How Is The Moon Classified?

The moon – your moon – means different things to different cultures, but nowhere on the globe is Earth's closest companion in the solar system treated with anything besides awe and reverence. It is all but impossible to imagine a sky without it, and literature and poetry would be devoid of untold amounts of imagery and allure.

In astronomical terms, the moon is classified as a natural satellite. It is the only one Earth has; as weird as things would be without it, imagine a sky crossed by multiple moons, as is the case for most other planets in the solar system with moons of their own.

The moon makes one trip around the Earth in just under a month; the relationship between the words "moon" and "month" is no coincidence.

The moon represents more than a bright object to point at and marvel over in the manner of a cherished pet. Without the moon, the phenomena of ocean tides would not exist, and everyone would have one less taken-for-granted thing to look forward to at the beach. The moon's astronomical impact is as strong as its romantic attraction and other workaday aspects, and learning more about it is always a rewarding and edifying endeavor.

The moon's diameter is about one-fourth that of Earth's – about 2,160 miles (3,475 kilometers). This makes the moon comparatively large for a planetary satellite. However, because volume increases with the third power of diameter, the Earth could fit about 50 moons inside it, and as the moon and Earth have a similar composition, the moon's mass is only about 1 percent of that of Earth. This means that the moon's gravitational field is far weaker than Earth's.

The moon orbits Earth at an average distance of 239,000 miles (384,000 kilometers) every 27.3 days, which means it takes light about 1.3 seconds to reach the moon from the earth's surface. It appears to take up about 1/2 of 1° of arc in the sky. (1° of arc is 1/360th of the sky's breadth. At any one time, you can see half of the sky, or 180° of it.)

One complete lunar cycle in terms of the moon's phases as viewed from Earth is a little longer – about 29.5 days. To simplify things a little, this "lag" results from the fact that with each new return of the moon to the same spot with respect to Earth, the Earth itself has moved about 1/12th of the way along its own orbital path around the sun.

• 27.3 days is a sidereal ("pertaining to stars") lunar month; 29.5 days is a synodic month. 

Interestingly, the sun (if you could look right at it) and the moon appear very close to the same size from Earth. This is because the sun happens to be both about 400 times more distant and 400 times larger in diameter than the moon. Astrophysicists have speculated that the moon was once part of Earth, breaking away early after their mutual formation some 4.5 billion years ago.

Because the moon always presents (roughly) the same half of itself to Earth during its revolution around "the Blue Planet," some sources may report that the moon does not rotate about its axis (with respect to the Earth). In fact, this is untrue. In reality, the moon's period of revolution exactly matches its period of rotation, which is what keeps almost half of it entirely hidden from view on Earth.

Consider what you would see over the course of a 29.5-day lunar cycle if the moon did not rotate with respect to the rest of the solar system. Instead of the same face and features, you and other Earthlings would gradually be treated to the full range of its surface as a slightly different portion of the lunar surface would face Earth with every new moonrise. Alas, humans must rely on photos taken from spacecraft to enjoy that visual experience.

The moon is considerably smaller than the great moons of Jupiter and Saturn's Titan, yet is the solar system's fifth-largest moon, nearly as big as the planet Mercury. This is amplified by the fact that Earth is far smaller than the gas giants Jupiter and Saturn; the Earth-to-moon diameter ratio is a very a small 4 to 1, making the pair vaguely like a double-planet system instead of a planet-moon duo.

A genuine understanding of the phases of the moon is both a useful skill for mastering the general geometry of orbiting bodies and the basis for an interesting "life hack" or two.

First, imagine the solar system from directly above. On this view, the massive, bright sun is far off to the left, illuminating the scene from that side. Place the moon so its starting point is directly on a line between the sun and Earth, and you'll see that the lit side of the moon is facing directly away from a viewer on Earth. This is a new moon, day 0 of a new lunar cycle.

Note that even though the moon is between the sun and the Earth, that doesn't mean there's an eclipse; the Earth and moon would need to be positioned just so for an eclipse to happen, and during most new moons, they aren't lined up in the right way.

