What Causes Moon Phases? – Demystifying the Wonders of Earth’s Only Moon

What causes Moon phases? And how do we define those differing changes throughout the month? These are the questions we’re tackling today. It’s all about lunar phases, the Moon’s orbit, and Earth’s orbit around the Sun. Since all the cosmic players constantly move, we get different views of the Moon.

From Earth, we consistently see the same face of the Moon, and that’s because as it orbits our planet, it also rotates. So it shows us the same side every night. But, the Moon regularly looks different. Sometimes, it’s a tiny sliver, while other times, it glows bright and enormous in the night sky. So, let’s dig into Moon phases to learn their causes and cycles.

The Science Behind Moon Phases: Unveiling the Lunar Cycle

You might first ask what the moon phases are. They refer to the Moon’s changing appearance as it orbits Earth. We see the changes because of the varying amounts of sunlight illuminating the Moon’s surface. But we see them from our unique perspective on Earth.

Whether viewing the Moon with your naked eye or studying it through a telescope, the 29.5-day lunar cycle includes several phases and transitional phases.

• New Moon
  • Waxing Crescent
• First Quarter
  • Waxing Gibbous
• Full Moon
  • Waning Gibbous
• Third Quarter
  • Waning Crescent

Knowing what phase the Moon is in is helpful for backyard astronomers wanting to observe stars, galaxies, or faraway planets. A Full Moon means there is so much light that you’ll only get clear views of planets. But the thin sliver of light from a Crescent Moon means you can see stars better. And if you’re into astrophotography, setting up the frame and getting just the right amount of light is half the battle in obtaining good images.

The above illustration of the moon’s phases places the Sun to the far right. The inner circle shows the Moon’s position throughout its cycle from above Earth’s North Pole. Yet, the larger Moon images indicate how we see the phases from here on Earth.

The Moon’s Orbit

The Moon lies approximately 239,000 miles (385,000 kilometers) away from Earth. Since it’s tidally locked to our planet, we only see one side of the Moon. It is the largest and the brightest object in our night sky.

Without the Moon, our home planet wouldn’t be very comfortable. As the satellite orbits our world, it helps moderate our climate by relieving some of Earth’s wobble on its axis. The Moon also causes tides in our oceans. And that creates a rhythm to life that guides humanity through the ages.

Scientists think the Moon formed several billion years ago after a small planet collided with Earth. This Mars-sized object broke off a chunk of our world where it and the impact dust and debris collected into Earth’s orbit and became the Moon.

Since the Moon doesn’t emit any light of its own, we see sunlight reflecting off its surface. As the Moon orbits Earth, we see the Sun shining in different places, so it appears that the Moon changes shape. The reality is that only our perception of the Moon changes. Its shape remains the same.

The Moon makes a complete rotation around Earth in 27.3 days. However, since Earth moves around the Sun simultaneously, it takes longer for the Moon to go through its eight moon phases. So, a lunar month is 29.5 days long.

Sunlight and Shadows Cause Moon Phases

Sunlight plays an enormous role in the Moon phases. We say the Moon is “waxing” when it appears to grow larger. On the other hand, we say it’s “waning” when it looks smaller. Then, when the Moon fully faces the Sun and is well-lit from our viewpoint, we call it a Full Moon. Finally, as the Moon orbits the Earth, its face is in shadow, and we can barely see it. So, we call that a New Moon.

Shadows and sunlight play into the dance between the Sun, Earth, and the Moon. The amount of the Moon’s surface that we can see depends on where we are in relation to the Sun. Understanding the lunar phases helps us know why the Moon appears to be changing.

An easy way to get up-to-date information on Moon phases is to check out NASA’s Daily Moon Guide. First, choose whether you’re viewing the Moon with your unaided eye, binoculars, or a telescope. Then, get details about the current Moon phase, how far away the Moon is, and some recommended viewing sites on the satellite.

The Daily Moon Guide shows images of what the Moon looks like in its current phase. So, don’t worry if it is cloudy or rainy because you can still get a good glimpse of the Moon. It just might be on your computer instead of from your backyard.

New Moon

During the New Moon phase, the Moon seems nearly invisible. The night side faces Earth, while the illuminated side of the Moon faces the Sun. Since the Moon is in the same area of the sky as the Sun, it rises and sets with the Sun. So, the Moon’s illuminated side faces away from Earth, making it hard to see at night. But the Moon is up and visible during the day!

