There are two more cycles to consider in this increasingly complex story of the astronomical cycles affecting Earth’s long-term climate. The first is another form of precession. Similarly to the precession of Earth’s axis, which marks out a circle on the star field every 26,000 years, the ellipse of Earth’s orbit around the Sun also precesses. That is, the orbit itself revolves, with the apsides of the orbital ellipse — the points of nearest and farthest approach — revolving about every 112,000 years relative to the fixed stars. Combining orbital precession with axial precession means that on average it takes about 23,000 years for the equinoxes to go through one cycle and return to the same calendar date. This affects climate by changing where on the orbit the seasons occur.
The other cycle, and the last one we will look at, is orbital inclination. It turns out that the plane of Earth’s orbit around the Sun is tilted relative to the Sun’s equator, and also relative to the Solar System’s so-called invariable plane. The invariable plane can be thought of as the rotational plane of the whole Solar System, mostly defined by the big gas giants: Jupiter, Saturn, Uranus and Neptune. The plane of Earth’s orbit is tilted relative to that by 1.57 degrees. The tilt of our orbit rocks up and down on about a 100,000 year cycle. That is, our tilt relative to the Sun and the rest of the Solar System is not constant, but changes over time. This affects climate by changing the apparent tilt of our axis relative to the Sun, affecting seasonal variation. Depending on the tilt of our orbit relative to the Sun, the same axial orientation relative to the fixed stars results in varying tilt relative to the Sun.
The Milankovitch cycles, and the others that weren’t known in his time, are well understood, and their effect on Earth’s climate is well accepted by climatologists. That doesn’t mean that we’ve got it all sewed up. There is the matter of the well established 100,000 year cycle in glaciation matching the 100,000 year orbital cycles, while those cycles have the weakest effect on climate. Then there’s an unexplained change from a 41,000 year ice age cycle that lasted for two million years, to the 100,000 year one that’s been in force for the last million. And there are others. There is still plenty of work for climatologists.
To recap: We have looked at the astronomical events that Milutin Milankovitch considered to be implicated in Earth’s Ice Ages. We began with a brief look at his life, then followed up with the three main cycles examined by him: orbital eccentricity, axial obliquity, and axial precession. And now we have tacked on orbital precession and orbital tilt. All of these things, and possibly others not yet discovered, interact in a complicated dance that results in the recurring cycles of glaciation. Milankovitch pointed out that the most important effect is the amount and intensity of insolation at the mid-latitudes, particularly in the northern hemishpere’s summer. Warmer summers tend to prevent the buildup of snow and ice on the big continental land masses.
A note of caution: the Milankovitch cycles are seized upon as an example of natural forces that affect climate change, often with the hope that they will negate the reality of our current climate change, or at least absolve us of our responsibility for it. This is a false hope. If it were true, then the cycles should be trending toward warmer, but they’re not. Orbital eccentricity is increasing, which should promote a cooling trend. Axial tilt is decreasing, also normally leading to cooling. If anything, Earth should be cooling. The fact that it’s warming should make it clear that something else is counteracting the astronomical effects. The most economical hypothesis would seem to be that we are releasing the solar energy captured and buried by plants hundreds of millions of years ago, by burning the resulting fossil fuels. Compounding this is the insulating effect of the carbon dioxide released in the process, leading to a warming of the atmosphere. Don’t blame Milutin.
Now that I’ve put you through all this, here’s a link to a nine minute video that brings it all together.