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 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 (Km) |
Orbital Radius (Km) |
Eccentricity | Orbital Period (days) |
Inclination (°) |
|---|---|---|---|---|---|
| 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.
Formation
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.
[ NASA / JPL / Caltech ]