Moon Phases

The sun illuminates one side of the moon and leaves the other side in shadow. Anyone can see that the side of the moon facing towards the Sun is bright, and the side facing away from the sun is dark. As the moon orbits the Earth we see these lighted and dark parts of the surface in changing proportions. The changing configurations of illumination on the moon's surface are known as lunar phases. Throughout human history people have used the phases of the moon as a calendar. There are approximately seven days separating the major phases of the moon, which serves as weeks, and each entire cycle is approximately one month. The word "month" is derived from the word "moon."

Lunar Surface, 2009. Photo by Brad Snowder.

New phase occurs when the Sun and Moon are on the same side of the Earth and we see only the dark side. On that day the Sun and Moon rise and set approximately together. The new moon is visible all day long.


Waxing Crescent phase occurs when the western edge of the Moon is lit but most of the surface visible from Earth is dark. The amount of illumination visible is growing from day to day during this phase which is what is meant by "waxing."


First Quarter phase occurs when the western half of the Moon is illuminated so that it looks like the letter "D". On that day the Moon rises at noon and sets at midnight.


Waxing Gibbous phase occurs when the Moon is mostly lit and the illuminated portion is egg-shaped (gibbous) with the eastern edge shaded. The amount of illuminated area visible is increasing from one day to the next which is what is meant by "waxing".


Full phase occurs when the Moon is on the opposite side of the Earth from the Sun and we see only the illuminated side. On that day it rises as the Sun is setting and sets and the Sun is rising. It is visible all night long.


Waning Gibbous phase occurs when the Moon is mostly lit and the illuminated portion is egg-shaped (gibbous) with the western edge shaded. The amount of illuminated area visible is decreasing from one day to the next which is what is meant by "waning".


Third Quarter phase occurs when the eastern half of the Moon is illuminated. On that day it rises at midnight and sets at noon.


Waning Crescent phase occurs when the eastern edge of the Moon is lit but most of the visible surface is dark. The amount of illumination is decreasing from day to day which is what is meant by "waning." During this time the illuminated portion of the Moon looks like the letter "C".

Names of the Moons

American Indian tribes gave names to each of the full moons to keep track of the passing year. The names are associated with the entire month until the next full moon occurs. The ones most commonly adpoted by colonial American settlers where those used by the Algonquin tribes, who lived in the region from New England to Lake Superior. Other Europeans settlers developed their own names.

Since a lunar month averages only 29 days, the dates of the full Moons changes from year to year, but here are some of the traditional titles.