How do satellites stay in orbit? Free fall or centripetal?

[waznboyd]

Hi, I just recently posted this on the astrophysics forum but thought it would probably not quite fit so i decided to move it here.

Hi, this is my first post. I have been trying to find an answer out of my own interest but is unable to do so. while searching I found this wonderful website. I hope you guys can help.

I came across a series of questions today and one of the question asks "What keeps satellites in orbit?" The answer choices were inertia, free fall, and centripetal acceleration (I knew it wasnt inertia so I am down to free fall and centripetal) I picked free fall. HOWEVER, the answer was actually centripetal acceleration. I think that, with the amount of speed and height, a satellite can travel faster than it can fall and thus continue try to fall but unable to do so because it is traveling too fast about the Earth's curvature and thus experience continuous freefall. Can someone explain this? Thanks

By the way, one of the results i came across is found on:
http://www.boeing.com/companyoffices...light/iss.html

Thanks for any explanation , I am quite curious.

 

[G01]

Hmmmm. Free fall is the name of the situation the satellite is in, but its not what keeps it orbiting. The centripetal acceleration causes the object to constantly accelerate toward the center of the Earth and thus orbit. Free fall is a result of this acceleration and the objects velocity going around the earth, not the cause of the orbit itself...That help?

 

[Mk]

Think of a stone tied to the end of a piece of string held in hand and whirled in a circle. The stone simulates the satellite, and the hand is the Earth. Centrifugal force pulls outward, but the taut string holds the stone in its circular orbit. If the speed of the stone is too low, the stone doesn’t move in a circle, but falls toward the hand holding the string.

There’s no air resistance in space, so as soon as a satellite has gained the right speed, it retains that speed because of inertia. Satellites fly in stable orbits, for which satellite speed and distance from the Earth are calculated accurately.

 

[rbj]

here's another thought (derived from Newton): think of a planet without an atmosphere and with a tall mountain in which an astronaut has a high powered rifle and shoots a bullet horizontally. gravity acts on the bullet and it starts to descend outlining a curved trajectory. if the muzzle velocity was just the right speed, the curvature of that initial trajectory would be the same as the curvature of the planet at that elevation. so the bullet would be "dropping", but the ground underneath it would be "dropping out" underneath the bullet at the same rate (due to the roundness of the planet) therefore the bullet would maintain the same elevation above the mean elevation of the planet's surface.

for a JAVA app demonstrating this, see: http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=24 .

 

[vanesch]

here's another thought (derived from Newton): think of a planet without an atmosphere and with a tall mountain in which an astronaut has a high powered rifle and shoots a bullet horizontally. gravity acts on the bullet and it starts to descend outlining a curved trajectory. if the muzzle velocity was just the right speed, the curvature of that initial trajectory would be the same as the curvature of the planet at that elevation. so the bullet would be "dropping", but the ground underneath it would be "dropping out" underneath the bullet at the same rate (due to the roundness of the planet) therefore the bullet would maintain the same elevation above the mean elevation of the planet's surface.
.

... and then the astronaut better gets off the mountain soon after he fired!




As to the original question: what keeps satellites in orbit ? Poles, no ? Aren't they all fixed on a very long pole ? just kidding.

What keeps satellites in orbit is gravity. If it weren't for gravity, they'd go straight off in a tangent. Gravity makes them follow a curved orbit. And, the Earth being the source of the gravity, it's in the center of the orbit. Now, if the Earth is smaller than the orbit, the satellite will not hit the Earth's surface, and keep on running on its orbit. On the other hand, if the orbit is smaller, then the satellite will hit the Earth's surface. In usual speak, we say it dropped down.

 

[waznboyd]

... and then the astronaut better gets off the mountain soon after he fired!




As to the original question: what keeps satellites in orbit ? Poles, no ? Aren't they all fixed on a very long pole ? just kidding.

What keeps satellites in orbit is gravity. If it weren't for gravity, they'd go straight off in a tangent. Gravity makes them follow a curved orbit. And, the Earth being the source of the gravity, it's in the center of the orbit. Now, if the Earth is smaller than the orbit, the satellite will not hit the Earth's surface, and keep on running on its orbit. On the other hand, if the orbit is smaller, then the satellite will hit the Earth's surface. In usual speak, we say it dropped down.

