Why does the Earth Orbit the Sun?

Question

I know its to do with Gravity. But my question is more broad than the sun and the earth. Why does the moon orbit the earth? earth orbit the sun etc. What is it in the gravitational pull that makes the satellite go around. As i understand it, gravity is a pull towards, not around. You try to make a coin float in the earth atmosphere and gravity will pull it down; it wont start orbiting. So surely instead of going around the sun we should just be getting ever closer, rather than spiraling in to the sun. Where is the pull/ sideways energy that brings us around?

Answer 1

Basically, when two unbalanced forces are acting on an object, it move in an ellipsis (try rolling a marble down a slope and following the arc it makes as it rolls along the ground), so gravity is acting on celestial bodies, but so is the original force of expulsion (so if the Moon was made from something colliding with Earth, it got pushed outwards, but gravity keeps a leash on it).
If you want to think big, think of spacetime, like a giant trampoline and the Sun for example makes a big dip, if you roll the Earth-ball around it at just the right speed and distance from the Sun-ball, it just orbits it.

Answer 2

A simple way of thinking about it is considering the forces acting on the earth (from the suns point of view).

When the solar system was initially formed the earth was not just sitting there it would have had a speed in some direction. Unless this speed was directly towards the sun or away from it the force due to gravity changes the speed in two dimensions (I know that there are three dimensions out there however it can be shown that the motion is restricted to a plane). In this case some of the force goes towards changing the speed towards the sun and some goes towards changing the speed along the direction the earth is already moving in. This acts to change the direction of motion but can never change it towards the sun since gravity acts towards the sun not in circles around it, which means that the part of the speed directed around the sun can never be zero.

Provided that the objects kinetic energy is less than its potential energy the orbit is bound and the earth can never escape the sun and hence it orbits it in a closed curve. The general case for orbit is an ellipse with the sun at one focus (not the centre of the ellipse), however a circle is also possible for a closed orbit. If the kinetic energy were greater than the potential energy, the earth would not be in a closed orbit around the sun but would have had either a hyperbolic or parabolic orbit with the sun at one focus. Comets have a very high speed and hence a very high kinetic energy so they often have hyperbolic orbits around the sun and are only ever seen once.

Jez

Answer 3

Yeah yeah yeah! I know all that stuff about balancing gravitational forces and centrifugal forces in a frictionless environment (and I am sure foo_fighters_fan_2002 does too).

However, it doesn't really explain "why"!

Why is the earth travelling at exactly the right speed counterbalance the gravitational pull of the sun and so stay in an apparently perfect orbit around it?

Its like balancing a billiard ball on a pin head - such a perfect balance - seems deeply unlikely to be stable and yet it is.

Now if some genius out there can answer that one, we really would have some enlightenment!

The only thing I can think of is that there could have been loads of other planets that were either travelling too slowly or too fast, but they are all gone now (either crashed into the sun or flown away from it) and only the stable ones remain.

Answer 4

Two things that start at rest relative to one another will fall directly towards one another: but two randomly-chosen astronomical bodies will almost always have some relative velocity. If that relative velocity is sufficient to overcome gravitational attraction, they'll pass and go their separate ways: if it's not, they will begin to orbit their mutual centre of gravity.

If you throw an egg sideways, then (ignoring air resistance) it will actually begin orbiting its mutual centre of gravity with the Earth. Unfortunately for the egg, the disparity in size means that unless it's thrown very fast, the orbit will go very close to the centre of the Earth - and the egg will hit the Earth's surface as part of the orbit. But it is orbiting, albeit briefly.

Most things that orbit do so outside the atmosphere, because otherwise drag reduces the relative velocity, the orbit shifts, and eventually the two objects collide. But on a planet with no atmosphere or surface irregularities, you could in theory have an object orbiting 1cm off the surface. It will always be falling towards the planet, but only fast enough to always fall just over the horizon - when the planet isn't there any more, and gravity is now pulling in a different direction.

In summary, gravity produces the pull towards; initial conditions produce the motion past; and it is the combination of the two which makes an orbit.

