How Moon Phases Work

Here's how the moon changes phases as it orbits the Earth, constantly changing the angle that sunlight hits the moon and is reflected, or not, to our eyes. CREDIT: Starry Night Software

Editor's Note: The moon will be full Saturday, May 9. But do you really know why? Today, SPACE.com launches this new weekly Night Sky feature, to publish on Wednesdays and complement our long-running Night Sky Friday, together designed to help beginning and seasoned skywatchers explore more.

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Fact or fiction? The phases of the moon are caused by the shadow of the Earth falling on the moon.

Fiction! This is probably the most commonly held misconception in all astronomy. Here's how the moon's phases really come about:

The moon is a sphere that travels once around the Earth every 29.5 days. As it does so, it is illuminated from varying angles by the sun. At "new moon," the moon is between the Earth and sun, so that the side of the moon facing towards us receives no direct sunlight, and is lit only by dim sunlight reflected from the Earth. As it moves around the Earth, the side we can see gradually becomes more illuminated by direct sunlight.

After a week, the moon is 90° away from the sun in the sky and is half illuminated, what we call "first quarter" because it is about a quarter of the way around the Earth.

A week after this, the moon is 180° away from the sun, so that sun, Earth and moon form a line. The moon is fully illuminated by the sun, so this is called "full moon." This is the only time in the whole month when the Earth's shadow is anywhere close to the moon. The Earth's shadow points towards the moon at this time, but usually the moon passes above or below the shadow and no eclipse occurs.

A week later the moon has moved another quarter of the way around the Earth, to the third quarter position. The sun's light is now shining on the other half of the visible face of the moon.

Finally, a week later, the moon is back to its new moon starting position. Usually it passes above or below the sun, but occasionally it passes right in front of the sun, and we get an eclipse of the sun.

So, the moon's phases are not caused by the shadow of the Earth falling on the moon. In fact the shadow of the Earth falls on the moon only twice a year, when there are lunar eclipses.

• More Night Sky Features from Starry Night Education

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions. Amateur astronomer Geoff Gaherty operates his own Foxmead Observatory in Coldwater, Ontario, Canada.

Other Moon Resources:

• Skywatcher's Guide to the Moon
• Where the Moon Goes as Phases Change
• Top 10 Cool Moon Facts
• Gallery: Moon Images