Everything in space is moving in some way, shape, or form. Understanding how and why those objects move, though, is a different story. In this article, we take a look at the ways that our Moon moves in space.
A Misunderstanding of Earth’s Rotation
Early astronomers saw the Moon flying through our sky once a day and were baffled by how fast it must move. With the belief that the Earth was the center of the Solar System, an object like the Moon would have to make a trip around the Earth once every single Earth day.
We know now that we’re not the center of the Solar System, and what we’re seeing each day is the Earth’s rotation. Sure, the Moon does move each day, but it’s nowhere near that fast.
The Law of Momentum
Perhaps you weren’t looking for a lesson in physics, but Isaac Newton had a few things to say about motion. In his first law of motion, he states that any object in motion will remain in motion in a straight line unless it is acted upon by another force.
Whether on Earth or in space, this law holds true. For the Moon, if it is in motion, it’s going to continue doing so unless something stops it. On Earth, we have the force of friction that brings moving objects to a stop.
In space, there is no such opposing force. There’s nothing to slow the Moon as it moves through space, and as a result, will continue doing so unless something unexpected happens.
What Started the Moon Moving in the First Place?
When the Solar System first formed some 4.5 billion years ago, scientists believe it was first a dense cloud of gas and dust. Then, a shockwave from the explosion of a nearby star condensed that cloud into a ring and started it spinning.
From that spinning ring, our Sun, planets, and moons slowly took shape. Even today, all planets and most other objects in the solar system revolve in the same direction.
The Moon’s Movement
Like a sub-par dancer, the Moon only moves in two ways: it revolves, and it rotates.
The primary way we see the Moon move through space is in its revolution around our Earth. The Moon moves in a slightly oblong shape around us, returning to the exact location in space once approximately every 27.5 days.
As it makes this journey, the Moon travels through different phases as perceived by all of us on Earth. Its revolution also means it appears in the sky at different times of day or night.
As it moves through space around the Earth, the Moon travels an impressive 2,288 miles per hour (3,683 kilometers per hour).
The Solar System is a pretty flat place, and the Moon’s orbit is no exception. It follows an imaginary line in the sky called the ecliptic and deviates very little from that line no matter where it is in its revolution around Earth.
We always see the same face of the Moon in the sky, no matter what time of year it is that we look. While this might lead you to believe the Moon does not rotate, you’ll be surprised to find out that this isn’t the case.
Actually, seeing the same side of the Moon each day is proof that it does indeed rotate. If it didn’t spin, we on Earth would be subject to a different view of the Moon each time it was visible.
Instead, the Moon’s rotation is tidally locked with the Earth. As it turns out, the Earth’s gravity is so strong that it’s as if the Earth is grabbing the Moon by its sides and not allowing it to freely spin.
Thus, the Moon does rotate, but it does so at precisely the same time it revolves around the Earth. That means as it takes the Moon 27.5 days to make a full revolution around the Earth, it also takes the Moon 27.5 days to complete one rotation. By that logic, a day on the Moon is the same as a year!
In the distant past, the Moon would have spun free of the Earth’s gravity and rotated much faster than we see today. It’s possible it could have even been something similar to the Earth’s rotation once every 24 hours. But, over time, the Earth’s gravity slowed it to what we see today.
What Keeps the Moon From Floating Off Into Space?
As mentioned earlier in the article, the Moon wants to travel in a straight line through space. Fortunately for us, the Moon doesn’t do this, or it would float off, and we’d never see it again.
As we mentioned earlier, the Earth’s gravity has a strong hold on the Moon. Where the Moon would naturally want to head off into the far reaches of space, our Earth holds the Moon in a steady orbit around our planet.
This gravitational force doesn’t oppose the Moon’s motion as it travels through space, so the Moon is allowed to continue traveling at the same speed around us.
The Moon is slipping away from Earth at just over 1.5 inches (3.8 centimeters) per year, but this is insignificant given that the Moon is 238,900 miles (384,470 kilometers) away from Earth. In fact, it will take millions if not billions of years for this to make any difference.
Barring a massive collision from some other celestial body (that would likely have significant repercussions for the Earth as well), the Moon will continue its same dance through the sky for many years to come.
Through this movement, we’re blessed with the ability to see the Moon through its different phases. On rare occasions, we are even able to view an eclipse. The stars are beautiful, but the Moon and its movement is what really makes our sky unique.