Earth has got only one moon – a rocky, cratered place, about a quarter the size of Earth and an average of 384,400 kilometers (238,855 miles) away. It is simply called – well, “the Moon” because people didn’t know other moons around other planets existed until Galileo Galilei discovered four moons orbiting Jupiter in 1610. Here are 10 amazing moon facts.
1. Moon is actually not very reflective
When you are in a remote area with little to no light pollution, you’ll be amazed at how bright the moon is – especially if it’s the full moon. Hence, one might think the moon is a very reflective object.
But, in fact, it’s not.
Astronomers use the term “albedo” to measure the reflectivity of a celestial object, and the Moon’s albedo is just 0.12, which means our satellite reflects back only 12% of the sunlight that falls upon it. This is just slightly higher than that of worn asphalt!
For comparison, Saturn’s icy mon Saturn’s moon Enceladus has an albedo of 0.99. It reflects almost all the sunlight that falls upon it back!
How can it be? How does our moon look so bright, despite it having an albedo of only 0.12?
In fact, it’s completely normal, because the Sun and moon appear the same size in Earth’s sky: the Sun’s diameter is about 400 times greater – but the sun is also about 400 times farther away. That’s how total solar eclipses occur when the Moon passes between the Sun and Earth.
So, if our Moon had an albedo of, let’s say 0.99, like Enceladus, it’d be almost as bright as Sun when it’s a full moon!
2. Moon is moving away from Earth
Today, on average, the Moon is 384,400 km (238,000 miles) away from the Earth. But that was not always the case. Our satellite was much closer in the past.
Every year, the Moon is moving away from Earth at almost 4 cm (1.575 in). This drift is driven by the effect of the Moon’s gravity on the rotating Earth: Because of the gravity of the Moon, tides raise in the oceans which causes drag and thus slows the Earth’s rotation speed. The resulting loss of angular momentum is compensated for by the Moon speeding up, and thus moving further away from us.
This drifting was practically confirmed by American and Soviet experiments, using laser-ranging targets placed on the Moon.
3. There’s no dark side of the moon
Despite the 1973 Pink Floyd album with that name, there’s no “dark side of the moon”. Because our satellite is not illuminated by the Earth, it is illuminated by the Sun. All the surface of the moon gets lit by the Sun as the Moon rotates.
4. The Moon is tidally locked to the Earth
“Tidal locking” is the situation when an object’s orbital period around another astronomical body matches its own rotational period. Our moon is a great example of that.
Why tidally locking occurs?
Any two astronomşcal bodies exert gravitational forces on each other. Since gravity gets weaker with distance (actually the square of the distance, see notes 1), there is always a slightly stronger tug on the sides of these objects facing each other.
This difference in tug slightly distorts the bodies, stretching them towards each other, and creating tides – “tidal flexing”.
Now, these objects are also spinning. In the Earth and Moon example, before the tidal locking occurred, as the Moon stretches while still spinning, the “tide” created by the tidal flexing doesn’t point straight at Earth. It will have spun a bit off-center.
Now, since the bulge is off-center, the gravitational force being exerted on it isn’t symmetric anymore. Because of that, that bump gets pulled on in the opposite direction the moon is trying to spin. That’s called “tidal friction”. Tidal friction causes a loss in the angular momentum and the spin of the Moon slows down (see Moon fact #2 above).
But, the moon never stops spinning: once its orbital period equals its rotational period, there’s no more tidal friction. The moon is “tidally locked” to the Earth.
To see a graphical explanation of why tidal locking occurs, see the video below.
An extreme example of tidal locking in our Solar System is the case of Pluto and its satellite Charon. Charon is such a large satellite compared to Pluto that they are tidally locked together. In general, if both the mass difference between the two bodies and the distance between them is relatively small, each may be tidally locked to the other. This means that Pluto only sees one face of Charon and vice versa.
Earth will be tidally locked to the Moon, too
Remember, the Moon also causes tidal friction on Earth (see Moon fact #2 above). Because of that, the Earth will be tidally locked to the moon in about 50 billion years – so the Earth-Moon system will be like the Pluto-Charon system.
But, in 50 billion years, there might be no Earth and Moon.
5. There is water on the Moon!
For a long time, the Moon was considered bone dry. But, now we know that there’s actually water on the Moon, frozen to ice.
Despite that water, the Moon is really dry. For comparison, the Sahara Desert has 100 times the amount of water that NASA’s SOFIA mission detected in the lunar soil.
6. Without Moon, life on Earth would be very hard
The Moon’s presence helps stabilize our planet’s wobble, which helps stabilize our climate. Life probably wouldn’t start on an Earth without Moon – or at least it would be very different.
Furthermore, the giant impact that formed the moon most probably changed the Earth’s path around the sun. With this new big astronomical object orbiting us, the gravitational pull the Sun exerts was also changed.
Without that impact and the moon, we may not be in the same place in our orbit. Our planet might be out of the habitable zone or Goldilocks Zone, meaning either it could be too hot or too cold.
7. The moon was likely formed after a Mars-sized planet crashed into the young Earth
The most widely accepted theory of the Moon’s origin is known as the Giant Impact Hypothesis (sometimes called the Big Splash, or the Theia Impact). It suggests the Mars-sized body, called Theia, impacted Earth, creating a large debris ring around Earth, and the Moon formed out of that debris. The hypothesis additionally suggests that this collision also resulted in the 23.5° tilted axis of the earth, thus causing the seasons.
There are other hypotheses about Moon’s origin, like the Synestia hypothesis, though.
8. Moon is the 5th largest satellite in the Solar System
It’s the largest satellite relative to its planet, though: the Moon is 0.273 times the diameter of Earth.
The next largest moon relative to a major planet (Triton of Neptune) has a diameter ratio of just 0.0546.
That title belonged to Charon, Pluto’s moon, being 0.52 the diameter of Pluto. Until 2006 though, when Pluto was reclassified as a dwarf planet.
Moon is even larger than Pluto, which has a radius of 1188.3 kilometers (738.38 miles).
9. We see 59% of the Moon’s surface from Earth
The moon’s orbit is slightly elliptical. This gives us peeks beyond its average eastern and western horizons. Furthermore, the axis of the moon is slightly tilted (1.5424°). This reveals more of the lunar north and south poles. With these extras, we can see 59% of the surface of the moon. 41% is still the “far side”, we cannot see it from Earth.
10. Some places on the Moon are the coldest places in the Solar System
During the lunar day, the temperature on the Moon can reach 127 °C (260 °F). During the night, temperatures can dip to -173 °C (-280 °F).
But, there are craters on the Moon that never sees sunlight: these “craters of eternal darkness” are extremely cold.
Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (-238 °C or -397 °F) and just 26 K (-247 °C or -413 °F) close to the winter solstice in the north polar crater Hermite. The latter is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto.
1. Gravitational force calculation
The equation to calculate the gravitational force between two objects with masses of m1 and m2:
F = G x (m1 x m2) / r2
- G is the gravitational constant (6.67408 x 10-11 m3 kg-1 s-2)
- r is the distance between the two objects
- F is the magnitude of the gravitational force between the objects.
- Moon on Wikipedia
- “Moon Albedo” on the Universe Today website
- “Interesting facts about the Moon” on the Royal Museums Greenwich website
- “Why is the Moon bright? Is Easter a full moon? How long does a full moon last? Your Moon questions answered by an astronomer” on The Conversation website
- Why is the Moon moving away from us? on the BBC Science Focus website
- There’s Water on the Moon? on the NASA Moon website
- What is the Temperature on the Moon? on Space.com