While it’s hard to see the Moon during this lunar phase, it’s one of the best times for dark sky observations. The New Moon is when you’ll want to set up your camera and tripod for astrophotography. Or dust off your telescope to view deep-sky objects.

New Moons and Lunar Eclipses

In its New Moon phase, we don’t usually get a lunar eclipse with the Moon passing directly between the Sun and Earth. That’s because of the Moon’s orbital inclination. From our perspective, the Moon only passes near the central star.

A lunar eclipse does occur as the Moon passes through Earth’s shadow. In the same way, solar eclipses happen when Earth passes through the Moon’s shadow. So, you might wonder why we don’t get lunar eclipses twice a month.

Since the Moon has a tilted orbit around Earth that is relative to our planet’s orbit of the Sun, we don’t get regular lunar eclipses. However, throughout the year, the Moon’s tilt is fixed with respect to stars, so it then changes with respect to the Sun. As a result, we see two annual lunar eclipses when the Moon is in a perfect position to pass through Earth’s shadow.

Waxing Crescent

This transitional Moon phase happens as the illuminated half mainly faces the Sun rather than Earth. We can only observe a small portion of the Moon during the Waxing Crescent phase. However, it grows more each day as the satellite’s orbit brings its dayside more into our view. During this lunar phase, the Moon also rises a few minutes later every day.

First Quarter

We can see about half of the satellite’s “sunny side during the Moon’s First Quarter phase.” You might hear it called a half-moon, but that doesn’t fully explain what you’re observing. In this phase, the Moon is a quarter of the way through its orbit of the Earth. The timing occurs about halfway between New and Full Moons.

From Earth, we can view half of the half (or one-quarter) of the side of the Moon reflecting sunlight. During this lunar phase, the Moon rises in the daytime sky around noon. Then, it rides high in the sky until midnight, when it sets. So, this is a good time of the moon cycle to get great views of the Mare Serenitatis or Sea of Serenity.

Below are three views of this lunar mare from the 1972 Apollo 17 mission’s mapping camera. In the left image, you can see the Apennine Mountains at the left, the Caucasus Mountains toward the middle, and the Supicius Gallus Rilles toward the bottom right.

The center photograph captures the ten-mile (16-kilometer) wide Bessel crater. Then, the last image packs in it a lot of exciting sites. It shows the eastern margin of the Sea of Serenity with the larger 59-mile (95-kilometer) diameter Posidonius Crater near the central horizon. Also, check out the southern Le Monnier and the Littrow crater to the right. Finally, this image shows Apollo 17’s Taurus-Littrow Valley landing site in the bottom right corner.

These sites are visible during a First Quarter Moon with a good-sized telescope.

Waxing Gibbous

The Waxing Gibbous lunar phase is halfway between the First Quarter and Full Moon phases. From Earth, we can see most of the Moon’s dayside again. Since waxing means “getting bigger,” the Moon starts appearing more prominent and brightly in the sky.

Full Moon

When the Moon is Full, the side facing Earth is entirely sunlit. Technically, we only see the dayside half, so we’re seeing a half-moon. But this lunar phase occurs when Earth-bound viewers see the Moon opposite of the Sun. So, we see the entire dayside.

Full moons rise about the same time as the Sun sets and then they set as the Sun rises. We usually observe the Moon as “full” for a couple of days before it moves along in the lunar cycle. However, there is more than one type of “fullness,” so let’s discover the different kinds.

Blood Moon

This type of full moon has a red glow and occurs during a total lunar eclipse. The Earth is in line between the Sun and the Moon, so it hides the satellite from sunlight.

During a Blood Moon, sunlight can only reach the Moon from the outer edges of Earth’s atmosphere. Blue light gets scattered due to air molecules, so the remaining light reflects in a red glow.

Other times, people use the name “blood moon” when it looks red because of smoky or hazy skies. Or, during the Fall, when leaves turn red, you might hear the term Blood Moon.

Harvest Moon

The Full Moon we see near the beginning of Fall is the Harvest Moon. The name comes from a time before electricity when people harvested crops late into the night by the light of the Moon.