Then in this case, the only force acting on it is gravity which can cause freefall and/or centripetal force right? Since the Earth is the source of gravity AND the center? Then wouldn't free fall and centripetal be the same thing: gravity?

 

[vanesch]

Then in this case, the only force acting on it is gravity which can cause freefall and/or centripetal force right? Since the Earth is the source of gravity AND the center? Then wouldn't free fall and centripetal be the same thing: gravity?

What's "free fall" ? Within Newtonian gravity (and there's no need to go beyond that here, so this viewpoint is implicitly assumed), free fall is the state of motion that results from the workings of the force of gravity, and only gravity, on an object. Free fall essentially means: ONLY gravity force, no other forces.
However, in order to work with forces in the first place, and to use Newton's equation m.a = F, you have to make sure that you apply this law in AN INERTIAL FRAME.

Now what is "centripetal" force ? It is an ARTIFICIAL FORCE that is introduced when you are working in a rotating reference frame and you'd like to pretend that it is an inertial frame. It is because m.a = F is a priori NOT correct in a non-inertial frame. However, you can PRETEND m.a = F to be correct, on the condition that you add an artificial force to F, which is the centripetal force.

But I invite you to look upon the satellite and the Earth in an INERTIAL frame (NOT rotating with the earth). Then you see the Earth rotating, and the satellite swirling around it - simply because of the force of gravity exerted by the Earth on that satellite.

 

[jtbell]

Now what is "centripetal" force ? It is an ARTIFICIAL FORCE that is introduced when you are working in a rotating reference frame

Are you sure you're not thinking of centrifugal (outward) force? Centripetal (inward) force is very real in this situation. It's just the gravitational force.

 

[DaveC426913]

The key here, I believe, is to ask the question: what WOULD the satellite do if otherwise left to its own devices?

A] fall down
B] go straight

If left to its own devices (it has an intrinsic velocity in a specific direction), the satellite would continue on a straight path forever. Going straight is the satellite's DEFAULT behaviour, thus the question is not "what keeps it from falling down?", the question is "what keeps it from flying off into the solar system?". The correct answer is B].

It is the Earth's gravity that pulls the satellite into a curve. Gravity is the force that provides centripetal acceleration.

 

[vanesch]

Are you sure you're not thinking of centrifugal (outward) force? Centripetal (inward) force is very real in this situation. It's just the gravitational force.

Eh, yes. Sorry. Mixed up the names - I've never been able to know which is which.

 

[pallidin]

Not everything in orbit stays in orbit. There are many factors which determines the potential decay and longevity.

 

[Mk]

What goes up ends up going down. Satellites end up burning in the atmosphere.

 

[rbj]

What goes up ends up going down. Satellites end up burning in the atmosphere.

geez, i dunno. those geosynchronous orbits are pretty far out there, not a lot of atoms to run into that slows the satellite down.

i suppose there is the density of stuff in space that is normal for our distance from the sun (what is it? a couple atoms per cubic meter?), but i don't expect the Earth's orbit around the sun to decay any time in my great-grandchildren's lifetime.

 

[vanesch]

What goes up ends up going down. Satellites end up burning in the atmosphere.

I don't think Voyager I and II are going to come down :-)

 

[rbj]

I don't think Voyager I and II are going to come down :-)

didn't know thems were satellites. i think they have exceeded escape velocity for the solar system. i thought i read that in 20,000 years or so, one of them will begin accelerating toward another system. that would mark the point where it last really leaves our solar system (when it "escapes" our system's gravity and is more influenced by another system's gravity.

maybe they'll "come down", but somewhere else, very far away.

 

[Mk]

didn't know thems were satellites.

Ah ha! They aren't satellites! Space probes!

 

[u83rn00b]

As you know, gravity holds on to the satellite. But if you could turn off gravity, the satellite would keep moving — but in a straight line. It would leave Earth. If you stopped the satellite and turned gravity back on, the satellite would fall straight down to Earth. But if you have both gravity pulling down and the speed of the satellite pushing out, the two balance out so that the satellite can go in a circle around Earth.

 

[YellowTaxi]

But I invite you to look upon the satellite and the Earth in an INERTIAL frame (NOT rotating with the earth). Then you see the Earth rotating, and the satellite swirling around it - simply because of the force of gravity exerted by the Earth on that satellite.