As described, the moon's cycle of phases is 29.5 days, a little longer than its actual orbital period because of the complex interplay between the movements of the Earth, moon and sun. After about a week, it is one-fourth of the way along its gravitationally compelled journey around Earth. In this configuration, the moon appears half-full, having grown to that size from a sliver of a half-circle ("waxing crescent") beginning on an Earth observer's right. This first quarter moon rises at about noon, underscoring the fact that because the moon must return to the same state every 29.5 days, it rises a little less than an hour later every day, or a little more than six hours later from week to week.

In the next seven days, the left side of the moon appears to fill out, and is now a "waxing gibbous" moon. It becomes a full moon when the Earth is on a line directly between the sun and the moon, and the moon is rising on the eastern horizon as the sun disappears over the western one; in other words, at about sunset. One week later, you again see a half-moon, now a third quarter moon rising at around midnight, but this time with the left side lit. Finally, the moon exists over the next week as a "waning crescent" figure before becoming a new moon once more.

• If you know the basis of the phases of the moon, if you can see it, you can figure out approximately what time of day it is and what direction you are looking in (especially if the moon is close to the horizon, or having just risen in the east or about to set in the west) with no other information. For example, if a half-moon with its right side lit is close to the horizon, it has either just risen, making it close to noon, or is about to set, making it close to midnight. Presumably you would know which it was!

In Western cultures, people have largely moved beyond treating the moon as a literal goddess and attempting to placate and please her with elaborate rituals. But make no mistake – modern society dearly craves unusual moon phenomena, and not just eclipses.

As a result, various types of full moons have become renowned and widely anticipated, as their appearance can be predicted many years in advance (weather phenomena like clouds interfering aside). You've probably heard of one or more of these full moon variants, which are:

Blood moon. This term, referring to an unusually reddish glow, lacks a precise astronomical basis. In most cases, it occurs simply as a result of a lunar eclipse, where the only moonlight is light reaching the moon's outer portions from Earth's atmosphere. But other things can cause a similar appearance, such as dust or haze.

Blue moon. The phrase "once in a blue moon" has a specific meaning and has nothing to do with the moon's color (if it did, the term would equate to "never," or so it seems). Rather, it refers to the phenomena of the second full moon of a calendar month. Since the lunar cycle just squeezes into a month with two or three days to spare, this happens about once every two and a half years.

Harvest moon. This term is rooted in the benefits to farmers in pre-electricity days of a full moon at the start of autumn, when it's time to pull up crops. Thus this type of full moon is, like a blue moon, a mathematical quirk, albeit one with real benefits.

Supermoon. The moon's average distance from Earth is 239,000 miles, but because its orbit (like all planetary and moon orbits) is slightly elliptical rather than circular, a full moon sometimes happens to occur when the moon is at its closest approach. This makes it appear noticeably larger, especially when it is close to the horizon for visual context.

On April 19, 2019, a full moon rose and garnered an unusual amount of media attention because this "pink moon" was close to being a supermoon. But this characterization is misleading: The name is rooted in folklore, not the moon's appearance.

A full moon that rises in April is known as a "pink moon" because some of the earliest flowers to bloom in the U.S. are pink-colored varieties, such as phlox. Anyone hoping to see the equivalent of a cotton candy disk in the sky was surely disappointed!

"Dark side of the moon" is a popular term, as well as the title of a famous album by the rock band Pink Floyd. But as you now realize if you didn't before, it is a nonsense term. One side of the moon always appears unlit from Earth, but just like Earth itself, all portions rotate through equal lengths of light and darkness. And it's extreme light and dark, with the temperature ranging from -233 to 123 C (about -387 to 253 F) thanks to the moon lacking an atmosphere to level temperatures out.

The entire surface is pockmarked by craters as a result of impacts by meteors and asteroids. This is more evident than on Earth, which has also been peppered with rocks over billions of years, but where weather and erosion can hide much of the damage; the moon has no weather to speak of.

The moon also demonstrates evidence of seismic activity – in other words, quakes, which for obvious reasons are not called earthquakes. The moon is believed to have a very hot, molten core, just as the earth does.

• As of 2019 (and as of 1971, for that matter) a total of 12 human beings have set foot on the moon, all of them American males. As of 2019, some wealthy entrepreneurs had already begun dabbling seriously in the idea of private (that is, non-NASA) trips to and from the moon. NASA has also announced that its Artemis program would focus on sending a crewed flight back to the moon in 2024. This crew will include the first woman to be sent to the moon.