Size of the Moon compared to the Earth
History Of The Moon
Millions of people around the world look up at night and marvel at a misty bright glowing round object – the Moon.
Far fewer understand the Moon’s own history, what a Moon day or night is like in comparison with a day or night on Earth, what kind of material the Moon is composed of or how important the Moon is to the Earth itself.
The history of the Moon is rich no matter how it is approached. The Moon has spawned many a myth and legend along with the many facts about it that are known today. Early religions incorporated the Moon’s phases into rites and rituals, and some still do this today.
Early astronomers keen to uncover the facts behind the Moon’s orbit and different phases contributed much to the modern history of the Moon that humankind is still compiling today. In this article, learn the history of the Moon and uncover a new wonder and appreciation for the Earth’s nearest neighbor.
What Is the Moon?
At its most fundamental, the Moon is an astronomical body. But that is not all. The Moon is also the Earth’s only permanent satellite.
The Moon is also the brightest object in the Earth’s night sky, second only to the Sun itself.
Earth’s Moon measures 2,195 miles (3,475 km) from end to end.
The Moon is called a terrestrial planet, a category it shares with the Earth itself as well as Mars, Venus and Mercury. (In contrast, the more distant planets are called Jovian or giant planets.)
Interestingly, a band of asteroids separates the Moon and its neighboring terrestrial planets from the outer giant planets.
The Moon is about 238,856 miles (384,402 km) away from the Earth. At this distance it takes about three full days for astronauts to travel from the Earth’s surface to land on the Moon.
The exact amount of time it takes to reach the Moon depends on the route taken. The majority of the moon’s surface is covered with regolith, which is a combination of very fine dust and rocky debris. This comes from billions of years of meteor impacts.
Get to Know the Moon
Today, astronomers know that the Moon is slowly moving away from the Earth. But at the rate it is traveling, about 1.5 inches (4 cm) per year, it will be a long time before the two part ways.
Currently it takes the Moon about 27 days and eight hours to make a complete rotation around the Earth.
The Moon and the Earth have what is called a “synchronous” rotation, which means the speed at which the Moon rotates around its own axis is the same as the speed at which the Moon rotates around the Earth.
This is why no one ever gets to see the other side of the Moon, which is sometimes called the “dark side.” And in fact, the correct name for the other side is the “far side” of the Moon, since “dark side” implies that the other side of the Moon doesn’t get sunlight, which it does get.
Interestingly, there are times when the Moon will wobble just a slight amount because its orbit is not completely circular. So sometimes it is possible to see just a tiny bit of the far side.
However, astronauts have also been able to capture photos and videos of the far side of the Moon using satellite technology.
The Moon has a monthly cycle just like the Earth. The Moon’s cycle is called the lunar or synodic cycle.
The monthly cycle measures the amount of time it takes for the Moon to move from one new moon to another new moon. Currently, it takes the Moon about 29 days and 5 hours, or 709 hours total, to do complete a cycle, or lunar month.
The way the Moon appears when viewed from Earth is called “phases.” Astronomers can use these phases to precisely predict how and when the Moon will appear in the sky, which is called a “lunar calendar.”
Sometimes the Moon is barely visible if at all, while other times it is huge and round and bright phase. The amount of visibility is caused by the relationship between the Sun, the Earth and the Moon. When more Sun is reflecting off the Moon’s surface, more of the Moon will appear to be visible in the sky. There are also supermoon cycles and you can read more about that in our dedicated supermoon facts post.
Phases of the Moon
The diagram below highlights the eight different phases of the Moon: Full Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter and Waning Crescent.
Phase 1: New Moon.
The New Moon phase is when the moon is least visible. It may be seen as a tiny sliver or crescent or not visible at all.
Phase 2: Waxing Crescent.
The Waxing Crescent phase arrives when the Moon appears more like a slim crescent (about one-eighth of the Moon’s full size) that is readily visible.
Phase 3: First Quarter.
In the First Quarter phase, one-quarter of the Moon is clearly visible from Earth.
Phase 4: Waxing Gibbous.
The Waxing Gibbous phase shows a full one-half of the Moon’s Earth-facing surface in the sky.
Phase 5: Full Moon.
The Full Moon phase is when the Moon is at its brightest, appearing as a full, round, sunlight-illumined circle in the sky.
Phase 6: Waning Gibbous.
The Waning Gibbous phase is the mirror opposite of the Waxing Gibbous phase.
Phase 7: Last Quarter.
The Last Quarter phase is the mirror opposite of the First Quarter phase.
Phase 8: Waning Crescent.
