# Eli5: If millions of years ago a day had only 22 hours, does science count those years differently, or is the standard always 24 hours? Aren’t we then missing a few years which we need to add up?

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Eli5: If millions of years ago a day had only 22 hours, does science count those years differently, or is the standard always 24 hours? Aren’t we then missing a few years which we need to add up?

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The rotation of the Earth doesn’t really change the time it takes to make a revolution around the Sun.

So, the length of a day doesn’t matter.

The Earth would have had ~398 days per year. The reason that days have gotten longer is that the presence of the moon has slowed the rotation of the Earth relative to its own axis. That does not affect the rotation of the Earth around the sun, which is what determines the length of a year.

The length of the day hasn’t always been the same. The length of the *year* has. A year isn’t defined by “how many days it takes to get back to the same point in the orbit,” it’s defined by “how *long* it takes to get back to the same point in the orbit.” The year 1,000,000,000 BCE was the same length as the year 2022 CE.

We’re also not counting years back then with any degree of accuracy. We have no idea “exactly when” the asteroid hit Chicxulub, all we can do is estimate it.

A year isn’t a set number of days, its the number of times the earth rotates around the sun. Even todays years aren’t 365 days exactly, that’s why we have leap years to fix the slight difference between the days and the years.

Also if you’re counting years that far back no one is looking at the days because no one was there to write down the days and how long they were.

It doesn’t matter as long as the sun lines up on the summer and winter solstice (longest and shortest day of the year.) If it doesn’t, then that means the seasons are all out of whack.

You can just measure things in “Current Earth Years”, it doesn’t matter if that length of time actually corresponds to anything.

We use years to talk about things that happen before the Earth existed and things that will happen when the Earth is long gone. It’s just a unit, you can do that, it doesn’t mean we’re missing anything.

As others have pointed out the time it takes for the earth to complete one rotation (a day) has changed, but that doesn’t effect the time it takes for the earth to go around the sun (one revolution).

But to answer your question, does science count those past years differently? No, and it doesn’t need to. When we talk about millions of years ago we are talking about it relative to our current time. 1 million years ago is 1 million of OUR years ago. We don’t care what the “date” was we are talking about because nothing that happened 1 million years ago has a specific date we care about. If you were magically transported to 1 million years ago you might have to setup a slightly different calendar, but we don’t care about it today.

The speed of the earths orbit around the sun IS slowing, but its on a time scale of roughly 0.0000048 seconds per year. That means in the entire existence of humanity (roughly 300,000 years) the year has gotten about 1.5 seconds slower. Meaning you could more or less use the current calendar even then. Even a million years ago the calendar year would only be 4.8 seconds faster. In the entire existence of the solar system, roughly 4.5 billion years the length of the solar year has changed by 6 hours. If you lived on earth when it was first formed (which would be difficult at best!) that would mean…wait a second, that means no more leap year! A year would be close to exactly 365 days! But wait, thats 365 MODERN days. It would actually be different due to the slowing of the earths rotation due to the moon. So complicated.

It doesn’t matter because when scientists say an asteroid hit the Earth 66 million years ago they’re not saying it hit on April 36th at 11:27am.

Why?

Well, nothing is that accurate but mainly due to the fact that the time it takes the Earth to go around the Sun isn’t dependant on the length of the day.

Science when talking about long lengths of time talks about years which are based on the length of time to revolve around the Sun a change in the length of days doesn’t alter this.