eli5: why are the orbits of celestial bodies elliptical?
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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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