This ^ is a geostationary orbit - so the satellite looks like its just sitting in the air with nothing whatsoever holding it up (no apparent orbital motion from any transverse velocity). We know there IS velocity, it's just that the Earth is rotating at exactly the same speed, so we can't see any motion.

But what's to say for certain that the Earth IS actually rotating and not the rest of the universe ?

 

[jtbell]

Foucault's pendulum and the Coriolis effect are two phenomena that indicate that the Earth is rotating. Or has someone come up with a calculation that predicts the same effects for a non-rotating Earth and rotating universe?

 

[YellowTaxi]

Mmm, wiki says Foucult's pendulum was first demonstrated in 1851
So, before then there was still no logical reason to be absolutely sure that the Earth was not the centre of the universe, even if following Newton's and keplers laws ? And 1850, that's only 150 years ago..

 

[vanesch]

But what's to say for certain that the Earth IS actually rotating and not the rest of the universe ?

Ah! Mach's principle.

 

[u83rn00b]

Hi every1, I am currently doing a posterboard and i need sum help...I need to know how satellites stay in space..i need to sdo an expierement then post pictures, questions, summarys, etc. Anyone have and ideas on how to signify how a satellite stays in space?
A friend of mine said that I could get a balloon, and a bowl of water. He said I should fill the balloon with air and the bowl with water, then from the bottem of the balloon pull it into the water. He claimed that if i pictured it upside down (the water as the Earth's gravity and the balloon as the satellite) would show how the satellite stays in space...Is this valid??or bs?? HELP ME!

 

[rcgldr]

"What keeps satellites in orbit?" The answer choices were inertia, free fall, and centripetal acceleration

You eliminated inertia too soon. A satellite stays in orbit because it's inertia (speed) resists the gravitaional pull by the right amount to maintain an orbit, as opposed to excessive speed where it's inertia causes it to leave the Earth's gravitational pull, or insufficient speed, where it will collide with the Earth's atmoshpere.

If the Earth was replaced by a point source with the same gravitational pull, then a satellites speed would always orbit the point source, and the size of the orbit would correspond to the total energy (sum of potential and kinetic (speed-inertia related)) of the orbiting satellite.

There is a maximum potiential energy for a gravitational field generated by a point source, and if the kinetic energy is greater than this, the satellite will escape and never return, and the path will be a hyperbola. If it's the same, the satellite will still never return, but the path will be a parabola. In the real world, it's very unlikely for the energy to be exactly the same, so this is more of a theortical case.

 

[u83rn00b]

How this expierement for my board? i got it fro NASA. :tongue2: THANK U NASA!:tongue2:

1. Cut a piece of nylon to put around the rubber ball.

2. Tie one end of a 3-foot length of string around the nylon.

3. Hold the other end of the string and begin to whirl the ball over your head.

The ball is held in its "orbit" around your head by the string, which is similar to the force of gravity that pulls satellites toward the Earth.

The forward motion of the ball is its momentum. If the "gravity" of the string were not acting on the ball, the ball would continue in one direction. The swinging of the ball gives it its forward motion. When these two forces are equal, the ball remains in orbit, without falling into or flying away from the Earth (you). A satellite's forward motion is controlled by rockets. When the rockets are not fired, inertia keeps the satellite going in one direction.

4. Discuss other examples of momentum and gravity, for example, swinging a bucket of water over your head without getting drenched; or riding amusement park rides where you turn upside down or in a loop without falling out.

5. Let go of the string.

What happens? (The ball flies outward, since gravity is now equal to zero.)

 

[brainstorm]

According to Newtonian gravity, objects accelerate toward each other at a rate of acceleration determined by the distance between the objects. In that case, a falling object may be described as accelerating due to gravitational force at an increasing rate. Acceleration is already an increasing rate of velocity, so the increase in gravitational force that occurs as the distance between the two objects decreases may be seen as an additional layer of "acceleration" in the sense that the force of gravity is increasing and causes the rate of acceleration of the falling object to increase, no?

If a satellite in orbit is also accelerating due to gravity, how can its velocity and altitude remain constant? Does the curvature of its orbital trajectory account for a constant increase in energy that supplants the tendency to accelerate?

The notion of space-time curvature suggests to me that perpetual orbital trajectories may actually be straight-lines in the Newtonian sense that an object in motion tends to stay in motion unless acted upon by external force. But is gravity then a force or purely curvature of space-time?

