Earth's Obliquity
A paper I read on astronomically calibrating pre-Cambrian sediments to get a more accurate chronology of important evolutionary events led me to look up where Earth’s rotational axis has pointed over geological time.
As it happens, Earth’s rotational axis is not at a 90° angle relative to the plane of its orbit around the sun. Right now, it’s 23.5° off, causing almost all of the current climate’s seasonal variation.
Above, an illustration from a 1952 book, Across the Space Frontier, which shows how the rotational axis of the earth isn’t square with the plane of the Earth’s axis. The book advocates for a space station in an orbit that would be at 90° to the plane of the Earth’s orbit. That’s the white ellipse with arrows directed to the south. Notice the Sun’s light shining on the polar regions above the arctic circle: this illustration takes place at or near the summer solstice.
Anyway, the angle Earth’s rotational axis makes with the plane of its orbit is called obliquity. I found more than a few papers about variation in obliquity during geological time. Here’s on of the more recent.
A 650-Myr history of Earth’s axial precession frequency
and the evolution of the EarthMoon system derived
from cyclostratigraphy
Yujing Wu1, Alberto Malinverno2, Stephen R. Meyers, Linda A. Hinnov
Sci. Adv. 10, eado2412 (2024) 16 October 2024
This paper is about frequency of the change in obliquity, how fast obliquity changes, not about the obliquity itself. Nevertheless, it has some interesting material.
These folks have a day in the Cryogenian period (650 million years ago) lasting 20.94 hours, so Earth has slowed a bit since then. While the obliquity varies due to Sun and Moon’s gravity interacting with an ever so slightly lumpy Earth, the rotation of Earth slowing down is due to tidal friction.
In the present-day ocean, about 3/4 of tidal energy dissipation takes place in turbulent bottom boundary layers of shallow seas, with the rest occurring in the deep ocean where internal tides interact with large bathymetric features
The oceans sloshing about causes Earth to spin slower and slower. Sloshing is affected by the size and shape of the oceans, which changes due to continental drift, and water level, which changes during ice ages. It’s somtimes suggested that the first land plants triggered the end-Ordovician glaciation, and that previously-subducted carbon released by volcanoes helps end ice ages. That means the biosphere changes the Earth’s obliquity to some degree.
In any case, the frequency of obliquity changes, sometimes rapidly, sometimes slower. The causes aren’t easy to tease out, but oceans have a lot to do with it.
About the tidal friction:
We neglect here the 0.2 TW of tidal energy dissipation in the solid Earth and the effect of solar tides, which contribute another 0.5 TW; both these minor contributions did not change substantially over the time span we investigate.
0.2 and 0.5 tera Watts dissipated continuously is a minor contribution.
Wikipedia claims the entirety of humans uses about 19.2 TW.
… current human civilization consumes approximately 18–20 terawatts of continuous power.