THE ORBIT AND rotational behavior of the Earth moving about the Sun affects the amount of solar energy received. Precession is the phenomenon where the Earth wobbles about on its axis, just like a spinning top. Consequently, the solstices and equinoxes occur earlier each year. The winter solstice occurs a few weeks before perihelion, the closest approach Earth makes to the Sun. Over time, the summer solstice will occur closer to perihelion and the summer heat will become even greater. This precession of the equinoxes takes 25,800 years to complete. During the period of a precessional cycle, the North Pole follows a circle in space, perpendicular to Earth’s imaginary axis of rotation. In concert with the eccentricity of the orbital path around the Sun, and the varying tilt of the Earth’s axis in space, precession is a factor of climate change affecting Earth’s average surface temperature over the long-term.
The Greek astronomer Hipparchus (c.190-c.120 b.c.e.) compared astronomical observations 169 years apart, and found that the Earth’s axis around which the night sky appeared to rotate shifted gradually. He discerned a 2-degree shift by looking at the Earth’s shadow on the moon during a lunar eclipse. From this observation, Hipparchus could determine the Sun’s position among the stars and identify Earth’s changing axial position.
Earth is not a perfect sphere and its equatorial bulge is a factor in the precession phenomenon. Centrifugal force generated during Earth’s daily rotation creates the bulge and provides the force that wobbles Earth ever so slightly. The gravitational forces of the moon (responsible for the tides) and the Sun (responsible for Earth’s orbit) on the bulge provide the impetus for Earth’s precession.
The Earth achieves perihelion on January 3rd, which is close to the Northern Hemisphere’s December 21st winter solstice. This timing of Earth’s closest approach to the Sun and the shortest day of the year reduces seasonal differences in the amount of solar radiation received by the Northern Hemisphere. The fact that the Northern Hemisphere is closer to the Sun in winter than in summer, produces a relatively warmer winter, thereby reducing the overall variability of the yearly seasons. By contrast, 11,000 years ago, the Earth reached its closest point to the Sun during the Northern Hemisphere summer, increasing both the summer heat and the overall seasonal variability of Earth’s climate.
Changes in Earth’s orbital eccentricity over time, by itself, has little impact on the total amount of radiation received. The effect is on the order of .01 percent. However, as the eccentricity cycle is modulated by the precession cycle, the effect can be greater or lesser as time passes. When Earth’s orbit is highly elliptical (eccentricity is high), the effect of precession on the seasons is strong. When Earth’s orbit is almost circular (eccentricity is low), the effect of precession on the seasons is negligible.
Milutin Milankovitch (1879-1958) understood these changing orbital parameters and saw the effect that the precession of the equinoxes had on climate, as well as the ebb and flow of the ice ages. When climate extremes were maximized by the right combination of factors, including the summer solstice occurring during perihelion, the time was ripe for an ice age. With the corresponding winters colder in the Northern Hemisphere, more snow fell, for a longer duration, overwhelming the extreme season of summer heat and, in turn, feeding the glacial ice sheets. Because snow is an effective reflector of sunlight, the snow-covered land-masses of the Northern Hemisphere did not warm as they once had when winter ended. Milankovitch argued that this was just enough to cause the periodic cycle of the ice ages.
Sediment and ice cores have been a useful tool to find Earth’s temperatures for the past half-million years. These datapoints highlight how rare Earth’s present warm spell is, allowing human civilization to flourish for the past 11,000 years. This relatively warm period is due, in part, to the wobble of Earth’s rotational axis over time. The associated precession of the equinoxes has aligned the Earth-Sun pattern such that the Earth’s Northern Hemisphere points away from the hot Sun while it is furthest away from it. This moderates both the summer heat and winter cold to provide a more livable climate.
