The Seasons and the Earth's Orbit

Essay

By: George H. Kaplan

Date: 2005

Source: Kaplan, George H. "The Seasons and the Earth's Orbit." Astronomical Applications Department, U.S. Naval Observatory. 〈http://aa.usno.navy.mil〉 (accessed December 14, 2005).

About the Author: Dr. George H. Kaplan is an astronomer at the U.S. Naval Observatory (USNO) in Washington, D.C., a position he has held since 1971. While at the USNO, Kaplan has worked on various projects within the Nautical Almanac Office, the Astrometry Department, and the office of the Scientific Director related to positional astronomy such as Earth rotation measurements, planetary orbit computations, VLBI (very long baseline interferometry) and connected-element radio interferometry, astrometry of the satellites of the planet Jupiter, optical interferometry, lunar occultations, the mathematics of celestial navigation, and the design of computer almanacs. Kaplan also acts as chief of the Science Support Division of the Astronomical Applications Department.

INTRODUCTION

While working for the U.S. Naval Observatory, George Kaplan described the Earth's orbit and its seasons with respect to their relationship to Milankovitch cycles. Kaplan provided in his paper an explanation of the major principles involved with the theory of Milankovitch cycles, which were developed by Milutin Milankovitch, a Serbian astronomer, geophysicist, and mathematician.

Milankovitch (1879–1958) worked throughout his professional career on his mathematical theory of climate change that dealt with the latitudinal and seasonal variations of radiation directed towards the Earth from the Sun. While working at the University of Belgrade, he developed a significant theory—now known as the Milankovitch theory, or the astronomical theory of climate change—which was based on orbital variations of the Earth and its association with Earth's past climatic variation (for a 600,000-year period prior to 1800). He is well known within the scientific community for carefully calculating the magnitude of Earth's precession, eccentricity, and obliquity. Milankovitch wrote his first book in 1920 based on his astronomical theory that highlighted the effects caused by the ice ages. Over twenty years later, in 1941, he published a more detailed version of his mathematical theory, which was later translated into the English language in 1969 under the title "Canon of Insolation of the Ice-Age Problem" and reprinted again in 1998.

Milankovitch cycles, which are incorporated within the Milankovitch Theory, are defined as three periodic variations—precession, eccentricity, and obliquity—in Earth's orientation with respect to the Sun.

Precessional motion is the counterclockwise, near-circular wobble (or motion caused by the change in the direction of the Earth's axis of rotation) that produces a cycle of about 21,000 years. The motion of eccentricity (or ellipticity) is the variation in shape, over about 100,000 years, in the Earth's orbit around the sun, which varies from a near circular orbit to a more elliptically shaped orbit. Obliquity (or nutation) is the variation of the Earth's axial tilt in a period of about 41,000 years, with respect to the plane for which it orbits the sun. These three variations are believed to produce significant cyclical changes in Earth's climate with a period of about 100,000 years.

PRIMARY SOURCE

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SIGNIFICANCE

The significance of the three dominant Milankovitch cycles follows from their possible influence on climate changes over an extended cycle of time of about 100,000 years. As reported by Kaplan, Milankovitch based his studies on the hypothesis that the present Ice Age, which occurred during the Pleistocene Epoch that lasted from about 1.6 million years to about 10,000 years ago, was caused mainly by cyclical changes in the Earth's orbit about the sun. Mutual variations in the Earth's precession, eccentricity, and obliquity, as proposed by Milankovitch, created seasonal changes in the amount and angle of radiation coming from the sun to the Earth. When periods of increased or decreased solar radiation occurred, they directly influenced the Earth's seasons, thus impacting its global climate. In particular, Milankovitch charged that these major variations in the motion of the Earth impacted the advance and retreat of Earth's glaciers during the ice ages.

In the future, Kaplan stated that the theory of the Milankovitch cycles will predict climatic changes due to the variation in the Earth's orbit that occur over thousands of years. However, Kaplan also explained that the climate system of the Earth may also take thousands of years to respond to those orbital variations.

In 1976, a study published in the journal Science reported that evidence within sediment cores retrieved from the bottom of the ocean supported Milankovitch's theory. A record of temperature changes taken from the sediment cores found that major variations in climate going back 450,000 years were closely related with changes in precession, eccentricity, and obliquity of the Earth's orbit. The study concluded that the Ice Age occurred during a period when Earth was going through different stages of orbital variation. However, other studies performed since 1976 do not directly support the contentions of Milankovitch.

The Milankovitch theory has proven its merit with respect to its results of the major astronomical cycles of the Earth's orbit—those of precession, eccentricity, and obliquity. However, its conclusions with respect to the three cycles shaping Earth's global, or at least hemispheric, climate variations remain controversial within the scientific community. The correlation between the slight orbital variations, proposed by Milankovitch, which drastically affect Earth's past climate, is still not completely verified by scientists.

Milankovitch cycles have frequently explained climatic change over the last tens of thousands of years, and recently they have explained climate shifts over periods of 1,000 to 15,000 years. Generally, the 21,000 precession cycle and the 41,000 obliquity cycle appear to be present in past records of climate, but the dominant 100,000 eccentricity cycle is less obvious within related records.

Researchers still doubt the general validity of the Milankovitch theory with respect to predicting changes of Earth's climate. It is uncertain whether or not Milankovitch cycles will be proven in the future to explain long-term climate changes found on the Earth. Whatever the result, the cycles have nothing to do with, and are unable to account for, climate changes made by the industrial and related activities of humans. As a result, if in the future Milankovitch cycles are scientifically proven to affect natural changes to climate, then it may be easier to prove the amount that climate is affected by artificially generated emissions from human activities such as greenhouse gases.

FURTHER RESOURCES

Books

Imbrie, John, and Katherine Palmer Imbrie. Ice Ages: Solving the Mystery. Cambridge, MA.: Harvard University Press, 1986.

Lutgens, Frederick K., and Edward J. Tarbuck. The Atmosphere: An Introduction to Meteorology. Englewood Cliffs, NJ: Prentice Hall, 1995.

Web sites

Earth Observatory Library, National Aeronautics and Space Administration. "On the Shoulders of Giants: Milutin Milankovitch (1879–1958)." 〈http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch.html〉 (accessed November 26, 2005).