The Earth’s rotation around the Sun is constant, has certain known characteristics, and is thought to be affected by the tidal pushes and pulls of other celestial bodies.
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First, for the known characteristics of the Earth’s orbit. The speed of the orbit is 108,000 km/h. Our planet travels 940 million km during one orbit. The Earth completes one orbit every 365.242199 mean solar days. The planet’s distance from the Sun varies throughout the orbit. Like a wandering minstrel, the Earth is never in the same place or the same distance from the Sun from day to day. When the Earth is closest to the Sun it is said to be at perihelion. This occurs around January 3rd at a distance of 147,098,074 km. When it is at its furthest distance from the Sun, Earth is said to be at aphelion. That happens around July 4th at a distance of 152,097,701 km.
Many of us picture the Earth circling the Sun in a perfectly round circle. That is not the case. It is more like an oval. Mathematicians and astronomers call this shape an “ellipse”. An ellipse can be long and skinny or it can be very round. Scientists need a way to describe how round or “stretched out” an ellipse is. They use a number to describe this, and call it the eccentricity of the ellipse. Eccentricity is always between zero and one for an ellipse. If it is close to zero, the ellipse is nearly a circle. If it is close to one, the ellipse is long and skinny. Earth’s orbit is almost a circle, it has an eccentricity of less than 0.02. That is why the distance from the Sun at perihelion and aphelion are very close.
An orbital perturbation is when a force or impulse which is much smaller than the overall force or average impulse of the main gravitating body and which is external to the two orbiting bodies causes an acceleration, which changes the parameters of the orbit over time. A small radial impulse given to a body in orbit changes the eccentricity, but not the orbital period. A prograde or retrograde impulse changes both the eccentricity and the orbital period. Notably, a prograde impulse given at periapsis raises the altitude at apoapsis, and vice versa, and a retrograde impulse does the opposite. A transverse impulse causes rotation of the orbital plane without changing the period or eccentricity. In all instances, a closed orbit will still intersect the perturbation point. Some scientists think that the Earth is subject to one or several of these forces as it orbits the Sun.
Check out the NASA page about the Earth. Here on Universe Today we have a great article about Earth’s orbit. Astronomy Cast offers a good episode about the Earth being unbalanced by a black hole.
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