Why Earth's Orbit Matters

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Way back in Lesson 3, we talked about how solar radiation from the Sun strikes the Earth differently in different seasons. For Northern Hemisphere summer, the Earth's north pole tilted toward the Sun and vice versa for the winter. This is why we have longer days in summer than in winter. The geometry of Earth's annual orbit around the Sun changes slowly over time. These changes are subtle, but they are persistent over thousands of years and have a profound impact on climate.

Obliquity

The first of these changes involves the tilt angle (or, more technically, the obliquity) of Earth's axis relative to its orbital plane. Today, the Earth's rotational axis is inclined at an angle roughly 23.5 degrees from the vertical to the orbital plane. This is why the tropics are located between 23.5°N and 23.5°S and why the Arctic and Antarctic circles are poleward 66.5°N and 66.5°S (90°-23.5°=66.5°), respectively. This angle of inclination is not fixed over time, however, and it varies between roughly 22.1 degrees and 24.3 degrees. Seasonality only exists because of the tilt; if not for the tilt, neither hemisphere would be preferentially tilted toward the Sun at any time of the year. Therefore, periods, when the tilt angle is greatest, are periods of heightened seasonality, while periods, when the tilt angle is smallest, have reduced seasonality. It takes roughly 41 thousand years (41,000 years) for the tilt angle to go through one full cycle of alternation between minimum and maximum values of the obliquity.

Video: Changes in Obliquity (Tilt) (:01) (No Audio)

Wobble

The second of these orbital variations involves the slow wobble (or, to use the more technical term, the precession) of the Earth's rotational axis. This is analogous to the wobbling of a gyroscope. One full wobble takes roughly 19-23 thousand years (19,000-23,000 years). The precession determines when the Northern and Southern Hemispheres are each tilted toward (summer) or away (winter) from the Sun.

Video: Axial Precession (Wobble) (:04) (No Audio)

The primary importance of this factor is that it determines whether the summer solstice in each hemisphere occurs when Earth is farthest (making summer a little cooler) or closest (making summer a little warmer) to the Sun. This factor only matters, then, because Earth's annual orbit around the Sun is not circular but slightly elliptical – which brings us to our last factor.

Eccentricity

The last of Earth's changing orbital parameters involves the ellipticity (or, to use the more technical term, the eccentricity) of the orbit. Another way to think about this in non-technical terms is the "ovalness" of Earth's path around the Sun. Earth's orbit is not circular but, instead, is slightly elliptical. The degree of ellipticity is measured by the eccentricity, which ranges from roughly zero (an essentially circular orbit) to a maximum of roughly 4% (a slightly elliptical orbit). It takes roughly 100 thousand years (100,000 years) for the eccentricity to go through one full cycle of alternation between low and high eccentricity.

Video: Changes in Eccentricity (Orbit Shape) (:07) (No Audio)

These orbital cycles were first discovered by a mathematician named Milutin Milankovitch. Recreating past and future values of these orbital parameters is straightforward using celestial mechanics (a branch of astronomy that deals with motions of objects in outer space) and Milankovitch did these calculations by hand back in the early 1900’s.

The most important part of these orbital parameters is that they impact where solar radiation hits the Earth. Based on his calculations, Milankovitch theorized that the amount of solar radiation hitting the Northern Hemisphere could swing by 20% depending on the relative phases of these cycles. This has significant impacts on the climate system. In the next section, we will discuss how these can lead to large swings in the Earth’s temperature during glacial-interglacial periods.

Let's review these three important concepts. Think about what they mean -- if you can't define it in your head, click to expand and make sure "you hammer it home!"

Obliquity (Tilt)

This refers to the angle between Earth's rotational axis and the vertical to its orbital plane around the Sun. Earth's tilt changes slightly over a cycle of about 41,000 years, varying between approximately 22.1° and 24.5°. Changes in obliquity affect the severity of the seasons: a greater tilt means more extreme seasons (hotter summers and colder winters), while a smaller tilt leads to milder seasons.

Precession (Wobble)

Precession is the slow wobble of Earth's rotational axis, similar to the wobbling of a spinning top. This wobble occurs over a cycle of roughly 19,000 to 23,000 years. Precession alters the timing of when each hemisphere is tilted toward or away from the Sun, affecting the timing of the seasons in relation to Earth's position in its orbit.

Eccentricity (Ellipticity)

Eccentricity describes the shape of Earth's orbit around the Sun, which changes from being more circular to more elliptical over a cycle of about 100,000 years. When the orbit is more elliptical, the distance between Earth and the Sun varies more throughout the year, influencing the amount of solar energy Earth receives and impacting long-term climate patterns.