Answer 5

Newtons Laws of motion (as determined by experiment and observation)
1. (Law of inertia): A body at rest remains at rest and a body in motion continues to move at a constant velocity unless acted upon by an external force.
2. A force F acting on a body gives it an acceleration a which is in the direction of the force and has magnitude inversely proportional to the mass m of the body: .
3. Whenever a body exerts a force on another body, the latter exerts a force of equal magnitude and opposite direction on the former. This is known as the weak law of action and reaction.

From the first law we have the earth tending to travel in a straight line perpendicular to the earth sun axis. The second law has the sun pulling on the earth and towards the sun. This pull causes a deviation in the earths straight line path. after an infinitesimal movement the sun is pulling on the earth at it's new location and this continues in a near circle. The deviations from circular are to do with the conservation of energy-as the earth gets closer to the sun so its kinetic energy increases and the pull of the sun deviates less so the earth starts to move away and the oposite is true for when the earth is moving away. The third law tells us that the earth is also pulling on the sun with a force of equal magnitude. The sun being so much more masive responds to this force proportionately less as given by the second law. But it can be shown that the two bodies orbit a common centre of mass rather than either orbiting the other.

Answer 6

The Earth is going around the Sun so fast that there is an equilibrium between the two. So that there isn't much of any movement towards the Sun.

There have been studies that show that the moon is moving away from Earth (like a centimeter or 2 a century/year?) So your theory works there.

The 'energy' is the fact the Earth WANTS to keep going straight. The best example I saw of this was with a blanket and the sun in the middle. Of course, the blanket would be depressed in the middle. If you push a small ball straight across the blanket, it will curve around the Sun. Granted, this is a simple explaination of Gravity but it DOES show that in spite of a body wanting to go straight, gravity of a huge body (like the Sun) will 'force' it into a curve.

Answer 7

There are two main reasons why the earth orbits the sun. First, when the earth was formed as a ball of matter having mass and gravitational attraction based on that mass, it was already revolving around the sun due to its gravitational attraction and there is no appreciable friction (wind resistance, etc.?) to slow the earth down.

The gravity of the sun (and the earth) bend space and instead of flying straight off into space the earth follows (and helps create) the curved path ahead. Astronomers have noted that as a star nears our line of sight on the other side of the sun (especially during total eclipses of the sun) the star light is bent by the sun's gravity and the star changes its apparent position relative to the field of background stars (just as Einstein predicted!). If you could spin a heavy weight around your head at the end of a rope inside a circular room and let the rope out until the weight just touched the (greased?) wall you could release the rope and the weight would continue to orbit you directed by the curved space around you (neglecting the fact it would also soon fall to the floor in our little experiment).

Answer 8

Due to gravitational forces between Sun and earth and
the velocity of the Earth.

Velocity of Earth is less than the critical velocity required for it to escape the gravitational force and go away from the sun.

Velocity of Earth is higher than the maximum velocity required for it to go and hit / meet the sun under gravitational pull in a sort of parabolic route.

Due to the above 2 reasons Earth Orbit the Sun.
For the same reason moon orbits the earth.

Same for the satellites etc.

Remember : Due to velocity and momentum of the earth it tries to move in the straight line (Newton's first and second laws of motion). So gravitational pull acts to make that straight line motion circular or rather curved.

So earth always tries to get away from the sun in a straight line due to its momentum. Remember Velocity and Momentum are vectors and they act in a straight line at any instant of time.
Sun tries to pull it towards itself.
Ultimately, a balance is struck between these two where the movement of earth is observed as an orbit around the sun (sort of elliptical / circular ?).

Answer 9

The gravitational pull keeps the Earth and the Sun close together and the earth goes around the sun in a big circle, this gives us seasons such as winter, summer etc.
The earth going around the sun is also known as orbiting.

Answer 10

If the gravitational pull of the sun is constant then would the centripetal force propelling the planets not reduce over time and the they will eventually get pulled in towards the sun? If this is not happening then there must be some other propelling forces being applied to, or generated from the planets themselves in order for them to be able to maintain their velocity and orbit. This then begs the question as to whether they are are in fact constructed space vehicles rather than naturally occuring bodies? The same theory would apply ot the moon - please discuss!