365 Days of Climate Awareness 17 - Milankovitch Cycles 2: Orbital Variations

 

365 Days of Climate Awareness

17 - Milankovitch Cycles 2: Orbital Variations

The earth orbits the sun in an ellipse which is subject to three known types of variation, two discovered by Milankovitch, one more recently. These oscillations are in the eccentricity--the flatness--of the orbital ellipse, the precession or rotation of the ellipse, and the tilt of the orbital ecliptic against the invariable plane, i.e. the central plane of the Milky Way Galaxy.

Earth's orbit is very nearly circular. Eccentricity (e) is a measure of the semimajor (long) axis against the semiminor (short) axis, with 0 signifying a circle and 1 a straight line. Earth's orbit bounces between an almost perfectly circular value of 0.000055, and a slightly more stretched value of 0.0679. Right now, earth's orbital eccentricity is 0.017 and decreasing. This beat in eccentricity is caused primarily by the gravitational influence of Jupiter and Saturn. There are several different components in this variation, each with different periods (as long as 413,000 years), but they combine to an overall period of roughly 100,000 years.

When the orbit is less eccentric--more circular--there is less change in the amount of energy reaching earth (the solar constant) throughout the year, due to the earth's more constant distance from the sun. At this eccentricity perihelion (when the earth is closest to the sun) results in 6.8% more insolation energy than aphelion (when the earth is farthest away). Currently perihelion occurs on January 3 and aphelion on July 4. When the orbit is more eccentric perihelion results in 23% more solar energy reaching the planet than aphelion.

Rhythmic variations in Earth's orbit.

The final effect has only recently been discovered and is not well quantified. The Milky Way Galaxy has a central plane on which star systems tend to orbit--roughly the same as Jupiter's orbital plane around the sun--and the plane of earth's orbit wobbles slightly with respect to that. Its cause has not been identified but right now the angle of inclination is 1.57 degrees.The ellipse itself rotates around the solar focus once every 112,000 years, known as apsidal precession. Like eccentricity, this is caused by the gravitation of Jupiter and Saturn. The combination of this with another cycle, the rotation of the earth's axis (axial precession), causes the seasons to very slowly advance through the calendar, starting slightly earlier every year, completing a cycle every 23,000 years. This combined effect is known as the precession of the equinoxes, and is the reason why constellations visible in certain seasons now are not the same as they were several thousand years ago.

Tomorrow: Milankovitch cycles 3: rotational effects.

Be well!