The earth and the moon are both bodies of mass, but the moon is much smaller than the earth. So why does the moon have any effect on the rotation of our planet? The answer lies in something called angular momentum, or simply, the speed with which an object rotates around an axis of its own length. This means that if you were to spin around in a circle while holding your arms out like you’re doing a limbo competition, you would have less inertia than if you were to hold your arms to your side while spinning around in that same circle.
What’s Gravity
Gravity is a force that acts between all masses in the universe. Everything in space is constantly being pulled towards one another. Since the Earth and moon are both massive objects, they are attracted to each other and pull on each other, too. As a result, we feel more gravity near the earth than near the moon. Scientists believe that this also contributes to why time moves slower as you go higher up into the atmosphere because there’s less matter around us so it’s easier for things to move away from us. When looking at the tidal forces on the earth due to the gravitational pull of the moon, we see that as much as 1/6th of our rotational kinetic energy may be lost due to friction with the tides. One theory holds that if not for these tidal forces slowing down our rotation, days would last just 24 hours long instead of 24 hours plus minutes and seconds. The opposite effect has been observed by astronomers studying planets orbiting close to their suns: Tidal forces slow them down and make their days longer. If the Earth were closer to the Sun, days would only be 10 hours long!
The Moon does have an impact on the Earth’s rotation but scientists don’t know how significant it is. We know that if the moon wasn’t here or was bigger then maybe nights wouldn’t exist! The moon slows down the Earth’s spin ever so slightly. On average, each day takes about 8 seconds longer than it otherwise would without the moon. While this doesn’t seem like much, over billions of years it can add up to quite a lot!
Newton’s Law of Universal Gravitation
Newton’s Law of Universal Gravitation (at least a year after his death) explained how forces between two objects could lead to motion. This law had three core propositions: that there is an attractive force between any two bodies in the universe; that this attraction is proportional to the product of their masses and inversely proportional to the square of their distance apart; and that this attractive force has instantaneous action at a distance, meaning it passes across empty space without any mediator or intermediary.
Gravity from the moon would not be enough to alter earth’s orbit around the sun but when combined with other factors such as tectonic plate movement, continental drift, and the obliquity of its axis, it can change the shape of our planet’s surface. The slight friction created by oceans moving against each other creates ocean tides on earth which would produce almost no water on one side if the moon were gone.
The Sun and the moon are perfectly spherical
The sun and moon are perfect spheres. They also have no effect on each other. As such, it is incorrect to say that the moon has an effect on the earth’s orbit as they do not share any gravitational effects with one another. If there were a gravitationally-bound object between the earth and the moon, this would actually pull them closer together which would cause tidal changes. If a heavy object was placed on or near the surface of our planet then this would cause us to lose momentum which would eventually slow down our rotation. However, because there is nothing in between the two bodies (moon and earth) their effect on one another is negligible at best so we can say that these two bodies do not share any gravitational force whatsoever.
In order for a gravitational field to be shared by both objects, the magnitude of each body must be close enough for its respective gravitational field to interact with the other. For example, if you dropped a rock from your hand and dropped a second rock from your other hand simultaneously, the two rocks would hit the ground at the same time even though you dropped them from different heights. That is because when you drop the second rock, its own gravitational field interacts with yours to make it fall faster than if it had been dropped from only your hand. There are three important things to keep in mind about this process: 1) all objects exert gravitational forces on all other objects 2) every single object shares some degree of gravitational force with every other object 3) the magnitude of each body’s field is proportional to its mass.
The center of gravity between sun and moon is beyond their surfaces
As a consequence, a moon orbiting a planet will exert no tidal forces on that planet. If a satellite is in an orbit with same-day contact (and hence zero angular momentum relative to the primary), then its distance from the primary can be increased by increasing its altitude. In such cases, the magnitude of external gravitational perturbations is reduced to below-zero values and may be neglected entirely in comparison with other effects. The direction of orbital precession caused by solar tidal torques is opposite to that caused by lunar ones. The sun rotates at more than 100 times the rate at which the earth rotates, so it appears stationary in our sky and we say it has an apparent or mean solar day. The sidereal day is longer than the mean solar day because there are stars rotating in the sky.
The sidereal day lasts about 23 hours, 56 minutes and 4 seconds, which means that for every 360 degrees the earth rotates around its axis one sidereal day occurs.
A star visible during daytime would have been seen close to sunset just a few hours before. Hence the length of this day varies, as observed in ocean tides: they are largest when nearest to new or full moon, since these positions cause maximally high spring tides due to the influence of both gravitational pulls; and smallest when at right angles to them – i.e., when at quadrature – since these positions cause minimally high neap tides due only to the pull of gravity.
Co-rotating bodies in free fall don’t experience any tidal forces
I cannot think of a situation in which a satellite will experience any tidal forces. This would only happen if they are not co-rotating with their host planet.
Even if you go to the moon and start throwing rocks at the earth, you will feel this pull because both bodies are rotating around one another. The planets, moons, asteroids and other solar system objects all co-rotate so that the mass is more evenly distributed. They rotate around each other as well as orbit around the sun or other object in the solar system. In the case of our Moon, it rotates synchronously while orbiting around us. It moves faster than we do so it feels a gravitational force on its surface but since it has no atmosphere and has no tectonic plates like we do on Earth there is nothing to absorb or resist these forces.
On average we are roughly 240 million kilometers from the moon. If you throw a rock straight up into the air, after falling back down onto the ground it will have fallen by about 7 meters! That’s how strong Earth’s gravity is.
An Interesting Phenomenon called Tidal Locking
Tidal locking is a pretty fascinating phenomenon that was first observed in 1736 by Sir George Hadley. He noticed that it takes as much time for one side of a planet to experience day and night as it does for the other side. This has since been confirmed on other planets and moons, such as Jupiter’s moon Io and Saturn’s moon Titan. The reason this happens is because there are two tidal forces acting on the rotating body. One force comes from gravitational attraction of the satellite while the other comes from centrifugal force due to its rotational velocity. The net result is an effect called tidal locking where they are locked into synchronous rotation so that only one face ever points towards each other. A special case of this is called true or direct tidal locking when both sides of the body always show the same face. In other words, they would not be able to rotate independently without outside interference or will. These sorts of tidal effects have been speculated about in relation to the Moon and Earth but never proven. And although tides are stronger near the ocean surface, their influence extends all the way down to depths of over 100 meters below sea level. So how do we know if lunar tides really do cause any change in the earth’s rotation?
A great question! We can look at anomalies in our measurements of star positions and see if anything jumps out. For example, did you know that every star is moving away from us at hundreds of kilometers per second? That’s because space itself is expanding.
