And how exactly did we find out how they move?
In: Planetary Science
Our orbits are pretty circular. The most elliptical is Mars with a value of 0.2, with 1 being the most elliptical.
But over the history of the solar system things like merging gas masses at different orbits, impacts of protoplanets into each other, planets passing through clouds of gasses… Pretty much anything that could affect the speed of the planet would lead to it being less circular in its orbit and those effects add up over time.
It’s a much more natural orbit. Although I’m sure there are planets out there with much more circular paths than we see in our solar system. But it basically works like this, any orbit that isn’t a perfect circle (impossible because of nature) is an ellipse. It’s also a more stable orbit, because if something pushed or pulled on something with a truly circular orbit, it would fall out of that orbit.
I believe Johannes Kepler discovered that previous were elliptical.
Probably something to do with the tilt of a planet’s axis, which is basically where the top and bottom points of the planet are, that experience the least movement from rotation
If you have a perfectly circular orbit and a speck of dust collides with the planet then it’s no longer a perfectly circular orbit. It stays an elliptic orbit. A circular orbit is a special case of an elliptic orbit. The orbits of the planets are not that far away from a circle, but there is no reason why they should be exactly circular.
Planets move in elliptical orbits instead of circular ones because of how gravity works. Gravity pulls the planet toward the Sun, but since the planet is also moving sideways, it ends up in a curved path. This path is usually an ellipse because of the way gravity weakens with distance (inverse-square law) and because planets move faster when they’re closer to the Sun and slower when they’re farther away. So, the combination of the gravitational pull, the planet’s speed, and its distance from the Sun naturally leads to an elliptical orbit.
A circular orbit is a perfect elliptical orbit. Nothing in nature is perfect; a better question would be “why are orbits elliptical in the first place?”.
The answer to that: two objects in space will always exhibit gravitational attraction towards each other. Imagine you had two objects in space, one more massive than the other. Eventually, gravity will bring these two objects together, and they collide.
Now imagine that you give these objects some relative velocity. It doesn’t matter if you push the smaller object or the bigger object; in space, the difference is irrelevant. If your initial velocity is in a direction that is NOT directly towards the other object and NOT directly away from the other object, you now have a situation where the two objects are still trying to be attracted to each other, but the relative motion means that the direction from one object to the other is constantly changing. Due to inertia, the more massive object will be much less mobile than the less massive object. With enough of a mass difference, we tend to model the more massive object as effectively stationary.
Imagine you were walking in a straight line. Now imagine you still tried walking in a straight line, and grabbed a stationary pole next to you. Because of the presence of the pole, you’re now walking in a curved path. This is basically what is happening to the big object and the small object.
Sometimes the smaller object has so much velocity and distance that the gravitational force of the bigger object isn’t enough to keep it. At some point then, the smaller object is basically “released” by the bigger object’s gravity if the velocity and distance are great enough. But otherwise, the smaller object is *constantly* in the bigger object’s influence. Such a path, then, has to be closed, and without any sharp corners. The only shapes that fit these are ellipses and circles.
ELI10: In reality, although we tend to model massive objects as stationary for simplicity, in reality, every single object in the universe is exerting some gravitational force on every other object. Thus, no orbit is truly perfectly circular. Given two objects, one orbiting the other, perturbations of an orbit can include other celestial bodies, effects of surface angular momentum on orbital angular momentum, oblateness of either object (e.g. if one or both of the objects are egg-shaped), etc.
nothing in the universe is “perfect” including circles. if you look at the orbits of the planets next to a circle, you cant tell they arent circles. https://britastro.org/wp-content/uploads/2018/05/Figure-4_4.jpg
but for precision math, circles arent enough
Ellipses can be many different sizes and shapes, but a circle can be only one, perfect one. The chances of a perfect circular orbit are very low.
Are you asking about a hold? These are elliptical. When we go missed (can’t land) we get sent into a holding pattern away from the airport. Each leg is generally 1 minute long.
Once you determine how you enter that airspace and are established you generally fly straight for one minute then turn 3 degrees a minute for one minute making a 180 degree turn. You repeat this over and over until you get a new clearance.
I reality is not bad because we have this pre-programmed in the flight plan and it’s just a button we push to make autopilot do what we need.
If this is what you’re asking about it does look like an elongated eclipse or race track.
Its theoretically possible, but it would need so perfect finely-tuned conditions to stay circular or even become it in the first place. Even a slight variation in gravity due to, lets say another body maybe an asteroid or another planet, would shift the planets trajectory enough so it deteriorates to an elliptical orbit.
Its something we would call metastable, in the sense that it only needs one tiny change to make it not circular anymore. In reality there is nothing so perfect in nature.
Eli5: imagine balancing your phone on its side. If you could find the almost-impossibly perfect balancing point it would stay there, its somewhat stable. Theoretically it could stay there forever, however, as soon as something moves it ever so slightly, even just the air around it or the ground underneath your building vibrating ever so slightly, it falls back to the real stable position of lying there. So its not fully stable, its metastable. Here the balanced position is the circular orbit, and the stable fallen down position is the elliptical orbit.
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