Every 24 hours, surface is bathed darkness & sunshine, because planet completes one day. Sunrise & sunset happen like clockwork as Earth rotates steadily on its axis. But what about Earth’s satellite, the moon, does it rotate, too?
The answer is yes, moon rotates, but it does such a lot more slowly than Earth does. A “moon day” is around 29.53 Earth days, consistent with NASA. In other words, whereas Earth completes one rotation every 24 hours, moon experiences a sunrise nearly every 709 hours.
As with many celestial bodies, moon’s rotating motion could also be a remnant from its creation. One theory, called giant-impact hypothesis proposes that around 4.5 billion years ago, a body closely the dimensions of Mars smashed into the still-developing Earth, consistent with NASA.
This theoretical object is called Theia and the heat from Theia’s impact may have created magma oceans by melting the crust & caused Earth to eject spinning vaporized particles into the space.
According to the giant-impact hypothesis, these clouds of dust & gas were spinning from the force of original collision. Eventually, these swirling particles came together because mass attracts mass and as the gas condensed, it actually began to spin faster.
Consider a figure skater, who tucks their arms in as they spin on the ice. The skater’s mass is more compact closer to their middle, therefore the skater speed-up. This is because their angular momentum is conserved, a product of the rotational force needed to rotate an object or the moment of inertia & angular velocity.
In other words, it takes more force to rotate an object farther from center of gravity. So, if the figure skater’s arms are out, they spin slower and when they change by tucking them in, they spin faster.
The moon has kept its angular momentum, ever since the original collision billions of years ago. “Two spinning bodies smacked into each other and laws of physics tell us that the huge resulting cloud would stay spinning.
Eventually, that spinning dust cloud condensed to form the spinning moon,” said Daniel Moriarty, a lunar geologist at the NASA Goddard Space Flight Center & the University of Maryland.
Unlike Earth, moon doesn’t have an environment, so there’s no air resistance to slow down moving objects, as such, when objects are spinning, they tend to remain spinning.
Of course, there are other theories for how Earth got its moon. One is the capture theory, in which moon was a wandering body like an asteroid that was captured by Earth’s gravitational pull.
In this theory, moon was created elsewhere in the solar system, then began to orbit Earth because it was passing by, consistent with NASA, so it could have already had its own spin as it was pulled into Earth’s gravitational field.
Another theory is co-formation theory, in which moon was created at the same time as Earth. In this hypothesis, two massive objects five times the size of the Mars crashed into one another. Earth & its moon then condensed out of the clouds of matter that resulted from the collision, NASA reported.
However, it’s Earth that sets the speed of the moon’s rotation. The moon completes one revolution in about 27 days roughly the same time it takes for the moon to orbit Earth: 27.32 days.
As a result, people on the Earth only ever see one side of the moon. If a moon day were any more or shorter, we might be able to see all of the moon’s surface as the moon orbited Earth.
In fact, orbit & rotation are not perfectly matched, because the Earth actually travels in an oval-like elliptical orbit. When moon is at the point of its orbit closest to Earth, its rotation is slower than its orbit, allowing us to see an additional 8 degrees of its surface that we usually would.
The illusion of the moon not rotating from our perspective is caused by tidal locking or a synchronous rotation in which a locked body takes as long to orbit around its partner as it does to revolve once on its axis due to its partner’s gravity. (The moons of other planets experience the same effect.)
Moreover, moon is not an ideal circle; it’s lopsided. “The moon isn’t symmetric,” Moriarty said. “There are differences in mass and density between the hemispheres of the moon facing toward and faraway from Earth. The difference is so big that one side of the moon its longer side stays ‘stuck’ facing Earth thanks to gravity.”
“So, because the moon orbits the Earth, it rotates to keep the same side facing-us,” Moriarty said. “One rotation takes the same amount of time together orbit, in other words, about a month.”
Put another way, Earth & the moon exert a gravitational pull on one another and the gravitational force exerted is usually strongest where the two bodies are directly facing one another, causing both Earth & the moon to stretch slightly as they’re pulled in the direction of the other.
As a result, moon is stretched into an elliptical shape with its longest axis being tugged to always be facing toward us. This is also what causes Earth’s tides to travel in & out every day.
In effect, moon may rotate very slowly, but its rotation is vital to important events, like the tides, on Earth.
Tidal locking also affects how planets & moons move. This means that the days on Earth & the moon were much shorter when these bodies first formed, because both Earth & the moon revolved at a way faster rate than they are doing currently.
A model by researchers at Harvard and SETI Institute even estimates that the early Earth had each day as short as 2.5 hours at the time of its collision with Theia. However, because of gravitational attraction constantly tugging on the moon’s longest axis to face towards the Earth, the Earth’s & moon’s days lengthened over time.