The Waning Crescent phase is the mirror opposite of the Waxing Crescent phase.
Why Isn’t the Moon a Planet like Earth?
In 2006, the distant moon Pluto was downgraded from a planet to a dwarf planet. Pluto measures (1,485 miles) 2,390 km from end to end.
The Moon is bigger than Pluto, so why isn’t the Moon considered a planet like the Earth?
All of this is governed by an organization called the International Astronomical Union (IAU). According to the IAU, a natural satellite cannot belong to two categories. In other words, the Earth’s Moon cannot be a moon and a planet, too. So the Moon is considered a moon and not a planet.
However, the Moon and Earth share a lot of planet-like qualities.
Here are three of the most fascinating shared qualities.
1. Both the Moon and the Earth have three layers (from outer to inner): mantle, crust, core. This means the Moon is formed like a planet forms.
2. Both the Moon and the Earth are still active geologically. This means both still have evolving rock surfaces. Moon rock formations are constantly changing.
3. At one time, the Moon had active volcanoes that produced lava, just like Earth still has active volcanoes today. This activity influenced the moon’s evolution just like it influenced Earth’s evolution.
What Caused the Moon to Form?
To date, scientists believe the modern Moon formed around 4.6 billion years ago, although some new data is beginning to suggest it formed even earlier than this.
The current reigning theory on how it formed is called the “giant impact hypothesis.”
This hypothesis states that when the planet that was to become the Earth collided with another planet that astronomers call Theia, which was about the same size as the planet Mars, the collision caused Theia to break apart into smaller pieces.
These pieces, in turn, gathered in around the Earth as it formed. The fragments were held together by the Earth’s gravitational pull as well as gravitational influences from neighboring planets. These diverse fragments eventually came together to form the Moon.
This means that the modern Moon may actually be composed of material that originally existed on earth as well as material that came from the planet Theia. Some theories suggest the fragments originally formed two moons that then become the Moon that exists today, while other theories state that only one moon formed from the start and that is the modern Moon.
Interestingly, more recent research (2016 and 2019) has put forth three new insights about the Moon’s formation and origins.
1. The early planet Theia may have actually come from outside the solar system.
2. The early planet Theia struck early Earth head-on instead of at a glance from the side.
3. Earth likely received most of its planetary water stores from Theia.
Additional hypotheses continue to exist regarding the Moon’s formation. Since no one was there when it happened, astronomers continue to revisit each theory as new evidence becomes available.
These are the five best-known and most enduring theories about the Moon’s creation.
1. The “Theia event” as just described.
2. Another event split a single planetary body into two new bodies: the Earth and the Moon.
3. An earlier protoplanetary disc (a spinning disc of gas and dust that often surrounds a newly-forming star) created both the Earth and the Moon at the same time.
4. An asteroid collided with the Earth and the fragment that broke away became the Moon.
5. The Moon was originally its own independent moon that was pulled into Earth’s gravitational field and trapped in a permanent rotation.
While some of these theories may sound far-fetched, each theory does have some amount of evidence.
Most of this evidence has come from comparing the Earth’s relationship with the Moon to the relationships other planets in the solar system have with their own satellite moons. Other important evidence has come from learning more about how stars and solar systems form.
What Is the Relationship Between the Moon and Earth?
As mentioned earlier, the Moon and Earth are in synchronous rotation.
It is this synchronous rotation that is responsible for the influence the Moon has on Earth’s tides and water supply.
The Moon, like the Earth, has its own gravitational force. The Moon’s gravitational force causes tidal activity on Earth that varies depending on where the Moon is in its rotation around the Earth.
While it can be mind-boggling to watch the ocean tides come in and flow out and realize this happens in large part because of the Moon (and also because of gravitational influence from the Sun), this is, in fact, the truth.
Basically, tides occur because the Moon is pulling the Earth’s water supply towards itself. This causes a “bulging” effect on either side of Earth. When the oceans bulge outward from Earth’s surface, they are bulging in the direction of the Moon. High tide is when the gravitational pull from the Moon is at its strongest.
What Is It Like on the Moon?
Unlike Earth, the Moon does not have an atmosphere. An atmosphere is made up of gaseous layers that may include hydrogen, carbon, oxygen and others. Heavier gases surround the terrestrial planets while lighter gases surround the giant planets.
The Moon isn’t surrounded by gases, which is why it does not have an atmosphere.
However, it may still have some water trapped inside its layers of rock. If present, this water is thought to be in the form of ice at either pole.
The Moon also has evidence of lava seas in its past history. There are vast pitted areas on the Moon’s surface where lava once flowed. These areas look darker than the rest of the planet’s surface.
