Moon Phase Explaination Page

For the diagram on the right, the Sun is off the right hand side. The Earth spins about it's axis about every 24 hours. At the same time the Moon is slowly going around the Earth every 29 days 12 hours 44 minutes calculated with respect to the sun.

The moon is visible by reflection of sunlight and has a slightly elliptical orbit, approximately 356,000 kilometers (221,600 miles) distant at perigee and 406,997 kilometers (252,950 miles) at apogee. Its mean diameter is 3,475 kilometers (2,160 miles), its mass is approximately one eightieth that of Earth.

New Moon
During a New Moon we can see NO portion of the Moon's surface that is lit. This phase typically marks the beginning of the Lunar Cycle which lasts 29.5 days. The Moon is between the Earth and the Sun.



First Quarter
During the First Quarter phase, we see the right half of the lit Moon.





Full Moon
During the Full Moon phase, we see the whole Moon lit. At this time the Earth is between the Sun and the Moon.




Last Quarter
During the Last Quarter phase, we see the left half of the lit Moon. After the last quarter, the Moon will slowly go to new Moon to repeat the 29.5 day cycle.




The side of the Moon that faces us is the only side we see from Earth.
The Moon is locked to the Earth so that the same side is seen all the time, other than +/- 9 wobble (nutation).
All the craters on the far side of the Moon we can never see from Earth. All the craters on the far side have Russian names! Russia had the first spacecraft to go around the far side of the Moon.