The lunar geologic timescale (or perhaps more properly the selenologic timescale) divides the history of Earth's Moon into six generally recognized geologic periods: Earth is the third planet from the Sun. ... For other moons in the solar system see natural satellite. ... In geology, a period or age is a time span of many millions of years that are assumed to have had similar characteristics. ...
The values for these dates remain in some dispute, as the divisions are based on the dates when various significant lunar geological events occurred and it is difficult to pinpoint them exactly. The majority of lunar samples from which the time scale was developed are basalts of Imbrian age. Samples from before and after this period are less abundant, and the older samples have been affected by violent impact events that make precise radiometric age determination difficult. The Copernician Period in the lunar geologic timescale runs from approximately 1100 million years ago to the present day. ... The Eratosthenian period in the lunar geologic timescale runs from 3,200 million years ago to 1,100 million years ago. ... In the Lunar geologic timescale, the Upper Imbrian epoch occurred between 3800 million years ago to about 3200 million years ago. ... In the Lunar geologic timescale, the Lower Imbrian epoch occurred between 3850 million years ago to about 3800 million years ago. ... The Nectarian Period of the lunar geologic timescale runs from 3920 million years ago to 3850 million years ago. ... The Pre-Nectarian Period of the lunar geologic timescale runs from 4550 million years ago (the time of the initial formation of the Moon) to 3920 million years ago, when the Nectaris Basin was formed by a large impact. ... Basalt Basalt is an extrusive igneous rock, sometimes porphyritic, and is often both fine-grained and dense. ...
In many lunar highland regions, planetologists cannot currently distinguish between Nectarian and Pre-Nectarian materials. These deposits are called Pre-Imbrian age materials, which encompasses both time periods.
The plane of the lunar orbit maintains an inclination of 5.145 396Â° with respect to the ecliptic (the orbital plane of the Earth around the Sun), and the lunar axis of rotation maintains an inclination of 1.5424Â° with respect to the normal to that same plane.
The points where the Moon's orbit crosses the ecliptic are called the "lunar nodes": the North (or ascending) node is where the Moon crosses to the North of the ecliptic; the South (or descending) node where it crosses to the South.
Lunar Prospector results, however, indicate the presence of hydrogen in the permanently shadowed regions, which could be in the form of water ice.
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