Orbital Parameters, Eccentricity

THE SUN SHOWERS the Earth in solar energy with varying intensity over time, while the Earth's orbital motion about the Sun affects how that energy is received. Eccentricity is the degree that the orbit deviates from a perfect circle. Seasons on Earth are modulated by the eccentricity of the orbital path around the sun, the precession effect, and the position of the solstices in the yearly revolution. Eccentricity plays not only a role in the diversity of the seasons, but also in the degree of climate change affecting Earth over the long term.

Johannes Kepler (1571–1630) discovered, through astronomical observations, that the Earth moved in an elliptical path around the sun. This could be expressed mathematically, and one of the parameters, eccentricity, ranges between a value greater than zero and a value less than one. A zero value produces a perfectly circular orbit, while unity ensures a parabolic encounter with the sun of a celestial body, such as a comet, that will never return. The smaller the value of eccentricity, the more circular is the orbit. Earth's eccentricity has been measured to be 0.0167 and so its orbit is not quite circular.

A not quite circular orbit means that the Earth is closer to the Sun at some times of the year, and farther away during other times. The perihelion, the closest approach to the sun, happens in January. This results in Northern Hemisphere winters that are slightly milder than if the orbit was perfectly circular. Over time, changes in the occurrence of perihelion, called the precession of the equinoxes, takes place every 22,000 years. This means that 11 millennia from now, the perihelion will occur in July instead of January and produce seasons even more severe than they are today.