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.
KEPLER’S LAW OF EQUAL AREAS
Kepler’s Law of Equal Areas says that an imaginary line drawn from the center of the Sun to the center of the Earth will sweep out equal areas in equal time intervals. This means that the duration of the seasons is proportional to the area of the Earth’s orbit swept between the solstices and equinoxes. Consequently, when eccentricity is greatest, seasons occurring furthest away, at aphelion, are longer in duration. Northern hemisphere summers are slightly longer than the winters, because summer occurs at aphelion, when the Earth is furthest from the Sun and moving at its minimum orbital velocity. In 2006, summer was almost five days longer than winter. However, changes in the Earth’s orbit over time will alter the location of the solstices and equinoxes. Eventually, Northern Hemisphere summers will become shorter, and winters will become longer, alternating duration, and changing how much solar radiation is absorbed by the Earth’s surface. Any cooling effect derived from such changes will be negated by an eccentricity that will be almost reduced by a factor of two. A reduction in the average orbital radius tends to raise average surface temperatures in both hemispheres and cause changes in climate for thousands of years. Global warming ebbs and flows over a millennial time scale.
Planetary eccentricity is modulated by the gravitational pulls among the planets in the solar system. Earth’s eccentricity alternates between nearly zero, to almost 0.0500. Consequently, the eccentricity of Earth’s orbit is governed by the mathematical timing of 100,000- and 400,000-year periods. This, in turn, affects the strength of the Earth’s seasons. Coupled with the varying tilt of the Earth’s axis over time, and the changing precession of the Earth’s orbit, the changing severity of summer and winter throughout the ages is thought to control the expansion and contraction of the polar ice sheets. Cooler summers in the Northern Hemisphere enable snow and ice to last longer. Since the greater land mass is located there, huge ice sheets over thousands of years are built up, covering the darker land and reflecting more sunlight. This process tends to lower the Earth’s average surface temperature. On the other hand, warmer summers in the future will contribute to shrinking ice sheets, uncovering of landmass, and the absorption of more solar energy rather than its reflection. The average surface temperature of the Earth will rise.
Whatever the conclusion as to the ultimate cause of global warming, the orbital mechanics of the solar system will play a big role over the long-term. Such factors as eccentricity are understood, but there is little humanity can do to control the motion of the Earth.
