Earth's Moon

Earth is the first planet from the Sun to have a moon. Despite being Earth's closest neighbor, the Moon is very different from the Earth. It has no atmosphere and the surface is billions of years old. The dark areas in the image below are called maria. They are regions that were struck by large asteroids in the past, which caused molten rock from beneath the crust of the Moon to seep out. The Moon has hardly any magnetic field, although rocks studied on the Moon show evidence that in the past, the Moon probably had a small, but stronger magnetic field than it has now.


Moon Factbox
Average distance from Earth (center to center) 384,400 km = 238,900 miles
Average orbital speed 3680 km/h
Diameter 3476 km = 2160 miles
Mass 7.349 x 10^22 kg = 0.0123 Earth mass
Average density 3344 kg/m^3
Escape speed 2.4 km/s
Surface gravity (Earth = 1) 0.17
Average surface temperatures Day: 130°C = 266°F = 403 K
Night: -180°C = -292°F = 93 K
Atmosphere Almost none

The Moon's Exterior

The Moon is much smaller than the Earth with a diameter of about 3476 km or 2160 miles. It is about 384,000 km or 238,900 miles from Earth.

The surface of the Moon is covered with craters because the Moon doesn't have an atmosphere to protect it from incoming asteroids and debris, and it has no plate tectonics that would shift the surface and cause craters to disappear over time.

The rocks brought back to Earth from the Moon are all between about 3 and 4 billion years old. They are all igneous rock, and most likely formed when the Moon was molten, and unlike rocks on Earth, contain absolutely no water.

The Moon's Interior

The Moon has a crust, upper and lower mantles, and and iron-rich core in the center. The average thickness of the crust is about 60 km on the side facing the Earth and about 100km on the back side of the Moon. 


The Moon's History

The most widely accepted theory for the creation of the Moon is called the Collisional Ejection Theory or Giant impact hypothesis. This theory is that when Earth was forming, a large, Mars-sized object slammed into it, causing a large mass to be ejected. This mass became our Moon.

Phases of the Moon

The Moon goes through phases every month, where different amounts of the Moon's surface appear bright. This is because as the Moon orbits the Earth, one side of the Moon is always facing the Sun. As the Moon's orbits, we see the Moon in different positions, and in these different positions we see varying amounts of the surface lit by the Sun.


Credit: This diagram based on a similar diagram

The lunar phase depends on the Moon's position in orbit around the Earth. This diagram looks down on Earth from north. Earth's rotation and the Moon's orbit are both counter-clockwise here with sunlight is coming in from the right. The times marked on Earth show the local time at various location around the Earth. This diagram shows, for example, that the full moon will always rise at sunset and that the waning crescent moon is high overhead around 9:00 am local time.

We always see the same face of the Moon because it is in a synchronous orbit with Earth. Each time it orbits the Earth it spins once on it's axis. As a result it keeps the same face toward Earth.


Lunar Eclipses:

As the Moon orbits the Earth, it occasionally moves into Earth's shadow. This happens only a handful of times each year because the Moon's orbit around the Earth is inclinde at a 5° angle to the plane of the Earth's orbit around the Sun. For the Moon to be in position to move into the Earth's shadow, it must be in the full moon position. On average two or three times a year the Moon happens to be at a point not far above or below the plane of the Earth's orbit during a full moon and a lunar eclipse can be observed from Earth. Most of the time, when the Moon is in full moon position, it is either above or below the plane of Earth's orbit and does not come into Earth's shadow.

The Earth casts two shadows called the umbra and penumbra. The umbra is a completely dark area where light from the Sun is totally blocked. The penumbra is an area where much, but not all light coming from the Sun is blocked. 


There are three main types of eclipses. A lunar eclipse is visible to all parts of the Earth where it is night time during the eclipse. Sometimes the Moon travels only through the penumbra. This is called a penumbral eclipse, and the Moon does not appear much dimmer than usual. These eclipses are not very noticable. Sometimes the Moon travels into the penumbra and partly into the umbra. Such an eclipse is called a partial eclipse. And sometimes the Moon travels through the umbra and appears to go almost completely dark. This is called a total eclipse. During a total eclipse, the moon will appear slightly red because some sunlight travels through the Earth's atmosphere and reaches the Moon.

Solar Eclipses:

A solar eclipse happens when the Moon blocks sunlight from reaching part of the Earth. A solar eclipse can only happen when the Moon is in the new moon position, and can only be observed for a few moment by observers in a small strip of Earth's surface called the path of totality. Outside the path of totality is a larger region where a partial solar eclipse can be observed.


Please note: It is very important to use a special filter approved for solar viewing when observing any type of solar eclipse. Permanent eye damage or blindness could result otherwise.

The Exploration of the Moon

Between July 20th, 1969 and December, 1972, there were 6 manned lunar landings. Lunar missions began in 1959 with unmanned lunar landers and orbiters. Since 1972, many unmanned landers and orbiters have been sent to the Moon.