Phases of the Moon

Brown bear with four cubs, Lake Clark National Park

Brown bear with four cubs, Lake Clark National Park, Alaska

The phase of the Moon is essentially the same everywhere on earth, independent of the position of the observer. During the full moon the moon is on the far side of the earth from the sun. The full moon rises at sunset and sets at sunrise.  The new moon sets and rises with the sun, making it especially difficult to see.

Beginning at the full moon, each night the moon rises 50 minutes later and the portion with sunlight decreases until in about two weeks the moon is almost invisible and rises and set in synchrony with the sun. This is the waning moon. During the next two weeks the moon rises during the daylight hours, a little later each day (waxing phase). During the waxing phase the sickle is turned to the west, during waning it is turned to the east. Eventually the moon becomes full and rises at sunset - and the 29.53 day cycle repeats.

The moon rises later each day because, when viewed from above the North Pole, both the earth and moon rotate in a counterclockwise direction. The full moon actually rises at 6 PM and sets at 6 AM. It doesn't obey the seasons the way the sun does.

Why can we usually see the dark portions of the moon? As observed by Galileo, earth light also shines on the moon and allows the dark portions to be visible. The light goes from the sun to the daytime portion of the earth to the dark portion of the moon and back to the dark (night) portion of the earth and there's still enought left for our eyes to see.

The moon is not symmetrical and gravity causes the same side to always face the earth. The side facing away from the earth is generally referred as the dark side of the moon, even through it is not always dark.

Figure 1. Phases of the moon (dynamic link to: http://aa.usno.navy.mil/graphics/Moon_phases.jpg)

The site: http://aa.usno.navy.mil/data/ has free calculators for sunrise, sunset, and phases of the moon.

 

 


Return to Window Outdoors