A Moment of Science

Years Ago, The Year Was 400 Days Long

Scientists have found fossil evidence from tropical corals that existed at that time, and they have about 400 daily growth rings per year.

multicolored coral under the sea

Photo: flightsaber (Flickr)

Could this coral be over a hundred years old?

Don: Yaël, do you notice the days changing length over the course of the year?

Yaël: Sure, Don, it stays light later in the summer and gets dark earlier in the winter.

Don: No, that’s not it. I mean the whole day feels longer.

Yaël: I’m not sure that’s possible, Don aren’t days always 24 hours long?

Earth’s Rotation

Well, it actually takes 23 hours, 56 minutes, and 4 seconds for the Earth to rotate on its axis. But in fact the Earth’s rotation is slowing down all the time because of the gravitational pull of the moon.

Does this mean that the days really do feel longer? Nope. The moon slows us down by about two milliseconds a century, so that even 1 million years ago days were only twenty seconds shorter.

Would People Notice?

I doubt I would have noticed that.

Maybe not a million years ago. But 400 million years ago days were 21 and a half hours long, which you would have noticed.

400 Days Per Year

What you really would have noticed is that the time it takes the Earth to get around the sun hasn’t changed, so that back then there were 400 days a year.

Scientists have found fossil evidence from tropical corals that existed at that time, and they have about 400 daily growth rings per year. So someone was enjoying all those extra sunny days.

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