# Earth's orbit

#### rajatgl16

Why earth's orbit is elliptical rather than being perfectly circular

#### NobodySpecial

It's very nearly circular - only 1.5% off

Being perfectly circular would require everythign to be absolutely perfectly balanced, the earth to be a perfect sphere and no other planets, debris, moons, solar wind etc to disturb it.

#### dacruick

In terms of Energy, there is only a specific kinetic energy that a planet can have to be orbiting in a circle. This energy is based on the distance to the planet, and the masses of the planets. However, if Earth has more than this energy, it will orbit elliptically.

In simpler terms of speed, the Earth simply is going too fast to orbit in a circle. If you slowed down the earth a bit it could be the right speed. If you increase the speed enough, the orbit will become hyperbolic and coincidentally the planet will leave the sun's orbit.

I'm not sure I'm being 100% clear so let me know if you have any questions

#### Janus

Staff Emeritus
Gold Member
In terms of Energy, there is only a specific kinetic energy that a planet can have to be orbiting in a circle. This energy is based on the distance to the planet, and the masses of the planets. However, if Earth has more than this energy, it will orbit elliptically.

In simpler terms of speed, the Earth simply is going too fast to orbit in a circle. If you slowed down the earth a bit it could be the right speed. If you increase the speed enough, the orbit will become hyperbolic and coincidentally the planet will leave the sun's orbit.

I'm not sure I'm being 100% clear so let me know if you have any questions
This is not quite right. The Earth's orbital speed changes over the course of its orbit; speeding up at perihelion and slowing down at aphelion. At perihelion is it moving too fast to maintain a circular orbit at that distance and at aphelion it is moving too slow. At a distance halfway between the two it is moving at just the right speed but in the wrong direction.

So what you need to do in order to circularize the Earth's orbit depends upon where it is in its orbit. You slow down at perihelion, speed up at aphelion and at its mean distance alter the direction of its path. Other points of the orbit would require a combination of change and speed and direction.

#### dacruick

This is not quite right. The Earth's orbital speed changes over the course of its orbit; speeding up at perihelion and slowing down at aphelion. At perihelion is it moving too fast to maintain a circular orbit at that distance and at aphelion it is moving too slow. At a distance halfway between the two it is moving at just the right speed but in the wrong direction.

So what you need to do in order to circularize the Earth's orbit depends upon where it is in its orbit. You slow down at perihelion, speed up at aphelion and at its mean distance alter the direction of its path. Other points of the orbit would require a combination of change and speed and direction.
Well of course, if you are in an elliptical orbit, your speed has to be inversely proportional to the distance from the sun. The easiest way to do this is in terms of energy though. Each orbit has an energy. The aphelion and perihelion will be the two extremes of the distribution of potential energy and kinetic energy.

#### DaveC426913

Gold Member
The shorter answer to the OP's question is that orbits are formed from a very, very wide array of proto-objects in terms of size, location and angular velocity. As they settle down into stable orbits (through collison, perturbation or ejection), their orbital momentum is averaged in the one remaining body.

In order for the remaining body's orbit to be circular, its final average velocity and its final average orbit would have to be perfectly balanced. It is simply highly improbable that so many events conspire to bring this about.

In general - all orbits are elliptical. This is default. They certainly start off this way in their infancy. A very few orbits have their parameters balanced (or have been subjected to forces that bring them into balance) so that their ellipticity is near zero.

#### rajatgl16

Is earth's orbit exactly constant or it changes somewhat with time.

#### D H

Staff Emeritus
In terms of Energy, there is only a specific kinetic energy that a planet can have to be orbiting in a circle. This energy is based on the distance to the planet, and the masses of the planets. However, if Earth has more than this energy, it will orbit elliptically.
To overcome Janus' objections, this is better stated in terms of mechanical energy (kinetic plus potential). A planet's total mechanical energy must be exactly equal to some value that is a function of planet mass, solar mass, and radial distance from the Sun for the planet to be in a circular orbit. Any deviation from this precise value and the orbit will not be circular.

Is earth's orbit exactly constant or it changes somewhat with time.
It changes over time. Describing an orbit in terms of an ellipse assumes the two body (two point mass) problem. The solar system comprises the Sun, eight planets, minor planets, and a slew of small miscellaneous objects. Jupiter and Venus, and to a much less extent all that other stuff, perturb the Earth's orbit.

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