One of the questions most often asked to astronomers is: What are moon phases? Most people know that the Moon appears to change shape over time. Does it look round and full? Or more like a banana or a lopsided ball? Is it up in the daytime or the nighttime? Throughout each month, the Moon appears to change shape while it appears in the sky at different times, including in broad daylight! Anyone can observe these changes as they happen. The Moon's ever-changing shapes are called "lunar phases."
Gradual Change Anyone Can Measure From the Back Yard
A lunar phase is simply the shape of the sunlit part of the Moon, as seen from Earth. Phases are so strikingly obvious that we almost take them for granted. Moreover, they can be observed easily throughout the month from the backyard or via a simple glance out the window.
The Moon's 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 the Lunar Phases
There are eight phases of the Moon to 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. Most of it is visible, except for a dark sliver that shrinks over the next seven nights. Look for the Moon at this time during the afternoon, too.
Full Moon: During the 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.
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: On occasion, the Moon's orbit brings it closer to Earth. Not every month has a Super Moon. Despite the hype about Super Moons in the media, it's difficult for the average observer to notice one, because the Moon might appear only slightly larger in the sky than normal. In fact, the well-known astronomer Neil deGrasse Tyson pointed out that the difference between a regular Full Moon and a Super Moon is analogous to the difference between a 16-inch pizza and a 16.1-inch pizza.
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 other Full Moon variation that often grabs media attention is a "Blue Moon." That's the name given 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. 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. It can be seen 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.
Making Lunar Phases at Home
Creating lunar phases is a great classroom or home science activity. First, set up a light in the middle of a darkened room. One person holds a white ball and stands a short distance from the light. He or she turns in a circle, just like the Moon does as it turns on its axis. The ball is illuminated by the light in ways that almost exactly match lunar phases.
Observing the Moon over the course of a month is a great school project, as well as something anyone can do on their own or with family and friends. Check it out this month!