Shop Windows to the Universe

The Universe at Your Fingertips 2.0 DVD from the Astronomical Society of the Pacific is in our online store, filled with Earth and space science resources.
A diagram which demonstrates why the moon goes through phases.
Click on image for full size

Phases of the Moon

The diagram shows the Earth, and the Moon in different positions along its orbit around the Earth. The Sun is off in the distance, lighting the Earth-Moon system. At any position, half of the Moon is reflecting light from the Sun (the light side of the Moon) and half is in shadow (the dark side). Also, half of the Moon can be seen from Earth (the near side of the Moon) and half can not (the far side). As the Moon moves around the Earth, the dark side and the far side overlap and produce the phases of the Moon as we are familiar with them.

When the Moon is between the Earth and the Sun (1), the near side of the Moon is the dark side. The Moon cannot be seen. We call this New Moon, the beginning of the cycle of lunar phases. When the Earth is between the Sun and the Moon (5), the near side is the light side. We call this Full Moon.

Halfway in between these times (3 & 7), only half of the near side of the moon is illuminated by the Sun. So we can only see one quarter of the Moon. We call these phases First and Third Quarters.

All the phases of the Moon have special names which indicate how much of the illuminated Moon can be seen from Earth, and whether this part is going to grow or shrink.

Last modified October 19, 2005 by Randy Russell.

Shop Windows to the Universe Science Store!

The Fall 2009 issue of The Earth Scientist, which includes articles on student research into building design for earthquakes and a classroom lab on the composition of the Earth’s ancient atmosphere, is available in our online store.

Windows to the Universe Community

News

Opportunities

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

1998 Perseids Meteor Shower

This summer (and its movies!) has brought with it an awful lot of speculation about things hitting or falling into the Earth. Well, the speculation is over! The Perseids meteor shower makes its annual...more

What are the flat surfaces on the Moon called? What is maria? What are lunar highlands?

What phase was the Moon in on December, 22 1962? How long does it take the Moon to travel from one phase to the next? Suppose that the Moon spun twice on its axis during each orbit around the Earth. How...more

Full Moon Names

You may have heard people refer to a Full Moon in the autumn as the "Harvest Moon" or the "Hunter's Moon". Native Americans in the eastern and northern parts of North America had special names for the...more

Orionid Meteor Shower

The Orionid meteor shower is one of several major meteor showers that occur on roughly the same date each year. The Orionids typically "peak" (are at their greatest level of activity) around October 21st....more

Leonid Meteor Shower

The Leonid meteor shower is one of several major meteor showers that occur on roughly the same date each year. The Leonids typically "peak" (are at their greatest level of activity) in mid to late November....more

Geminid Meteor Shower

The Geminid meteor shower is one of several major meteor showers that occur on roughly the same date each year. The Geminids typically "peak" (are at their greatest level of activity) in mid December....more

Quadrantid Meteor Shower

The Quadrantid meteor shower is one of several major meteor showers that occur on roughly the same date each year. The Quadrantids "peak" (are at their greatest level of activity) in early January. In...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF