We have all been taught that the seasons are caused by thetilt of the Earth's axis of rotation - the 23.4° offset ofthe axis from a direction perpendicular to the Earth's orbitalplane. The direction of the rotational axis stays nearly fixed inspace, even as the Earth revolves around the Sun once each year.As a result, when the Earth is at a certain place in its orbit,the northern hemisphere is tilted toward the Sun and experiencessummer. Six months later, when the Earth is on the opposite side ofthe Sun, the northern hemisphere is tilted away from the Sunand experiences winter. The seasons are, of course, reversed forthe southern hemisphere.
The solstices mark the two dates during the year on which theEarth's position in its orbit is such that its axis is mostdirectly tilted either toward or away from the Sun. These arethe dates when the days are longest for the hemisphere tiltedtoward the Sun (where it is summer) and shortest for the oppositehemisphere (where it is winter).
However, there is a complication. The Earth's orbit is very close tobeing a perfect circle, but not quite. It is somewhat elliptical, whichmeans that the distance between the Earth and the Sun varies over thecourse of the year. This effect is too weak tocause theseasons, but it might have some influence over their severity. Theremainder of this page explains this possibility.
The Earth reaches perihelion - the point in its orbit closest tothe Sun - in early January, only about two weeks after the Decembersolstice. Thus winter begins in the northern hemisphere at about thetime that the Earth is nearest the Sun. Is this important? Is there areason why the times of solstice and perihelion are so close? It turnsout that the proximity of the two dates is a coincidence of theparticular century we live in. The date of perihelion does not remainfixed, but, over very long periods of time, slowly regresses (moveslater) within the year. There is some evidence that this long-termchange in the date of perihelion influences the Earth's climate.
We can measure the length of the year in several different ways. Thelength of the year from equinox to equinox (equivalently, solstice tosolstice) is called thetropical year, and its length is thebasis for our Gregorian (civil) calendar. Basically, the tropical yearis the year of a complete cycle of seasons, so it is natural that we useit for ordinary purposes. But we can also measure the length ofthe year from perihelion to perihelion, which is called theanomalistic year. On average, the anomalistic year is about 25minutes longer than the tropical year, so the date of perihelion slowlyshifts over time, regressing by about 1 full day every 58 years. Thedate of perihelion thus moves completely through the tropical year inabout 21,000 years.
It is important to note that we are talking about long-term trendshere. There are small year-to-year variations in the dates and times ofsolstice and perihelion due to our leap-year cycle and the effect of theMoon on the motion of the Earth. See our page onEarth's Seasons for the exactdates and times of these events for current years.
Most of the difference in the average lengths of the two kinds ofyear is due to the very slight change in the direction of the Earth'srotation axis in space from one year to another. We usually thinkof the Earth's axis as being fixed in direction - after all, italways seems to point toward Polaris, the North Star. But thedirection is not quite constant: the axis does move, at a rate ofa little more than a half-degree per century. So Polaris has notalways been, and will not always be, the pole star. For example,when the pyramids were built, around 2500 BCE, the pole was nearthe star Thuban (Alpha Draconis). This gradual change in thedirection of the Earth's axis, calledprecession, is causedby gravitational torques exerted by the Moon and Sun on thespinning, slightly oblate Earth.
Because the direction of the Earth's axis determines when theseasons will occur, precession will cause a particular season (forexample, northern hemisphere winter) to occur at a slightlydifferent place in the Earth's orbit from year to year. At thesame time, the orbit itself is subject to small changes, calledperturbations. The Earth's orbit is an ellipse, and there is aslow change in its orientation, which gradually shifts the pointof perihelion in space. The two effects - the precession of the axisand the change in the orbit's orientation - work together to shift theseasons with respect to perihelion. Thus, since we use a calendaryear that is aligned to the occurrence of the seasons, the date ofperihelion gradually regresses through the year. It takes 21,000years to make a complete cycle of dates.
We would not expect the 21,000-year cycle to be very important climatologicallybecause the Earth's orbit is almost circular - the distance to the Sunat perihelion is only about 3% less than its distance at aphelion.That is, whether perihelion occurs in January or July, it seems unlikelythat our seasons would be much affected. At least, that is the casenow; but the eccentricity of the Earth's orbit (how elliptical it is)also changes over very long periods of time, from almost zero (circularorbit) to about three times its current value. The eccentricity of theorbit varies periodically with a time scale of about 100,000 years. So,it would be reasonable to suppose that if the 21,000-year perihelionshift cycle were to have any effect on climate at all, it would only beduring the more widely-spaced epochs when the orbital eccentricity wasrelatively large. That is, climatologically, the 100,000-year cycle ofeccentricity shouldmodulate the 21,000-year cycle ofperihelion.
In fact, Mars has an orbit much more eccentric than the Earth's,and its perihelion cycle (which has a period of 51,000 years) doesapparently have a significant effect on climate and prevailing winddirection there.
There is another important cycle that has the potential toaffect the Earth's climate; it is a 41,000-year variation inobliquity, the tilt of the Earth's axis with respect to adirection perpendicular to its orbital plane. This variation isdifferent from precession - the two motions are at right anglesto each other - and astronomically is a much smaller effect. Theobliquity varies by only a few degrees back and forth, and thecurrent value of 23.4° is near the middle of the range.However, climatologically, the obliquity variation has thepotential to have a fairly direct effect on seasonal extremes.After all, it is the obliquity that causes our seasons in the firstplace - if the Earth's axis were perpendicular to its orbitalplane, there would be no seasons at all.
The astronomical cycles described above are calledMilankovitch cycles after Milutin Milankovitch, a Serbianscientist who provided a detailed theory of their potentialinfluence over climate in the 1920s. Milankovitch's work was anattempt at explaining the ice ages, and it built upon previousastronomical theories of climate variation postulated by JosephAdhemar and James Croll in the 19th century. Although theMilankovitch theory is well-grounded astronomically, it remainscontroversial. The theory predicts different effects at differentlatitudes, and thus its use as a predictor of global (or at leasthemispheric) climate change is not unambiguous. The exactmechanisms by which the relatively modest variations in the Earth'sorbit and axis direction might result in such large effects as theice ages are not well established. The theory's popularity hastended to vary depending on the type of long-term climatologicaldata that has been available and the method used to establish atime scale for the data.
The 21,000-year perihelion cycle and the 41,000-year obliquity cycledo in fact appear to be present in the climatological record. But thedominant climate cycle that is seen has a period of about 100,000 years.Although this coincides with the period of change in the eccentricity ofthe Earth's orbit, the theory outlined above does not predict that weshould see this period directly - the effect of eccentricity shouldappear only as a modulation of the 21,000-year perihelion cycle. Themechanism by which the Earth's orbital eccentricity could affect theclimate in such a direct and important way is not known, although recentevidence (published in 2000) indicates that atmospheric carbon dioxidemay play a leading role in amplifying the orbital effect. However,some researchers still have doubts about the association between the100,000-year climate cycle and orbital variations. Thus, many questionsremain about long-term climate variations and their relationship, ifany, to astronomical causes.
A very readable book on the whole subject of ice ages and thedevelopment of the astronomical theories for theirorigin isIce Ages: Solving the Mystery by John Imbrie andKatherine Palmer Imbrie (1979, Enslow Publishers, New Jersey). Thebook obviously does not cover the latest research, but provides anexcellent background and historical context.
| last modified: 14 June 2011 14:04 |  |
