#fun fact the theia collision might also be why we have such a strong magnetic field which is another important thing for life

[lactose-intolerant-nihilism]

The length of a day is determined by the Earth's angular momentum, the length of a year by the shape of its orbit. Gravitational interactions alter both of those things, they just do it so slowly that it's imperceptible on human timescales.

The most important interaction to the change in Earth days is that with the moon - we notice its tidal interactions most in its influence on the ocean, but they're interacting with the entire planet, and they're leeching a tiny amount of our angular momentum away into the moon every cycle. For complicated orbital dynamics reasons, the moon's angular momentum increasing pushes it out to a higher orbit, so over time the moon gets further away and the day gets slower.

When the Earth and moon first formed out of the proto Earth-Theia collision back in the early solar system, the moon was twenty times closer than it is today and a day was only four hours long.

(In fact a period later on where a resonance between the sun and the moon caused the day's length to get stuck at 19 hours for around a billion years may well be the reason multicellular life first developed, and the rise of oxygen-breathing lifeforms changing the atmosphere and causing the amount of sunlight reflected away (since that also affects the angular momentum) may have been what broke it and let the day start slowing down again, but that's another story)

Fantastic reconstruction of an Archaeopteryx I saw at Southampton Fossil and mineral show. It was part of an exhibition of fossil replicas exhibited by Southampton University.