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The Moon


Diameter : 3,500 km
Distance from Earth 363,295 km (perigee), 405,503 km (apogee)
Time to Complete One Orbit (Sidereal) 27.3 days
Time to Complete One Orbit (Synodic) 29.5 days
Speed in Orbit about Earth 1.02 km/h
Rotation Period 1 Sidereal Month


Earth has opposite phases

400 BC meton

Sidereal Month and Synodic Month

The Sidereal Month, i.e. with respect to the stars is

27 1/3 days - 27d 7h43m11.5s

Synodical Month 29.5

The Moon travels east with respect to the stars, meaning that it rises later each night. The time lapse between moon rise on successive nights is known as the retardation. This retardation can be of the order of an hour, although the Harvest Moon in September only rises about 15 minutes later each evening. This variation is due to the inclination of the Moon's path to the ecliptic.


Because of elliptical orbit, a libration of longitude allows our perspective to vary by 7°.

Because of the Moon's imclination to ecliptic of 5° and its spin axis is 1.5° to this, a libration of latitude of up to 6.5° occurs.

Diurnal libration occurs because the Earth is not a point but an extended body. An observer on the Earth can see up to about 1° extra around the edge.

Altogether we are able to view 59% of the surface at one time or another.

Lunar Eclipse

During an eclipse the moon will be full.

The maximum number of solar and lunar eclipses possible in one year is 7.

Eclipse can only occur at the nodes These nodes move slowly westwards (regression of the nodes) because of the Sun's gravitational pull.

When a lunar eclipse is seen it is visible from a complete hemisphere of the Earth (unlike a solar eclipse). It can be either a total eclipse, when the Moon is in the Earth's umbra, or a partial eclipse, when the Moon is in the Earth's penumbra (which is not too easy to detect).

Totality can last for up to 1 hour 44 minutes. By virtue of the bending of some sunlight thru the Earth's atmosphere, the Moon is always visible during a lunar eclipse. The color is often described as 'coppery' - the exact nature varies from one eclipse to the other.

Eclipses were used by the ancient Greeks to deduce that the Earth was a sphere. This was mentioned by Aristotle (around 300s BC). (by 200BC, they had a fairly accurate measure of the size of this sphere, see Eratosthenes).


moon worst view at full moon. Seem best near the terminator when the floor is filled with shadow, either fully or partially.




Bombardment by heavy object terminator mass 1.2% that of Earth's

Moon showing Apollo Landing Sites

The map is 'the wrong way up' showing the view which would be seen in a telescope.

Crab Nebula

Big Red Spot on Jupiter