We recognize two troublesome, but nevertheless fascinating, aspects of physical reality. One, nothing is constant. Everything changes over time. Eventually, even entities of unfathomably long duration exhibit a shift or alteration. These changes are the result of the second aspect: that all things are interconnected. No phenomenon occurs in isolation. For instance, regard our landscape in mid-September. One might notice the gathering of desiccated leaves along with browning grass that will all too soon become frost-coated. Six months from now we’ll hear the first murmurs of flowing water under layers of thinning stream ice. Both of these examples, the former ominous, the latter joy-inducing, result from the continually changing orientation of Earth’s northern hemisphere relative to the Sun. The Sun itself remains quite constant over a period as comparatively brief as an Earth year.
Today’s wandering astronomer takes us on a wandering sojourn through both short and long time periods to understand how other facets of Earth’s orbit affects Earth’s weather and ever changing climate.
SHORT TERM
As Earth’s distance from the Sun changes throughout the year, the solar constant, defined as the amount of the Sun’s electromagnetic radiation that Earth receives, changes, as well. The average value of the solar constant, or its value when Earth is 1 AU from the Sun, is 1,360 W/meter squared.
However, because Earth travels along an elliptical orbit, its distance from the Sun varies from a minimum of 0.984 AU (perihelion) to a maximum of 1.016 AU (aphelion.)* As we would expect, the solar constant is at a maximum at perihelion:
AU=astronomical unit. Earth’s mean distance from the Sun, an AU, approximately 93 million miles, has been precisely defined by the International Astronomical Union as 149,597,870,700 m. 0.984 AU is about 91.7 million miles; 1.016 AU is about 94.5 million miles.
The chart above shows how the solar constant, or solar flux density, varies throughout the year. The solar constant is at a maximum when the year begins because Earth reaches perihelion around January 3rd or 4th each year. The minimum is reached when Earth reaches aphelion (July 3 – 5). The average value occurs when Earth is at exactly 1 AU from the Sun, which occurs around April 5th and October 5th.
LONG TERM
Serbian astronomer/geophysicist Milutin Milanković (1879–1958) studied how three aspects of Earth’s orbital motions affect incoming solar radiation. These aspects are eccentricity, obliquity and precession.
Eccentricity
All the planets in our solar system travel along elliptical orbits. Eccentricity measures an ellipse’s departure from circularity. An ellipse with zero eccentricity is a perfect circle. As the eccentricity value increases from 0 toward 1, the ellipse becomes increasingly more elongated. (When the eccentricity equals one, it becomes a parabola.)
Earth’s current eccentricity equals 0.0167. However, this value changes over long time periods. Refer to the graph below.
The thick dark line represents Earth’s orbital eccentricity. The 0 line corresponds to the year 2007. Notice that during the next 28,000 years, Earth’s orbital eccentricity value will decrease to an estimated minimum of 0.0034.
While Earth’s distance directly affects the solar constant, the orbital eccentricity also affects the seasonal durations. Earth moves fastest when at perihelion. Since Earth is at perihelion during early winter (N. Hemisphere) winter is currently our shortest season (89 days) while summer is the longest (93 days). However, as the eccentricity decreases, the seasonal durations will almost even out.
We experience seasonal changes, of course, because of the next factor:
Obliquity
Earth is currently “tilted” on its axis by approximately 23.5 degrees relative to the ecliptic, or the plane of Earth’s orbit around the Sun. However, over a 41,000 year period, this obliquity value will veer from a minimum of 22.1 degrees to a maximum of 24.5 degrees. When the tilt is less, the summers are not as warm. This heat reduction can promote increased glaciation as the region where snow does not entirely melt during the warm season expands.
The obliquity reached a “local” maximum value of 24.2 degrees 9500 years ago and is currently decreasing.
Another consequence of this gradual but inexorable obliquity decrease is the very gradual contraction of the tropics:
The “tropics” refers to the region between the Tropic of Cancer (23.5 degrees N) and the Tropic of Capricorn (23.5 degrees S). The Sun will only pass through the zenith at the latitudes within this area. As the Sun tends to maintain a higher angle throughout the year, the tropics are the hottest areas on Earth. However, because of the changing obliquity, the Tropical boundaries are shrinking by about 14 meters per year. When the obliquity is at a minimum, the Tropic of Cancer will be at 22.2 degrees N and the Tropic of Capricorn will be at 22.1 degrees S.
Precession
Finally, we mention Earth’s “wobble.” As Earth rotates, it also experiences an axial precession, like the wobble of a spinning top, due primarily to the gravitational influence of the Sun and moon. One precessional cycle lasts about 25,771 years. The main consequence of this precession is to flip the seasonal orientations from one hemisphere to another so that we’ll be closer to the Sun during the northern hemisphere summer/southern hemisphere winter.
The combination of these cycles produces the Milkankovitch Cycles: See graphic below, courtesy of “Universe today.” According to his theory, these cycles change the amount of incidental sunlight on Earth, which profoundly affects Earth’s climate patterns.
As for those effects, I defer hastily to a climatologist.
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