 

[ruko]

Are you sure you're not thinking of centrifugal (outward) force? Centripetal (inward) force is very real in this situation. It's just the gravitational force.

Inertia plus the gravity well of the Earth permit satellites to orbit. Centrifugal force is in fact, inertia.

 

[brainstorm]

Inertia plus the gravity well of the Earth permit satellites to orbit. Centrifugal force is in fact, inertia.

When would you call an object "at rest?" From a terrestrial perspective, I would say that an object standing still on the ground is "at rest" but from a relativity perspective I would venture to say that the same object is accelerating (potentially) toward the center of the planet.

If gravity is a force that causes objects to (tend to) accelerate toward areas of lesser space-time dilation, an object standing still on the ground could also be viewed as an object with the tendency to remain in motion, which has been obstructed from doing so by the ground.

An object in free-fall, on the other hand, is an object not being impinged upon by an outside force, except to the extent gravity is acting on it. However, if space-time is actually curved, with the object simply following the path of least resistance through it, then gravity would not be viewed as an external force acting on the object, right?

A mixed-scenario where an object in motion is not being acted upon by outside force, but is also not in orbit/free-fall might be a situation in which the object is following a path between two or more gravitational centers that cancel out each other's influence on something moving between them.

In that case, would the object slow down as a result of space-time contraction, despite not falling into orbit around either? Or would it maintain constant velocity and course, i.e. the gravitational fields canceling each other out with regard to the object moving between them?

Can an object ever be accurately described as being "at rest," or is any object always moving relative to other objects - unless it is being impeded by equal and opposite force from them?

 

[D H]

According to Newtonian gravity, objects accelerate toward each other at a rate of acceleration determined by the distance between the objects. In that case, a falling object may be described as accelerating due to gravitational force at an increasing rate.

The latter is true only if the distance between the two objects is in fact decreasing.

If a satellite in orbit is also accelerating due to gravity, how can its velocity and altitude remain constant? Does the curvature of its orbital trajectory account for a constant increase in energy that supplants the tendency to accelerate?

What increase in energy? The total mechanical energy of an orbiting satellite is constant in Newtonian mechanics. You should learn Newtonian mechanics before you start delving into general relativity (as you did in a later post). You need to learn to walk before you can learn how to run.

The notion of space-time curvature suggests to me that perpetual orbital trajectories may actually be straight-lines in the Newtonian sense that an object in motion tends to stay in motion unless acted upon by external force. But is gravity then a force or purely curvature of space-time?

In Newtonian mechanics gravitation is a force. In general relativity it is something different.


======================

Inertia plus the gravity well of the Earth permit satellites to orbit. Centrifugal force is in fact, inertia.

It is best not to invoke centrifugal force when explaining orbits. What inertia are you talking about? (More on this question later.)

There is no need to invoke the concept centrifugal force when describing orbits from the perspective of an inertial (non-rotating) frame of reference. That's a good thing; there is no centrifugal force in a non-rotating frame of reference. That orbits are conic sections for an inverse square law force falls right out of the math.

That said, one can explain circular orbits using centrifugal force. From the perspective of an observer rotating at orbital rate, an orbiting satellite is suspended. It is not moving. There is a non-zero centrifugal force in this the rotating frame of reference. Since the orbiting satellite is not moving, the net apparent force on the satellite must be zero. Since the object is not moving in this frame of reference, it has zero inertia (momentum) in this frame.

To say that inertia is centrifugal force is nonsense for (at least) two reasons. Firstly there is a units mismatch. Inertia has units of mass*length/time. Force has units of mass*length/time². Inertia and force are incomparable quantities. Secondly, you are mixing frames.

 

[brainstorm]

What increase in energy? The total mechanical energy of an orbiting satellite is constant in Newtonian mechanics. You should learn Newtonian mechanics before you start delving into general relativity (as you did in a later post). You need to learn to walk before you can learn how to run.

Walk? I'm crawling on the shoulders of giants:) But I do believe I understand the basics of what I'm crawling on. If Newtonian physics views an object in orbit as a straight-line trajectory being consistently pulled into a curve, doesn't the deviation from the straight-line require energy?