The Harvest Moon differs from other full Moon names since it doesn’t always fall in the same month. Since it relates to the autumnal equinox, it can occur in September or October. It just depends on how the Gregorian calendar and lunar cycle align.

Here’s a fabulous view of a Harvest Moon on September 30, 2023, from the International Space Station. You can see it between the station’s external hardware.

Super Moon

When the Moon is full and at its closest (perigee) orbit to Earth, we see it as a Supermoon. The Moon’s orbit around Earth is elliptical, which means the oval shape brings the satellite closer to or farther from Earth as it circles.

• The perigee is an average distance from Earth of approximately 226,000 miles (363,300 kilometers.)
• The apogee (furthest point in the ellipse) takes the Moon about 253,000 miles (405,500 kilometers) away from Earth.

Supermoons occur during the time the Moon is closest to Earth and at its fullest. So, we see it as larger and brighter than a “regular” full moon.

Blue Moon

There are at least twelve full moons in the year. But, if a second one happens within a month, we call it a Blue Moon. You’ve likely heard the saying, “once in a blue moon…” It means that whatever you’re talking about is rare.

Blue Moons don’t look any different than regular full moons. Instead, the term stuck in the 1940s when astronomers discussed the “extra” full moon that happens once every two and a half years. 

Interestingly, 2018 hosted two Blue Moons two months apart. What made it even more special was that one of them was a lunar eclipse. Two Blue Moons in one year actually does only happen once in a Blue Moon! It won’t happen again until 2037.

You might wonder what causes Blue Moon phases, so here’s the breakdown. Earlier, we discussed that the lunar cycle takes only 29.5 days to complete. That adds up to twelve full cycles in 354 days. However, our calendar year contains 365/366 days, so we see a thirteenth Full Moon every two and a half years. We call the extra one Blue.

On August 30th and 31st of 2023, we saw a Super Blue Moon. It was a Full Moon. Since the Moon was at its closest point in its orbit around Earth, it was a Supermoon. And since it was an extra occurrence, it was a Blue Moon. We hope you had a chance to see it!

Monthly Full Moon Names

In addition to the types of full moons, many cultures also give names to each monthly Full Moon. Before the Gregorian calendar, people marked time with lunar and solar cycles. The examples here come from Native American tribes that Colonial Americans adopted.

• January: Wolf Moon (wolves howling at the lack of winter food,) Old Moon, and Ice Moon 
• February: Snow Moon, Storm Moon, Bear Moon, and Hunger Moon 
• March: Worm Moon (worm trails in thawing ground,) Chaste Moon, Death Moon, Crust Moon, and Sap Moon (maple tree tapping)
• April: Pink Moon (early blooming wildflowers,) Sprouting Grass Moon, Egg Moon, and Fish Moon
• May: Flower Moon (spring blooming,) Hare Moon, Corn Planting Moon, Milk Moon
• June: Strawberry Moon (harvesting time,) Rose Moon, Hot Moon (beginning of summer)
• July: Buck Moon (male deer begin regrowing antlers,) Thunder Moon (summer storms,) and Hay Moon (harvesting time)
• August: Sturgeon Moon (fishing time,) Green Corn Moon, Grain Moon, and Red Moon
• September: Full Corn Moon (harvesting time, even at night,) Harvest Moon
• October: Hunter’s Moon (hunting for summer-fattened game,) Travel Moon, and Dying Grass Moon
• November: Beaver Moon (trapping beavers or beavers building dams for winter) and Frost Moon
• December: Cold Moon (winter is coming,) Long-Night Moon, and Oak Moon

YouTube Link: Moon Phase Animation

Waning Gibbous

The transitional Waning Gibbous moon phase happens between a full and third (or last) quarter moon. Waning means that the moon’s appearance is “shrinking,” so it looks less full even though it is still more than half-lit.

It rises later in the night than a Full Moon, too. For example, you might see it rise sometime between sunset and midnight. This lunar phase looks like a misshapen full moon when it is low in the night sky.

The Waning Gibbous moon phase happens as the Moon starts heading away from Earth and back toward the Sun. So, its opposite side reflects sunlight, making the lighted side look smaller. Actually, though, the Moon’s orbit only carries it out of view from our Earth-bound perspective.

