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Milankovitch (1879-1958) gave voice to the idea that Earth's long-term climate changes are due to periodic changes in the Earth's orbit and obliquity (Earth's axial tilt).
According to this theory, three major astronomical cycles play a vital role in bringing about the Ice Ages and the interglacial periods in between:
1) Perihelion (aphelion) cycle of about 21,000 years
2) Obliquity (tilt of Earth's rotational axis) cycle of about 41,000 years
3) Orbital eccentricity cycle of about 100,000 years
No, I did not forget about the 26,000-year precession (of the equinoxes) cycle! But should you want to know why I did not list it, I invite you to read the note at the bottom of the page.
Perihelion (Aphelion) Cycle
In this scenario, we're talking about the realignment of the perihelion point with a given solstice point. At the June solstice, the Earth's northern axis tilts most toward the Sun for the year, and on the December solstice, the northern axis maximally tilts away from the Sun.
Image credit: Wikimedia Commons. Click here for a larger diagram
At perihelion, the Earth comes to closest to the Sun for the year, and at aphelion it swings farthest from the Sun for the year. In our day, the Earth reaches perihelion in early January, or about two weeks after the December solstice (the Northern Hemisphere's winter solstice and the Southern Hemisphere's summer solstice). And the Earth swings to aphelion in early July, about two weeks after the June solstice.
Because the Gregorian calendar tries to align itself with the seasonal year, the equinoxes and solstices fall on or near the same dates each year. However, perihelion and aphelion advance about one calendar date later in about 58 (seasonal) years, or about 365 calendar days later in 21,000 years.
According to the mathematical wizard Jean Meeus, perihelion last coincided with the Northern Hemisphere's winter solstice (and with the Southern Hemisphere's summer solstice) in the year A.D. 1246. Looking into the future, perihelion will align with the Northern Hemisphere's spring equinox (Southern Hemisphere's autumn equinox) around the year A.D. 6430.
Image credit: Gary Benson
Then roughly 5,250 years after perihelion aligns with the March (spring) equinox, it'll align with the June (summer) solstice. Then about 5,250 years after perihelion aligns with the June (summer) solstice, it'll align with the September (fall) equinox. Then another 5,250 or so years after perihelion and the September (fall) equinox coincide, perihelion will have gone full circle, again coinciding with the December (winter) solstice point around A.D. 22246 (A.D. 1246 + 21,000 = A.D. 22246).
The NOAA Paleoclimatalogy article Astronomical Theory of Climate Change speculates, "Cool summers in the northern hemisphere, where most of the land mass is located, appear to allow snow and ice to persist to the next winter, allowing the development of large ice sheets over hundreds to thousands of years. Conversely, warmer summers shrink ice sheets by melting more ice than the amount accumulating during the winter."
Based upon the aforesaid statement, I suppose that means the coincidence of aphelion and a Northern Hemisphere summer should help to drive Earth toward another Ice Age. We'll overlook for now the impact of the Anthropocene epoch (Age of Humans) and the burning of fossil feuls on climate change.
Obliquity (Axial Tilt) Cycle
The tilt of the Earth's rotational axis varies from about 22 to 24.6o and back over a period of about 41,000 years. We're now about midway between the two extremes, with the present axial tilt at about 23.44o. The tilt is now decreasing, to reach a minimum in another 10,000 or so years.
Again, less tilt means less intense summer sun at both Arctic and Antarctic latitudes. Presumably, the cooler summers enable the build-up of more massive ice sheets.
The Earth's orbit around the Sun isn't a perfect circle but a slightly oblong ellipse, with Earth being farthest from the Sun at aphelion and closest to the Sun at perihelion. The greater the eccentricity, the more oblong the Earth's orbit, and the lesser the eccentricity, the closer to circular. The eccentricity of Earth's orbit is said to vary from about 0.00 (circular) to 0.06 (greatest eccentricity). At present, eccentricity is somewhere around 0.167 and decreasing.
Let's give some concrete examples on how eccentricity affects the distance of perihelion and aphelion. Take the eccentricity figure (0.167) and multiply it by the Earth's mean distance from the Sun (93,000,000 miles) to find out the Sun's distance at perhelion and aphelion. Multiplying 0.167 x 93,000,000 = 1,553,100 miles. That's approximately 1.5 million miles, so the Sun is about 1.5 million miles closer than the mean 93 million miles at perihelion, and about 1.5 million miles farther than the mean distance of 93 million miles at aphelion.
Image credit: Wikimedia Commons. Click here for a larger chart of the four inner planets (perihelion points: green; aphelion points: red)
At greatest eccentricity, we again multiply eccentricity by the Earth's mean distance from the Sun: 0.06 x 93,000,000 miles = 5,580,000 miles. That means at greatest eccentricity, the Sun is about 5.58 million miles closer at perihelion than the mean distance of 93 million miles, yet aphelion resides about 5.58 million miles farther away than the mean distance of 93 million miles.
After reading numerous articles on the Milankovitch cycles, I find surprisingly little explanation on how eccentricity drives glaciation. This web page claims "the climate becomes more severe as eccentricity increases." If I have read between the lines correctly, the perfect scenario for bringing about an Ice Age: Aphelion coinciding with a Northern Hemisphere summer, minimal axial tilt and high eccentricity. But I have yet to find any source that readily and succinctly states in what way the three intermingling cycles bring about glaciation.
In conclusion, I quote the passage from the U.S. Naval Observatory article on the Milankovitch cycles, "Many questions remain about long-term climate variation and their relationship, if any, to astronomical causes."
Note on the Precession Cycle
The precession (of the equinoxes) cycle is not synonymous to the perihelion cycle. After extensively reading a number of articles on the Milankovitch cycles, it appears that many confuse one cycle for the other. The 26,000-year precession cycle refers to the westward movement of the equinox and solstice points along the ecliptic relative to the backdrop stars, or the constellations of the zodiac. Or, if you prefer, you can say precession finds the north polar axis, when extended outward to the stellar sphere, drawing a circle around the north ecliptic pole in about 26,000 years.
Image credit: NASA
The perihelion cycle, on the other hand, finds perihelion (and aphelion) moving eastward along the ecliptic relative to the backdrop stars in a period of about 114,000 years. I'm pleased to report that I just found this web page which gives the perihelion cycle relative to the stars. Once again, perihelion realigns with a particular solstice (or equinox) point in periods of about 21,000 years.
The previously recited web page also stated, "A competing theory, recently revised and redefined, holds that a major climatic factor in the last one million years is the changing of the Earth's orbit with respect to the invariable plane (a 100,000-year cycle). Around maximum, this inclination is thought to dip the Earth's orbit low enough that the dust cloud between the Earth and the Sun reduces the amount of sunlight reaching Earth and triggers a glacial cycle."
copyright 2011 by Bruce McClure
|Ice Age Cycles Video by National Geographic|
December 2011 Feature * February 2012 Feature