How Geologists Found Earth's Orbit In Stone [8:03]

YouTube ^ | August 12, 2026 | Geo.Sassie (Saskia Elliott | Geoscientist)

[SunkenCiv]

At Kimmeridge Bay [Dorset, UK] on the Jurassic Coast, the rocks have a bit of a pattern to them. And there's a good reason for that! It's a record of changes in Earth’s climate that were driven by something happening far beyond Dorset: the changing geometry of Earth’s orbit around the Sun... Because Earth's orbit changes over time in cycles! Welcome to Milancovich cycles! 

This is far from the only rock unit that shows this, and in fact the Blue Lias around Lyme Regis is also pretty famous for this, but I found this cool paper that really investigated it so it was an interesting spot to talk about it. 

How Geologists Found Earth's Orbit In Stone | 8:03 
Geo.Sassie (Saskia Elliott | Geoscientist) | 12.1K subscribers | 40,702 views | August 12, 2026

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TOPICS: Astronomy; Science
KEYWORDS: astronomy; catastrophism; geology; gradualism; milankovitch; milankovitchcycles; saskiaelliott; science

[SunkenCiv]

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YouTube transcript reformatted at textformatter.ai *may* follow.

[SunkenCiv]

Abstract: Three independently measured variables (magnetic susceptibility, photoelectric factor and total gamma-ray) obtained from throughout the type Kimmeridge Clay Fm in Dorset (Southern England) were used to identify regular metre-scale, sedimentary cycles. Spectral analysis demonstrates that for long stratigraphical intervals the cycles are expressed as large-amplitude cycles of 1.87–4.05 m wavelength and smaller-amplitude cycles of around half that wavelength. These cycles are interpreted to record orbital obliquity and precession, respectively. The much larger amplitude of the inferred obliquity cycles compared with the precession cycles may indicate a high-latitude climatic forcing transferred to lower latitudes via sea-level variations. Orbital tuning indicates that the Early Kimmeridgian (sensu anglico) lasted at least 3.6 Ma (95 longer-wavelength cycles) and the Late Kimmeridgian at least 3.9 Ma (103 longer-wavelength cycles). The first detailed productivity estimates for the Kimmeridge Clay Fm, on a cycle-by-cycle calculation, indicate that average productivity of the type Kimmeridge Clay (220 g m−2 a−1) was less than the average productivity on modern continental shelves. The high average organic carbon content of the type Kimmeridge Clay (3.8% total organic carbon) cannot be attributed to high average productivity. However, the average organic carbon content is consistent with low siliciclastic mineral dilution of organic matter and/or elevated preservation linked to reduced bottom-water oxygenation.

• Cyclostratigraphy, orbital tuning and inferred productivity for the type Kimmeridge Clay (Late Jurassic), Southern England | Graham P. Weedon | Angela L. Coe | Ramues W. Gallois | July 2004 | https://doi.org/10.1144/0016-764903-073

[SunkenCiv]

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[Red Badger]

Space Aliens driving Space SUVs.............

[BenLurkin]

They’re smokin’ Lucky Strikes, and wearing nylons too.

[SunkenCiv]

Present-day nautilus shells almost invariably show thirty daily growth lines (give or take a couple) between the major partitions, or septa, in their shells. Paleontologists find fewer and fewer growth lines between septa in progressively older fossils. 420 million years ago, when the moon circled the earth once every nine days, the very first nautiloids show only nine growth lines between septa. The moon was closer to the earth and revolved about it faster, and the earth itself was rotating faster on its axis than it is now. The day had only twenty-one hours, and the moon loomed enormous in the sky at less than half its present distance from earth.

• When the Days Were Shorter [10/04/2004]

[Red Badger]

https://www.youtube.com/watch?v=kXc5Oe_kj8k

[Eli Kopter]

Interesting story SC. It is a fact that when objects orbit other masses, the orbit slows down over time. In the formula, mass is constant, so velocity changes. In the case of earth, the wobble changes angles as the orbit slows. So the revolution of the earth lessens slightly over time as it slows down, which means the year is getting shorter. We see differences in magnetic forces that are associated with these fluctuations, and in fact, Magnetic shifts are visible in the rock record. It has occurred a few times and is responsible for some of the Climate changes that are noted. However, the Earth has been in a constant state of Climate Change for 4.8 billion years, depending on who you talk to.

[Abathar]

"So the revolution of the earth lessens slightly over time as it slows down, which means the year is getting shorter. "

Well said in your post, but I believe that would make the year longer, not shorter however.

[alexander_busek]

420 million years ago, when the moon circled the earth once every nine days, the very first nautiloids show only nine growth lines between septa. The moon was closer to the earth and revolved about it faster, and the earth itself was rotating faster on its axis than it is now. The day had only twenty-one hours, and the moon loomed enormous in the sky at less than half its present distance from earth.

Nota bene: 420 million years ago was at the tail-end of the Silurian Period; the Devonian Period would begin less than one million years later.

According to Kepler's Third Law of Planetary Motion:

The square of a body's orbital period is proportional to the cube of the length of the semi-major axis of its orbit around its primary.

Thus, if the Moon's distance d from the Earth (semi-major axis) is halved, its orbital period p (27.32 days) would be reduced to p _* √(d³) = p _* 0.35 = 9.7 days (i.e., modern days of 24 hrs each).

So your math checks!

However, the initial premise, i.e., your estimate of the Earth-Moon distance 420 million years ago, is highly inaccurate. Around 500 million years ago (in the Cambrian Period - which preceded the Ordovician, Silurian, and Devonian Periods), the Moon was about 370,000–375,000 km from the Earth (i.e., about 96% of its modern-day value).

Even 2.5 billion years ago, the Moon was already a respectable 52 Earth-radii (i.e., 331,630 km) away from the Earth: The lunar nodal tide and the distance to the Moon during the Precambrian era. That's a whopping 86% of the modern-day value.

To approach one-half of the current Earth-Moon distance, you'd have to go back to the middle of the Hadean Era - just after the solidification of the Earth's crust, and not long after the formation of the Moon itself.

Regards,

[Eli Kopter]

I was wondering who would catch that.

[SunkenCiv]

The whole point is, the nauliloid fossil septa show real data, as opposed to standard assumptions.

[CurlyDave]

The whole point is, the nauliloid fossil septa show real data, as opposed to standard assumptions.

The only thing it really tells us is that the septa are fewer in the older ones. There are many confounding factors. The growth rate of the organism inside the shell, nutrient concentration in the water, shell precursors in the water, genetics of the organisms, etc.

[SunkenCiv]

Wrong.

[no-to-illegals]

Than You Both for the mathematical connections/calculations

[no-to-illegals]

My oh my …. Dropped “k” in Thank. LOL. Undoubtedly a variable in posting?