DECEMBER 2007

environment news

The Earth's Orbit

Milankovic Theories

  Milutin Milankovitch was a Serbian scientist who showed that the earth's orbital "cycle" has additional modulations that make it fluctuate considerably. They correspond to three variables (according to Milankovitch) in the earth's orbit.

    - Most important of these variations is the eccentricity cycle of 93,408 years (according to Milankovitch) - which is the variation of the Earth's orbit from its almost circular path.

    - The second of the orbital cycles is the change in the tilt of the earth's equatorial plane in relation to its orbital plane over a period (according to Milankovitch), averaging 41,000 years.

    - The third orbital phenomenon is the 25,920-year precession cycle which causes a gradual shift of the Earth's polar (poles) alignment.

 

In the past (at least) 100 years, the Earth's orbit (according to the Orbital Variance Theory) has moved us a bit closer to the Sun.  When you consider that the Earth's tilt on her axis (approximately 23 degrees) is what causes seasonal changes (Summer to Winter) it is quite reasonable to believe that a gradual orbital variance will produce dramatic climate fluctuations (ice ages) over the eons

Astronomer Milutin Milankovitch developed the mathematical formulas upon which these orbital variations are based. He hypothesized that when some parts of the cyclic variations are combined and occur at the same time, they are responsible for major changes to the earth's climate (even ice ages). Milankovitch estimated climatic fluctuations over the last 450,000 years and described cold and warm periods. Though he did his work in the first half of the 20th century, Milankovich's results weren't proven until the 1970s.

A 1976 study, published in the journal Science examined deep-sea sediment cores and found that Milankovich's theory corresponded to periods of climate change. Indeed, ice ages had occurred when the earth was going through different stages of orbital variation.

Milankovic Theory

Milankovitch dedicated his career to developing a mathematical theory of climate based on the seasonal and latitudinal variations of solar radiation received by the Earth. Now known as the Milankovitch Theory, it states that as the Earth travels through space around the sun, cyclical variations in three elements of Earth-sun geometry combine to produce variations in the amount of solar energy that reaches Earth:

1. Variations in the Earth's orbital eccentricity—the shape of the orbit around the sun.

    

2. Changes in obliquity—changes in the angle that Earth's axis makes with the plane of Earth's orbit.

    

3. Precession—the change in the direction of the Earth's axis of rotation, i.e., the axis of rotation behaves like the spin axis of a top that is winding down; hence it traces a circle on the celestial sphere over a period of time.

Together, the periods of these orbital motions have become known as Milankovitch cycles.

Milankovitch Cycles in Paleoclimate

Milankovich cycles are cycles in the Earth's orbit that influence the amount of solar radiation striking different parts of the Earth at different times of year. They are named after a Serbian mathematician, Milutin Milankovitch, who explained how these orbital cycles cause the advance and retreat of the polar ice caps. Although they are named after Milankovitch, he was not the first to link orbital cycles to climate. Adhemar (1842) and Croll (1875) were two of the earliest.

A "wobbling top": The Milankovitch theory

A Serbian mathematician named Milutin Milankovitch was intrigued by this puzzle of climate change, and in the 1930s he presented a theory that might explain it. Milankovitch studied climate records, noting differences over time. He theorized that global climate change was brought about by regular changes in Earth's axis, tilt, and orbit that altered the planet's relationship to the Sun, triggering ice ages.

Earth doesn't rotate perfectly like a wheel about an axis; it spins like a wobbling top. Every 22,000 years, Milankovitch calculated, there is a slight change in its wobble. Every 100,000 years, there is a change in Earth's orbit about the Sun. Its almost circular orbit becomes more elliptical, taking Earth farther from the Sun. And finally, Milankovitch discovered, every 41,000 years there is a change in the tilt of the planet's axis, moving either the Northern or Southern Hemisphere farther from the Sun.

These cycles mean that at certain times there is less sunshine hitting Earth, so there is less melting of snow and ice. Instead of melting, these cold expanses of frozen water grow. The snow and ice last longer and, over many seasons, begin to accumulate. Snow reflects some sunlight back into space, which also contributes to cooling. Temperatures drop, and glaciers begin to advance.



