When the Earth was young, the sun was colder. Billions of passing years burned away the hydrogen, and gravity squeezed more luminosity from the shrinking core.

When the Earth was young, the days were shorter, and the Moon was closer. Eon by eon, the Moon slowed down Earth’s rotation with its gravitational drag, and drifted slowly outward.

The wheels within wheels wobble and turn, rattle and roll chaotically.

Around the great singularity at the center of the galaxy the sun circles, four times in a billion years passing through a sprawling, spiraling arm. Undulating up and down in this orbit, every 32 million years the sun passes through the galactic disk, dense with star dust. The climate cools and warms.

The Earth’s path around the sun stretches and contracts between ellipse and circle, the eccentricity pulsing every 100 thousand years, shifting the aphelion and perihelion, modulating the seasons. Every 41 thousand years, the Earth teeters on its axis. The tropics drift, the seasons intensify and weaken, the northern and southern hemisphere ice sheets grow and recede asymmetrically. Every 23 thousand years, the pole wobbles twixt Vega and Polaris, and the seasons change phase with perihelion and aphelion, becoming milder or harsher.

And there are yet smaller wheels within these wheels: solar flare activity rising and falling every 11 years with the 22-year magnetic cycle, itself waxing and waning every 87 years, and so on.

Cosmic cycles and wobbling orbits, punctuated by random bombardments and internal convulsions of the Planet that change greenhouse gases and ocean circulation: continental drift, volcanoes, seismic release of methane hydrates, and the occasional evolutionary surprise:

Land plants stripping CO₂ from the atmosphere…

Homo sapiens putting it back.