Think of a roller coaster entering into a curve from a straight run. In the straight run, all the roller coaster's momentum is in the forward direction. Once it enters into the curve, the inertia transitions into centripetal force, no?

Disregarding the friction of the track, etc. - doesn't the coaster require energy to change direction through the curve. In the straight run the coaster is an object in motion tending to stay in motion without impingement. In the curve it is being impinged upon to deter it from its straight line path, hence the centripetal force.

Admittedly, this is getting confusing but it seems like an object on a circle track expresses energy differently than one on a straight track.

In orbit, this is different because there is no straight-line path from which to deviate. For a satellite to take a straight path, it would have to cut across a lower altitude, which would require acceleration and then deceleration as it returns to the same altitude again. The curvature of the orbital path is the "straightest" trajectory insofar as momentum and velocity remain constant.

Do you still think I'm getting something wrong?

 

[D H]

If Newtonian physics views an object in orbit as a straight-line trajectory being consistently pulled into a curve, doesn't the deviation from the straight-line require energy?

No, yes, and no.

The first no answer: Strictly speaking, a force is required.

The yes answer: For conservative forces, the force is the gradient of the potential energy.

The second no answer: Let's start with a circular orbit. The force is always perpendicular to the velocity vector, so no work is done. There is no change in kinetic energy, and because the object is at a constant distance, there is no change in potential energy, either.

For a non-circular orbit, total mechanical energy is still a constant of motion. Gravitation is a conservative force. Total mechanical energy is a constant for any conservative force.

 

[brainstorm]

No, yes, and no.

The first no answer: Strictly speaking, a force is required.

The yes answer: For conservative forces, the force is the gradient of the potential energy.

The second no answer: Let's start with a circular orbit. The force is always perpendicular to the velocity vector, so no work is done. There is no change in kinetic energy, and because the object is at a constant distance, there is no change in potential energy, either.

For a non-circular orbit, total mechanical energy is still a constant of motion. Gravitation is a conservative force. Total mechanical energy is a constant for any conservative force.

Ok, just for clarity's sake take a comparative example of two rockets.

Rocket A is in circular orbit around the sun. Rocket B is traveling at the same speed as Rocket A far from any significant gravity well.

If Rocket B wants to replicate the curvature of Rocket A's path, it has to expend fuel for constant trajectory corrections. It can do this, presumably, without increasing velocity beyond that of Rocket A in orbiting unpropelled around the sun, right?

So isn't Rocket B's fuel expenditure and propulsion energy and work? And isn't that energy and work equivalent to the effect of the sun's gravity on Rocket A's trajectory in the absence of propulsion?

 

[ruko]

The latter is true only if the distance between the two objects is in fact decreasing.


What increase in energy? The total mechanical energy of an orbiting satellite is constant in Newtonian mechanics. You should learn Newtonian mechanics before you start delving into general relativity (as you did in a later post). You need to learn to walk before you can learn how to run.



In Newtonian mechanics gravitation is a force. In general relativity it is something different.


======================



It is best not to invoke centrifugal force when explaining orbits. What inertia are you talking about? (More on this question later.)

There is no need to invoke the concept centrifugal force when describing orbits from the perspective of an inertial (non-rotating) frame of reference. That's a good thing; there is no centrifugal force in a non-rotating frame of reference. That orbits are conic sections for an inverse square law force falls right out of the math.

That said, one can explain circular orbits using centrifugal force. From the perspective of an observer rotating at orbital rate, an orbiting satellite is suspended. It is not moving. There is a non-zero centrifugal force in this the rotating frame of reference. Since the orbiting satellite is not moving, the net apparent force on the satellite must be zero. Since the object is not moving in this frame of reference, it has zero inertia (momentum) in this frame.

To say that inertia is centrifugal force is nonsense for (at least) two reasons. Firstly there is a units mismatch. Inertia has units of mass*length/time. Force has units of mass*length/time². Inertia and force are incomparable quantities. Secondly, you are mixing frames.

What inertia you ask? How about a 1000 pound satellite traveling around the Earth at 17,000 MPH. Do you think this would have some inertia? How can you possibly say this satellite has zero momentum or inertia in any reference? This is nonsense. If this were true the satellite would fall to the earth.