BIOGRAPHY: Milutin Milankovic was born on May 28, 1879, at Dali near Osijek. He received a degree from the School of Civil Engineering in June, 1902, having submitted a project for a reinforced concrete bridge.  His thesis, Theorie der Druckkurven (Theory of Pressure Lines), was noted for its original approach; it was published in 1907, in the eminent German scientific, non-technical, review Zeitschrift für Mathematik und Physik, Bd. 55.
In 1905, Milankovic took up practical work and joined the then famous firm of Adolf Baron Pittel Betonbau-Unternehmung in Vienna.  He built dams, bridges, viaducts, aqueducts and other structures in reinforced concrete throughout the Austria-Hungary of the time.  Milankovic continued to practice civil engineering in Vienna until the autumn of 1909 when he was offered the chair of applied mathematics (rational mechanics, celestial mechanics, theoretical physics) in Belgrade.  The year 1909 marked a turning point in his life.  Though he continued to pursue his investigations of various problems pertaining to the application of reinforced concrete, he decided to concentrate on fundamental research.

Turbulent events took place as soon as he had settled down in Belgrade when the Balkan Wars were followed by World War I. When World War I broke out (he was just married), he was interned in Nezsider and later in Budapest, where he was allowed to work in the library of the Hungarian Academy of Sciences.  As early as 1912, his interests turned to solar climates and temperatures prevailing on the planets.  Throughout his internment in Budapest he devoted his time to the work in this field and, by the end of the war, he had finished a monograph on the problem which was published in 1920, in the editions of the Yugoslav Academy of Sciences and Arts by Gauthiers-Villards in Paris, under the title Théorie mathématique des phénomènes thermiques produits par la radiation solaire (Mathematical theory of thermic phenomena caused by solar radiations).

The results set forth in this work won him considerable reputation in the scientific world, notably his curve of insolation at the Earth's surface.  This solar curve was not really accepted until 1924 when the great meteorologist and climatologist Vladimir Köppen with his son-in-law Alfred Wegener, introduced the curve in their work Klimate der geologischen Vorzeit (Climates of Geological Past).  After these first tributes, Milankovic was invited, in 1927, to cooperate in two important publications: the first was a handbook on climatology (Handbuch der Klimatologie) and the second a handbook on geophysics (Guttenberg's Handbuch der Geophysik).  For the former, he wrote the introduction Mathematische Klimalehre und astronomische Theorie der Klimaschwankungen (Mathematical science of climate and astronomical theory of the variations of the climate), published in 1930 in German and in 1939 translated into Russian.  Here the theory of planetary climate is further developed with special reference to the Earth.

For the second textbook, Milankovic wrote four sections developing and formulating his theory of the secular motion of the Earth's poles and his theory of glacial periods. Fully aware that his theory of solar radiation had been successfully completed and that the papers dealing with this theory were dispersed in separate publications, he decided to collect and publish them under a single cover.  Thus, in 1941, on the eve of war in his country, the printing of his great work Kanon der Erdbestrahlung und seine Anwendung auf das Eiszeitenproblem (Canon of Insolation of the Earth and Its Application to the Problem of the Ice Ages) was completed, 626 pages in quarto, in Cemian, published in the editions of the Royal Serbian Academy. This work was translated into English under the title Canon of Insolation of the Ice-Age Problem, in 1969 by the Israel Program for Scientific Translations and published for the U.S. Department of commerce and the National Science Foundation, Washington, D.C.

Objections were raised in the 50's against the Milankovic theory of ice ages; these objections came mainly from meteorologists who claimed that the insolation changes due to the changes in the Earth's orbital elements were too small to perturb significantly the climate system.  However, in the late 60's and 70's, investigation of the deep-sea sediments and theoretical works in celestial mechanics and climate modelling showed that Milankovic's view was correct and that the astronomically induced changes in insolation, received by the Earth from the Sun, was indeed the primary cause for the waxing and waning of the Quaternary ice sheets.

In addition to his scientific work, Milankovic always showed great interest in the historical development of  science.  In addition to a textbook on the history of astronomy, the wrote two books on a popular level: Through Space and Centuries fictionalized the development of astronomy while the other, entitled Through the Realm of Science, dealt with the development of exact sciences.

Milankovic also published a three volume autobiography in Serbo-Croatian, Recollection, Experiences and Vision, which was never translated.  For this reason his son, Vasko Milankovic, has completed a beautiful biography: My father, Milutin Milankovic.

Milankovic was elected a corresponding member of the Serbian Academy of Arts and Sciences in 1920, a full member in 1924, a corresponding member of  the Yugoslav Academy of Arts and Sciences in 1925 and as member of the German Academy of Naturalists "Leopoldine" in Halle; he was also a member of many scientific societies and related to organizations, both in Yugoslavia and abroad.

He died on December 12, 1958, in Belgrade.