Wax On, Wane Off: A Guide to All the Lunar Phases
Your guide to the moon—and its many phases.
It's the brightest object in the night sky. Our moon is our closest planetary neighbor and has inspired countless astronauts to venture into space, been the muse of many writers and sparked the imaginations of science fiction writers around the world.
We can see the moon because light from the sun is reflected off of its surface. But it doesn't always look the same. When Earth is between the moon and the sun, we see a fully lit moon. A waxing and waning moon is growing or shrinking in illumination, respectively. When the moon slips between Earth and the sun, we see a new moon, shrouded in darkness. Because the moon is a sphere like Earth, half of its surface is always illuminated by the sun. The demarcation line between the light and dark side of the moon is called the terminator.
It takes the moon about one month—29.5 days on average—to complete a full lunar cycle, called lunation, and it should come as no surprise that the word "moon" and "month" have the same root.
Here's a quick guide to the moon's phases.
Waxing Crescent
The small sliver of moon that we see in the night sky at the beginning of its growth cycle is called a waxing crescent. It will continue to grow in illumination.
First Quarter
First and third quarter moons occur when the Moon sits at a 90 degree angle between the Earth and Sun. The half of the Moon that we see is illuminated by the sun.
Waxing Gibbous
Gibbous is a term used to describe a moon that is almost fully lit. A waxing gibbous is a moon that is continuing to become more and more illuminated.
Full Moon
During a full moon, the Earth is positioned between the sun and the moon, so the moon is fully lit by the sun, illuminating its many craters and high plateaus.
Why isn't Earth's shadow shown on the lunar surface during the full moon? The moon orbits Earth on a different orbital plane than the one on which Earth orbits the sun.
On occasion, these orbital planes align when the moon swings through Earth's shadow. This is called a lunar eclipse.
Waning Gibbous
Waning gibbous describes the moon when its illumination is shrinking. less and less of it will be lit up in the night sky.
Third Quarter
Like the first quarter moon, this half moon is sits at 90 degree angle between the Earth and Sun. This time, the other half of the Moon is illuminated.
Waning Crescent
The waning crescent moon's illumination is shrinking. It is drawing closer and closer to a new moon each night.
New Moon
The new moon, a mysterious moon.
When the moon falls between the sun and the Earth, the sun illuminates the back side of the moon so that it appears dark from Earth's perspective.
Similar to the full moon, the moon doesn't cast a shadow on Earth, an event known as a solar eclipse, because of that 5 degree difference in the Earth and moon's orbital planes.
Jennifer Leman is a science journalist and senior features editor at Popular Mechanics, Runner's World, and Bicycling. A graduate of the Science Communication Program at UC Santa Cruz, her work has appeared in The Atlantic, Scientific American, Science News and Nature. Her favorite stories illuminate Earth's many wonders and hazards.
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