The above mosaic shows the landing site of Apollo 15 (1971 July). The mountain is Hadley Delta at the foothills of Montes Apenninus ¡Map 14. Astronauts called it a "featureless mountain" (quite true in this picture). They found lunar scenery almost white or black. They also reported that distances on the Moon were hard to estimate, partly because the Moon's surface curves more sharply than that of the Earth and hence the horizon is closer, partly because there is no atmosphere and hence no softening of shadows. Note that on the airless Moon, the sky is dark even at daytime.

Here is the shadow effects experienced by the Apollo astronauts on the Moon: http://science.nasa.gov/headlines/v2006/03ian moonshadows.htm

Although the Moon is physically a barren world compared to our vivid Earth, yet there are lots of surface features visible in telescopes. See these funny highlights:

Funny Features on the Moon

Under certain illuminating conditions, the lunar features may look conspicuous and funny in telescopes. Below are the glimpses of them. Their details are traceable from the |MAP| pages.

Funny Features on the Moon 1. Crab shell 2. Tortoise 3. Pancake 4, Tropical fish 5. Sword 6, Elephant with swirling nose 7. ETface 8. White-head snake 9 Man walking with torch 10, Cat face 11. Spectacles 12. Lobster 13 Rope-jumping rabbit 14. Peanut shell 15, Helmet (South is up.)

Age of the Moon

The age is the number of days that has elapsed since the last new moon. In average, the new moon repeats every 29.53 days; this period is called synodic month or one lunation. Because the Moon's and Earth's orbital speeds are not uniform, the first quarter, the full moon and the last quarter may occur slightly earlier or later than the indicated age. It is also possible to have two full moons in a calendar month, e.g. 2004 August and then 2007 June (once every 33 months). "Blue Moon" is an ambiguous term for the second full moon in the same calendar month. Rarely the Moon is tinted blue unless there is an excessive amount of dusty particles in the atmosphere.

In the drawing below, note also the Moon must orbit about the Earth more than 3600 from new moon to next new moon.

Perigee and Apogee

The angular diameter of the Moon is about 0.50, equivalent to viewing a pencil thickness at an arm's length. It is not constant but changes according to the instantaneous Moon-observer distance. Thus the Moon appears about 1.5 % bigger at zenith (overhead) than at horizon, though this change is quite small. To an observer on the Earth's equator, the Moon's angular diameter can vary up to 34.1 arcminutes at perigee (closet to Earth) or down to 29.8 arcminutes at apogee (farthest from Earth), a total change of almost 14 %. The average is 31.6 arcminutes, when the Moon is 378,000 km (30 times Earth's diameter) from the observer. A lunar crater of 2 km diameter and facing Earth gives a visual angle of about 1 arcsecond.

Greatest high tide, also called perigean spring tide, will occur when the Moon is at perigee and when the Sun, the Earth and the Moon (whether new moon or full moon) are aligned to reinforce gravitational interactions. Such high tides are not uncommon and are notable at some coastal spots in Hong Kong, e.g. on 2005.01.10~11, the new moon was at perigee; the river in Shatin overflowed the bank during the high tide.

Actually the Earth and the Moon move in slightly elliptical orbits about the center of mass of the system which acts as if all the mass were concentrated there. The Earth and the Moon are always in opposite sides of the center of mass. They go around this center once every sidereal month (27.322 days).

The center of mass is about 1900 km below the Earth's surface when the Moon is at perigee, and 1400 km below the Earth's surface when the Moon is at apogee.

The center of mass is not stationary. It follows the Moon's orbital motion to sweep an elliptical loop inside the Earth globe.

Over time, the friction of tidewater slows down the Earth's axial spin by 0.0017 second per century. This loss of spin is absorbed in the Moon's orbital motion by the principle of conservation of angular momentum. It causes the Moon to spiral away from the Earth at average of 3.8 cm per year. However, the Moon remains with the same face (nearside) locked towards Earth. The Moon's distance shall not increase forever. It would stabilize billion years later, probably at 550,000 km. By that time, one lunation would be lengthened to 1.8 times of present; the Earth's rotation would be dramatically slowed down; or perhaps the Sun might have expanded to a red giant swallowing up the Earth and the Moon !

(The Moon's elliptical orbit is exaggerated to show its perigee and apogee.)

River overflowed a bank during perigean spring tide. This picture was taken shortly after tidewater retreat.

(The Moon's elliptical orbit is exaggerated to show its perigee and apogee.)

River overflowed a bank during perigean spring tide. This picture was taken shortly after tidewater retreat.

The Earth-Moon System

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