Lets say I'm driving my car and I make a turn to the left. In England I would turn right. On the bench seat next to me is a ball free to roll. When I make the turn what happens to the ball? The inertia of the ball traveling at the speed of the car wants it to continue in a straight line doesn't it? Is this inertia or this fictitious centrifugal force? What stops the ball from continuing on in a straight line? The door. What keeps the ball hugging the door as I turn? The same thing that produced the roll, inertia. What stops a satellite from continuing on a straight line due to the tremendous inertia it acquired in the acceleration stage? The gravity well of earth.
www.britannica.com/EBchecked/topic/.../centrifugal-force[/URL]

 

[D H]

What inertia you ask? How about a 1000 pound satellite traveling around the Earth at 17,000 MPH. Do you think this would have some inertia?

You are looking at the satellite from the perspective of an inertial frame of reference. Those who insist on using centrifugal force to explain satellite motion are implicitly looking at the satellite from the perspective of a co-rotating frame of reference. The satellite has zero velocity in such a frame.

How can you possibly say this satellite has zero momentum or inertia in any reference? This is nonsense. If this were true the satellite would fall to the earth.

A good example are the geosynchronous satellites. A simple, fixed dish works just fine for receiving a high bandwidth signal from a geosynchronous satellite. From the perspective of an Earth-fixed observer, those geosynchronous satellites are not moving. Compare to a polar orbiting weather satellite. While an omni-directional receiver on the ground suffices for the low bandwidth signals broadcast by those satellites, receiving the high bandwidth signals requires the use a (very expensive) tracking antenna. Those polar orbiters are moving with respect to the Earth.

So how to explain how those geosynchronous satellites don't just fall to the Earth? From the perspective of an inertial observer, those satellites are moving. They are following a curved path. That means some net force must be in play. However, from the perspective of an Earth-fixed observer, those geosynchronous satellites are stationary. The apparent net force on those satellites must be zero. An Earth-fixed observer is a observing from the perspective of a rotating frame of reference, and it is those rotating frames of reference in which fictitious forces such as the centrifugal force arise. A geosynchronous satellite is stationary with respect to an Earth-fixed observer because the centrifugal force cancels the gravitational force.

 

[brainstorm]

DH, your post is insightful. I think the confusion comes from the fact that an orbiting satellite is in equilibrium for its position in the gravity well. "Centrifuge force" implies that something has an inertial tendency outward from the center, whereas an orbiting satellite does not have such a tendency except to the extent that a hypothetical situation is imagined where other matter in the gravity-well suddenly disappears or escapes, or gravity diminishes for some other reason.

Another part of the confusion is caused by normative distinctions between potential and kinetic energy, I think. If you think of a rotating cloud of disassembled dust or other particles instead of a planet with tensile integrity, all gravity would take place in the form of free-falling momentum, i.e. kinetic energy. This is assuming no friction between any of the particles, since any friction would create some level of potential energy, presumably.

So to the extent that such a rotating dust-cloud is wholly in free-fall in its very rotation, there is no basis for saying that one thing is orbiting another. Everything is orbiting the fulcrum, again ignoring any turbulence or frictions between particles.

So is/should interpretation of the forces and energy-behavior of the satellite be considered relative to an observer frame of reference, or should the object simply be treated according to its own inertial characteristics within its own frame of reference?
From a Newtonian perspective, I believe it would be treated as an object at rest since an object in orbit is basically analogous to an object at rest on Earth, discounting potentialization of gravity for the Earth-bound object pressed up against the ground due to tensile friction and sluggish rotation of the planet.

In relativity, however it would only be treated as stationary to the extent it is not framed in reference to any other object. Once it is framed relative to the Earth, sun, moon, etc. space-time curvature comes into play because its position and characteristics are considered in relation with other objects. Within its own frame it is stationary and the movement and characteristics of other objects appear relative to itself.

 

[ruko]

So how to explain how those geosynchronous satellites don't just fall to the Earth? .

Easy. Geo satellites are in orbit traveling at about 6900 MPH with an altitude of about 22,000miles. The exact same mechanism holds for satellites in low or high Earth orbit. It just so happens this altitude and speed produces an orbital period that matches Earth's period of rotation. To us the geo looks like it is motionless but it is definitely moving very fast.

Something I'm curious about geosats. Are the antennas on them directional? If so the satellite would have to rotate 360 degrees on its axis during every orbit to keep the antennas facing the earth. Just like the moon keeps the same side to Earth by rotating during the journey.