eli5: why are the orbits of celestial bodies elliptical?
In: 1
Because f=gmm/r2
Orbits are elliptical because the force of gravity decreases as the square of the distance between objects. Newton proved this.
Edit: to clarify a bit, Kepler showed that the orbit of Mars and other objects was elliptical, based on observations mostly from Tycho Brahe. Newton proved that only a square-diminishing force would produce such orbits.
It’s just more likely.
A circle is an ellipse. It’s just a special one where the “horizontal stretch” is the same length as “the vertical stretch”. But there are an infinite number of ellipsis shapes, and only one possible circle. So it’s just more likely by far that any orbit will be a normal ellipse rather than the very special circle ellipse.
Ellipse is just one possibility of an orbit, it can actually be any conic section (only approximately in reality)
The question why is hard to answer without some arithmetic, but I’m going to try to simplify as best I can.
Imagine being in space around a bigger body and moving with respect to that body. It will draw you to itself bc of gravity. That means your path will be slightly bent by that force. After a while, you moved a bit on that bent trajectory and the planet will be somewhere else from your POV (I don’t know how to better put it. When you pass a car, before you have to look forward to see that car, after you will have to look back). That means the direction of gravity will also have changed from your perspective, curving your path even further. In the end and absent any disturbances, you will arrive at the starting point with the same speed and then make another round.
The math about how gravity works that in theory orbits all follow a certain shape.
The force of gravity declines with the square of the distance. aka the “inverse square law”.
If you plug that into some equations and ignore real world factors and things like general relativity, you get one standard equation about how two object should move in regards to one another on account of gravity.
You can group that into categories: circle, elliptical orbit, parabolic trajectory and hyperbolic trajectory. Circle and parabolic arch are just mathematical possibilities that appear when the numbers work out just right, which they don’t in real life.
A circle is just a special type of an ellipsis, like square is a special type of rectangle.
A circle is when mathematically the ellipsis has zero “eccentricity”, (you can think of that as how stretched the ellipsis is.). The closes the eccentricity gets to one the more stretched the ellipsis gets.
Once the eccentricity hits one you don’t have an ellipsis at all any more but a parable, if you go beyond on you get a hyperbole.
Parable and hyperbole means that it isn’t a proper orbit because the thing won’t come back around again.
So with circle only a mathematical possibility and the other stuff only working once and never coming round again, you are left with only ellipsis in real life.
A guy named Kepler worked that out in the early 1600s.
The vast majority of celestial bodies in our solar system have elliptical orbits. This includes planets, asteroids, comets and even some moons. An elliptical orbit is simply an oval-shaped path that a body follows as it revolves around another body. The reason why most orbits are elliptical has to do with the laws of gravity and motion.
In general, any two objects will be pulled towards each other by the force of gravity. The amount of this force depends on how massive each object is and how far apart they are from each other. If you think about what would happen if there were only two objects in space – say a planet and a satellite orbiting around it – then you can see that the satellite would be constantly pulled towards the planet by gravity. However, at the same time, the satellite is also moving through space so it has momentum that keeps it moving forwards (this is called its orbital velocity).
If there were no other forces acting on them then these two objects would eventually crash into each other because gravity would pull them together while their forward momentum would carry them apart again (think of throwing a ball up into the air – eventually it will come back down to Earth becausegravity pulls it down while its forward momentum slows down and reverses direction). In reality though, there are always potential for other forces to act upon these kinds of systems such as friction or air resistance (in planetary atmospheres), which can change things significantly!
But in cases where there aren’t any significant outside forces acting on a system like this, we find that orbital paths tend to follow an ellipse rather than being perfectly circular. This happens because an ellipse represents the shape that minimizes gravitational potential energy whilst still allowing for enough kinetic energy to maintain orbital velocity; thus resulting in more stable long-term orbits for both celestial bodies involved
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