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Comment by philliphaydon | Hacker News Reader

> Earth turned faster at the end of the time of the dinosaurs than it does today, rotating 372 times a year, compared to the current 365, according to a new study of fossil mollusk shells from the late Cretaceous. This means a day lasted only 23 and a half hours, according to the new study in AGU’s journal Paleoceanography and Paleoclimatology.

I'm curious, how do they decide that the earth spun faster on it's axis rather than the earth taking longer to orbit the sun?

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>how do they decide that the earth spun faster on it's axis rather than the earth taking longer to orbit the sun?

It's both effects really. Celestial bodies' orbits do undergo decay and also their rotation undergoes decay. The question is which happened to which degree, and I think the rotational slow down is the dominant effect in Earth's case.

Orbits decay due to various forms of drag. The long-term rate of decay of orbits in the Solar system is relatively well established.

Rotation also is slowed down due to drag, but in our case there's another major force: the tidal influence from the Moon. Earth's Moon is a relatively large companion (at 1.23%[1] by mass). Both bodies influence each other tidally, and that influence saps away rotational energy and also Moon's orbital energy; the Moon already got tidally locked to Earth. Aside of that there's a (smaller) tidal influence from the Sun, which again saps Earth's rotational energy.

[1] https://www.wolframalpha.com/input/?i=mass+of+the+moon+%2F+m...

Is the Earth's magnetic field affected by its rotation speed?

Since the magnetic field is tied to rotation of the planet’s core, I would expect it to slowly weaken as it caught up with the lack of rotation of the surface.

Because the solar system has reached the current stability a long time ago. If any of the planets had such fluctuations on their orbits so close in the past(relatively), we wouldn't be here

I'd like to know too how they can confidently state that Earth's orbit does not change. Would it mean we'd spiral into the sun or out of orbit if it did?

From Newtonian mechanics, the orbit can only change by applying a force from somewhere, and a big move would require a lot of energy. And there's no evidence of such an event in the geologic pas; if it was triggered by an impact, it would be far larger than the one which killed the dinosaurs.

Addendum: far larger than the one that created the moon.

There's a reasonably thorough discussion of this topic, from an...interesting...perspective, in this essay: "How to Destroy the Earth" https://qntm.org/destroy

Without any outside input of energy we probably can't really escape the Sun's gravity well.

Oddly enough it's pretty tricky to steer the Earth into the sun as well, but we should be losing minute amounts of energy that will eventually put the Earth closer to the Sun. This probably won't happen before the Sun explodes though.

Rotational speed of Earth is 1,000 mph at the equator where it is moving fastest. Other parts of the mass are moving slower.

The entire Earth orbits the Sun at 67,000 mph -- around 67X faster. And note that it is the entire mass of the Earth moving at that speed, not just the equator.

Changing the rotation speed by 1% is a whole lot easier than changing the orbital speed by 1%.

Changing rotational speed or revolutional speed matters more on tidal forces and other sources of drag, which for the latter are much less.

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