The moon, Earth's only natural satellite, is large as moons go. It is fifth in diameter among planetary satellites, more than two-thirds as large as Mercury, and more than three times the diameter of the largest asteroid. It is, in fact, over one-fourth the size of the earth, with a diameter of 2160 miles (3476 kilometers).


Since the moon is a relatively near neighbor, we can measure its distance easily by geometrical methods. The average is 238,857 miles (384,403 kilometers).


Next to the sun, the full moon is the brightest object in the heavens. However, its surface is rough and brownish and reflects light very poorly. In fact, the moon is about the poorest reflector in the solar system. The amount of light reflected by a celestial object is called the albedo (Latin: albus, white). The moon relects only 7% of the sunlight that falls upon it, so the albedo is 0.07.


The phases of the moon are caused by the relative positions of the earth, sun, and moon. The moon goes around the earth, on average, in 27 days 7 hours 43 minutes.

The sun always illuminates the half of the moon facing the sun (except during lunar eclipses, when the moon passes thru the earth's shadow). When the sun and moon are on opposite sides of the earth, the moon appears "full" to us, a bright, round disk. When the moon is between the earth and the sun, it appears dark, a "new" moon. In between, the moon's illuminated surface appears to grow (wax) to full, then decreases (wanes) to the next new moon.

The edge of the shadow (the terminator) is always curved, being an oblique view of a circle, giving the moon its familiar crescent shape. Because the "horns" of the moon at the ends of the crescent are always facing away from the setting or rising sun, they always point upward in the sky. It is fun to watch for paintings and pictures which show an "impossible moon" with the horns pointed downwards.

The Far Side

People often refer to "the dark side of the moon", but there is no such thing. The sun shines on all sides of it in turn. However, there is a "far side of the moon" which is never seen from the earth. Over the eons, the gravitational forces of the earth have slowed down the moon's rotation about its axis until the rotational period exactly matches the revolution period about the earth.

You can see this effect by using two round objects such as softballs. Hold one of the balls stationary, to represent the earth. Now move the other ball around the "earth" without twisting your wrist. You will see that people on the "earth" would see all sides of the "moon". However, if you slowly spin the "moon" on its trip around the "earth", you will see that you can time it so only one side of the "moon" is ever seen from the "earth". That's why the features you see on the face of the moon never change.



Related Public Forum Threads

Moon-Landing: Was it real? - Do you believe the moon-landing ever took place, or do you think it was just a big cover-up?


1. - NASA Explorations
2. - National Space Science Data Center
3. - NASA Data Center Data Sheet

Orbit: 384,000 km
Diameter: 1738.1 km
Mass: 10e24 kg
Rotational period: 27 days 7 hrs. 7 mins.
Orbital period: 27 days 7 hrs. 7 mins.
Mean orbital velocity: 1.023 km/sec
Orbital eccentricity: 0.0549
Orbital inclination: 5.1454 degrees
Escape velocity: 2.38 km/sec
Visual geometric albedo: 0.12
Magnitude: +0.21