from the Moon's gravitational field. For the Earth this speed is about 11 kilometres per second (7 miles per second).

Albedo

This is the percentage of sunlight that is reflected by the Moon. Its dark rocks reflect only 12% of the light falling on them.

Apparent Magnitude

Apparent Magnitude tells how bright an object is as seen from the Earth.