Third Quarter

Like a First Quarter Moon, the Third (or Last) Quarter Moon looks more like a half moon. The opposite half is now illuminated compared to the First Quarter. The Sun shines on half of the Moon’s surface, but we actually see half of the illuminated half of the Moon (one-quarter.)

This lunar phase has the Moon rising around midnight and setting about noon. Check out this fantastic image of a Third Quarter Moon in Kazakhstan. It shines above the Soyuz MS-11 spacecraft in 2019. Astronauts David Saint-Jacques, Oleg Kononenko, and Anne McClain returned to Earth after living and working aboard the International Space Station for 204 days.

Waning Crescent

This next intermediate moon phase falls between the Third Quarter and New Moons when the lunar cycle starts all over. Waning Crescent Moons are almost back to the orbital point where the dayside faces the Sun directly. So, from our perspective on Earth, the Moon looks like a thin curve or sickle.

This Moon rises just before sunrise and sets just after the Sun. You can see it the best in the early morning hours, just before dawn.

During a Waning Crescent Moon, you are likely to see Earthshine. That’s when you can see a dull glow lighting up the darker part of the Moon. It happens when sunlight reflects off the Earth’s surface and shines on the Moon. You might also hear Earthshine called the da Vinci glow because Leonardo da Vinci first (in recorded history) explained the phenomenon.

The Moon Phases and Astrophotography

There are four main Moon phases, each with strengths and weaknesses in astrophotography. Whether you’re making plans for backyard observations or going exotic with a telescope rental in Chile, choose the right time in the lunar cycle to capture your best images.

New Moon

Because the Moon is almost invisible to us during the New Moon phase, it is an excellent time to take advantage of dark skies with little light.

Now is the time to take star trail images. Hopefully, you won’t have nearby city lights, so you can leave your shutter open for super long exposures without worrying about overexposure. We’re talking about hours-long exposures. If you focus on the North Star, you’ll see star trails and rings around it.

The dark nights during the New Moon and even some Waxing Crescent nights are also an excellent opportunity to capture Milky Way and other galaxies or nebulae images. Any deep-sky broadband imaging or faint objects are best when little to no moonlight interferes.

First Quarter Moon

First Quarter and Waning Crescent Moons are still okay for capturing star trails. However, more light enters the sky, so the fainter stars may not appear as well. You’ll also need to take shorter exposure, up to about fifteen minutes. That way, you’ll still get some concentric trailing without overexposing the image.

This time in the lunar cycle might be a great time to learn or improve your light painting technique. You’ll have enough light to see what you’re doing without tripping, but it is still dark enough to get some exciting night photography. While this isn’t so much about astrophotography, exploring nighttime imaging is still a fun time.

Full Moon

Moonlight can illuminate nightscapes, giving you a stunning image of stars while still seeing the foreground below. Rising full moons provide the perfect opportunity to capture details both on Earth and in the skies above.

These nights are also a good time to take images of planets since they’re brighter than deep-sky objects. Choose objects that aren’t directly in the Moon’s path so it doesn’t compete for the shot. Shorter exposures work best. Two to five minutes may be as long as you dare, but play around to see what works best in your conditions.

Check out this Full Moon picture that captures the lights of cars on a freeway and the brightly lit New Orleans skyline. It is a great example of just the right amount of exposure to get the best of Earth and the sky.

Third Quarter Moon

As the lunar cycle continues, it returns to a quarter moon. Third-quarter images are similar to the First, where you might experiment with some star trails and light painting. Viewing conditions are light enough to move around outside without fear of tripping. So, use your imagination to capture the shots that speak most to your aesthetic.

Conclusion: What Causes Moon Phases

As we wrap up this article on what causes Moon phases, remember that the Moon doesn’t actually change sizes. Instead, our perception of it from our viewpoint on Earth shows us differing views of the Moon. 

Whether you’re hoping for a night of stargazing or planning an astrophotography outing, it helps to understand the Moon phase. In addition, being familiar with the Moon’s rising and setting times throughout the lunar cycle helps in your planning.

There are four main Moon phases and four transitioning stages. For astrophotographers, each stage of the night sky offers opportunities for capturing the wonder of the cosmos. Think about what you’re aiming for as you plan your images. If you want long star trails, a new moon is perfect. But a moonlight landscape can really stand out under a full moon’s brightness.