Moon Phases

4 keys to understanding moon phases

Moon phases: Top view of half-lit Earth and half-lit moon with lines between dark and light sides aligned.
Click here to see animation. As seen from the north side of the moon’s orbital plane, the Earth rotates counterclockwise on its rotational axis, and the moon revolves counterclockwise around Earth. Thus, the circular motions result in the moon phases we see in our sky. Not to scale. Image via NASA/ Wikimedia.

Moon phases change shape every day

Why does the moon seem to change its shape every night? It’s because the moon is a world in space, just as Earth is. Like Earth, the sun always illuminates half of the moon; the round globe of the moon has a day side and a night side. And, like Earth, the moon is always moving through space. So as seen from our earthly vantage point, as the moon orbits around Earth once each month, we see varying fractions of its day and night sides: the changing phases of the moon. How can you understand moon phases? Here are four things to remember.

28-panel composite with a moon in each panel varying in phase through a lunar cycle.
View at EarthSky Community Photos. | Meiying Lee in Taipei, Taiwan, made this composite of a full lunar cycle in October 2021 using images collected over the years. Meiying wrote: “Some people think that the moon can only be seen at night. In fact, if you look up at the sky, you will often find that not only the moon can be seen during the day […] This combined photo is the daytime moon I collected for nearly 5 years. It contains all the daytime moons of all ages. In addition to the rich and beautiful colors, the most special thing about this photo is that you can see the relationship between the moon and the sky.” Thank you, Meiying!

1. When you see the moon, think of the whereabouts of the sun.

After all, it’s the sun that’s illuminating and creating the dayside of the moon.

The fact is, moon phases depend on where the moon is with respect to the sun in space.

But don’t just take our word for it. Go outside. No matter what phase of the moon you see in your sky, think about where the sun is. It’ll help you begin to understand why the moon you see is in that particular phase.

Chart showing new moon between Earth and the sun.
At new moon, the sun, Earth and moon are aligned in space, with the moon in the middle. And the moon’s night side – its darkened hemisphere – directly faces us and we don’t see the moon. Chart via John Jardine Goss/ EarthSky.
Diagram: Moon, Earth, and sun in alignment with moon on far side of Earth from the sun.
At full moon, the sun, Earth and moon are aligned in space, with Earth in the middle. And the moon’s day side – its fully lighted hemisphere – directly faces us. Chart via John Jardine Goss/ EarthSky.

2. The moon rises in the east and sets in the west, every day.

It has to. The rising and setting of all celestial objects is due to Earth’s continuous daily spin beneath the sky.

So, when you see a thin crescent moon in the west after sunset, it’s not a rising moon. Instead, it’s a setting moon. In fact, it rose earlier in the morning soon after the sun rose.

3. The moon takes about a month to orbit the Earth.

Although the moon rises in the east and sets in the west each day (due to Earth’s spin), it’s also moving on the sky’s dome each day due to its own motion in orbit around Earth.

This is a slower, less noticeable motion of the moon. And it’s a motion in front of the fixed stars. So, if you just glance at the moon one evening – and see it again a few hours later – you’ll notice it has moved westward. In fact, that westward motion is caused by Earth’s spin.

By the way, the moon’s own orbital motion can be detected in the course of a single night. But you have to watch the moon closely, with respect to stars in its vicinity, over several hours.

Additionally, the moon’s eastward, orbital motion is easy to notice from one day (or night) to the next. It’s as though the moon is moving on the inside of a circle of 360 degrees. The moon’s orbit carries it around Earth’s sky once a month, because the moon takes about a month to orbit Earth.

So, the moon moves – with respect to the fixed stars – by about 12 to 13 degrees each day.

4. The moon’s orbital motion is toward the east.

Each day, the moon moves another 12 to 13 degrees toward the east on the sky’s dome. Then, Earth’s rotation takes a little longer to bring you around to where the moon is in space.

Thus, the moon rises, on average, about 50 minutes later each day.

The later and later rising times of the moon cause our companion world to appear in a different part of the sky at each nightfall for the two weeks between new and full moon.

Then, in the two weeks after full moon, you’ll find the moon rising later and later at night.

Diagram with line of Earths and moons, and below panels with 9 phases of the moon.
View larger. | The moon’s (and Earth’s) orbit in one lunar month (new moon to new moon) as seen from north of the ecliptic – Earth’s orbital plane. The sun is at the top, outside the illustration. New moon is at extreme right and left. Full moon is at center. Click here for larger chart and more detailed explanation. Image via Wikipedia/ Attribution-Share Alike 3.0 Unported.

More details on individual moon phases at the links below

Follow the links to learn more about the various phases of the moon.

New Moon
Waxing Crescent
First Quarter
Waxing Gibbous
Full Moon
Waning Gibbous
Last Quarter
Waning Crescent

Plus, here are the names of all the full moons.

Finally, here are the dates and times of 2022 moon phases.

Bottom line: The moon is a world in space just as Earth is and half of it is always illuminated by the sun. As the moon orbits Earth, we on Earth’s surface see varying fractions of its lighted face, or day side. These are the changing phases of the moon. Four tips to understanding moon phases, here.

Posted 
October 14, 2022
 in 
Moon Phases

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