The Moon

Neither Mercury nor Venus possess natural satellites of their own, so the Earth is the first body (from the Sun) to possess one. The same hemisphere of the Moon always faces the Earth. Until the beginning of the space age no one had seen the far side, which was first imaged by the Russian probe Luna 3 on 7th October 1959.

The Moon

The Moon is the second largest natural satellite relative to the parent planet in the Solar System, and its very presence may have induced regularity in the rotation of the Earth and aided the evolution of life upon it. The table below provides some information on the Moon, including orbital characteristics (Illingworth, 1994).

Satellite Diameter
Radius (Km)
Eccentricity Orbital
Period (days)
The Moon 3 476 384 400 0.055 27.32 23.4


Phases of the Moon

The animation below (not to scale; looking down on the north pole) illustrates the Moon's motion around the Earth relative to the Sun, explaining its phases as observed on Earth.

Phases of the Moon animation



The formation of the Moon has been a point of debate for many years in the astronomical community, but the most convincing and generally accepted theory is that a large Mars-sized body collided with a primieval Earth soon after it was formed, throwing a huge cloud of debris into orbit around the Earth. Over several millions of years, it collected together and became the Moon.


Geological History

The following table shows the lunar stratigraphic timescale, in which the times shown are in billions of years before the present (Ga). It also shows the main events of each eon. (Sources: Greeley, 1994; Illingworth, 1994)

Time (Ga) Eon Events
1.00 - 0.00 Copernican  Dates from the formation of the crater Copernicus
 Little or no volcanism
 Some cratering, rays visible on most large craters
3.15 - 1.00 Eratosthenian  Characterised by the formation of craters similar to Eratosthenes
 Further effusion of high-titanium basaltic lava early in this eon, forming the last 1/3 of the visible lunar maria
 Some low-titanium lava-emplacement at around 2 Ga
 Cratering, rays not visible
3.85 - 3.15 Imbrian  Dates from the formation of the Imbrian Basin
 KREEP volcanism ends before 3.5 Ga
 Orientale Basin forms, along with a few other large basins
 Widespread effusion of basaltic lava to the surface, forming approximately 2/3 of the visible lunar maria
3.92 - 3.85 Nectarian  Dates from the Nectaris Basin impact and the formation of its ejecta blanket, known as the Janssen Formation
 Contains four times the density of large impact craters as the younger Imbrian Basin
 Formation of other large impact basins, such as Crisium and Humorum
 Some volcanic activity
4.57 - 3.92 Pre-Nectarian  Dates from the formation of the Moon, and the last phases of its accretion
 High density of cratering and impact basins
 High-aluminium lunar-sea lavas emplaced
 KREEP volcanism begins at about 4.1 Ga [K = potassium; REE = rare earth elements; P = phosphorous]
 Heating caused by high impact rate causes the outer few hundred kilometers to melt and consequent development of an anorthositic crust
 Deep-seated volcanism by the end of the eon


Missions to the Moon

For a list of missions to the Moon see the links page.

Earthrise (12 KB)

[ NASA / JPL / Caltech ]