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