What Are Moon Phases?

Lunar phases
This image shows the phases of the Moon and why they happen. The center ring shows the Moon as it orbits around the Earth, as seen from above the north pole. Sunlight illuminates half the Earth and half the moon at all times. But as the Moon orbits around the Earth, at some points in its orbit the sunlit part of the Moon can be seen from the Earth. At other points, we can only see the parts of the Moon that are in shadow. The outer ring shows what we see on the Earth during each corresponding part of the moon's orbit. NASA

Next time you're out and see the Moon, notice what shape it is. Does it look round and full? Or more like a banana or a lopsided ball? Throughout each month, the Moon appears to change shape while it appears in the sky at different times. You can observe these changes as they happen, and they may even surprise you. The shapes are called "lunar phases".

Gradual Change You Can Measure from Your Back Yard

A lunar phase is simply the shape of the sunlit part of the Moon as seen from Earth.

That shape changes for the following reasons:

  • The Moon orbits Earth.
  • Both Earth and the Moon orbit the Sun.
  • The Moon's orbit is the same length as the time it spins on its axis (about 28 Earth days), which means that we see the same part of the lunar surface all month.
  • The Sun illuminates both Earth and the Moon.

Get to Know Your Lunar Phases

There are eight phases of the Moon that you can track each month.

New Moon: During New Moon, the side of the Moon facing us is not illuminated by the Sun. At this time, the Moon is not up at night, but it is up during the day. We just can't see it. Solar eclipses can occur during the new moon, depending on how the Sun, Earth, and Moon line up in their orbits.

Waxing Crescent: As the Moon waxes (grows) into its crescent phase, it begins to show up low in the sky right after sunset. Look for a silvery-looking crescent. The side facing the sunset direction will be lit up.

First Quarter: Seven days after New Moon, the Moon is in first quarter. Only half of it is visible for the first half of the evening, and then it sets. 

Waxing Gibbous: After First Quarter, the Moon appears to grow into a gibbous shape. You can see most of it, except for a shrinking sliver over the next seven nights.

 Look for the moon at this time during the afternoon, too. 

Full Moon: During full moon, the Sun lights up the entire surface of the Moon that faces Earth. It rises just as the Sun sets and disappears beneath the western horizon when the Sun rises the next morning. This is the brightest phase of the Moon and it washes out the nearby part of the sky, making it difficult to see stars and faint objects such as nebulae. 

Supermoon: Ever hear of a Super Moon? That's a full Moon that happens when the Moon is closest in its orbit to Earth. The press likes to make a big deal about this, but it's really a very natural thing. A "Super Moon" happens as the Moon's orbit brings it closer to Earth on occasion. Not every month has a Super Moon.

Lunar eclipses occur only at full moons because the Moon is passing directly between Earth and the Sun in its orbit. Due to other perturbations in its orbit, not every full moon results in an eclipse. 

The full moon can appear slightly larger sometimes, creating what's called a Super Moon. Most people really can't tell the difference between them. Still, it's a great chance to observe the Moon! 

The other full Moon variation you might hear about is a "Blue Moon".

That's the name give to the second full Moon that occurs in the same month. These don't happen all the time, and the Moon certainly doesn't appear blue. Full moons also have colloquial names based on folklore. It's worth reading about some of these names; they tell fascinating stories about early cultures.

Waning Gibbous: After the glorious appearance of Full Moon, the lunar shape starts to wane, meaning it gets smaller. It's visible later at night and into the early morning, and we see a steadily shrinking shape of the lunar surface that's being lit up.  When you do spot the Moon, the side that is lit up is facing toward the Sun, in this case, the sunrise direction. During this phase, look for the Moon during the day—it should be in the sky in the morning. 

Last Quarter: At Last Quarter we see exactly half the sunlit surface of the Moon and it can be in the early morning and daytime sky.

 

Waning Crescent: The last phase of the moon before returning to New Moon is called Waning Crescent, and it is exactly what it says: a steadily-shrinking crescent phase. We can see only a small sliver from Earth.  It's visible in the early morning and by the end of the 28-day lunar cycle, it has vanished almost entirely. That brings us back to New Moon to start the new cycle.

Make Your Own Lunar Phases

You can demonstrate this to yourself by setting up a light in the center of the room. Take a white ball in your hand and stand a few feet away from the light. Next, turn yourself in a circle, as if you are the Moon spinning on its axis. Watch how the ball is illuminated as you turn. 

Observing the Moon throughout a month is a great school project, as well as something you can do on your own or with family and friends. Check it out this month!