Facts about the Moon
- There is no dark side of the Moon. Both sides of the Moon see the same amount of sunlight, however because the Moon is tidally locked to Earth, only one face of the Moon is ever seen from Earth. This is because the Moon rotates around its axis in exactly the same time it takes to orbit Earth. The side we see from Earth is reflected by sunlight, while the “dark” side has only been seen by the human eye from a spacecraft and lies in darkness.
- The rise of fall of tides on Earth is caused by the Moon. Two bulges exist due to the gravitational pull the Moon exerts. One is on the side facing the Moon and the other on the side facing away from it. These bulges move around the oceans as the Earth rotates which causes the high and low tides found across the globe.
- The Moon is slowly drifting away from Earth. Every year, the Moon moves roughly 3.8 cm further away from Earth. Scientific estimates suggest this will continue to happen for 50 billion years. At that point, the Moon will take 47 days to orbit the Earth, compared to the current time of 27.3 days.
- You weigh much less on the Moon. A common fact about the Moon is that it has much weaker gravity than Earth’s gravity. This is because its smaller mass and you would weight one sixth (about 16.5%) of your Earth weight while on the Moon.
- Only 12 people have ever walked on the Moon. It started with Neil Armstrong in 1969 as part of the Apollo 11 mission moon landing and ended with Gene Cernan in 1972 on the Apollo 17 mission. A total of 12 American males have walked on the Moon. Since 1972, all lunar missions have been unmanned spacecraft.
- The Moon will be visited by man again. NASA space agency has plans to set up a permanent space station on the Moon for further lunar exploration, and man may walk on the Moon again sometime around 2020-21.
- The USA considered detonating a nuclear bomb on the Moon in the 50s. A secret project during the height of the cold war – codenamed “Project A119”, also known as “A Study of Lunar Research Flights” was planned as a “show of strength” for the United States at a time when they were falling behind the USSR in the space race.
- There is no atmosphere on the Moon. There is no protection for the lunar surface from cosmic rays, meteorites, asteroids, comets, or solar winds. This is why the Moon has such huge temperature variations and it is covered with impact lunar craters. The lack of atmosphere also means no sound can be heard on the Moon and the sky is always black.
- The Moon has quakes. The gravitational pull of Earth causes small moonquakes several kilometres beneath the surface of the moon – causing ruptures and cracks. It is believed that, like Earth, the Moon has a molten core.
- The Moon is the fifth largest natural satellite. It is much smaller than the major moons of Saturn and Jupiter at 3,475 km in diameter, but the Moon is the largest in relation to the size of the planet it orbits. Earth is about 80 times the volume of the Moon, yet they are the same age. A popular theory is that the Moon was once part of the Earth and formed from a chunk broken off by a huge object that collided with Earth while it was still young.
- The “Man in the Moon” is an optical illusion seen when looking at the Moon’s surface from Earth. It is a result of the contrast between the lighter lunar highlands and darker lunar plains.
- The diameter of the Moon is the same distance from New York City to Phoenix, Arizona.
- There are over 500,000 craters on the Moon’s surface.
- A lunar eclipse is when the Earth passes between the Sun and the Moon and a shadow is cast on the Moon.
- A solar eclipse is when the Moon passes between the Sun and the Earth – causing a shadow to project onto the Earth’s surface.
More information and facts about the Moon
The Moon is unique in that it is the only spherical satellite orbiting a terrestrial planet. The reason for its shape is a result of its mass being great enough so that gravity pulls all of the Moon’s matter toward its center equally.
Another distinct property the Moon possesses lies in its size compared to the Earth. At 3,475 km, the Moon’s diameter is over one fourth that of the Earth’s. In relation to its own size, no other planet has a moon as large.
For its size, however, the Moon’s mass is rather low. This means the Moon is not very dense. The explanation behind this lies in the formation of the Moon. It is believed that a large body, perhaps the size of Mars, struck the Earth early in its life. As a result of this collision a great deal of the young Earth’s outer mantle and crust was ejected into space. This material then began orbiting Earth and over time joined together due to gravitational forces, forming what is now Earth’s moon. Furthermore, since Earth’s outer mantle and crust are significantly less dense than its interior explains why the Moon is so much less dense than the Earth.
When viewed from Earth, the many impact lunar craters found on the lunar surface are visible. The reason for this is simple. Unlike the Earth, the Moon is not geologically active, and so it does not possess an atmosphere nor does it possess volcanic activity. Consequently, the Moon does not undergo resurfacing as does the Earth.