Earth's day lengthens by 2 milliseconds per century

[DrKrettin]

That's the conclusion reported in this article in the Grauniad

 

I find it hard to believe they can reach this conclusion from ancient measurements, but I haven't read the original article.

[swansont]

That's the conclusion reported in this article in the Grauniad

 

I find it hard to believe they can reach this conclusion from ancient measurements, but I haven't read the original article.

 

 

It's wrong. The day lengthens by about 2 milliseconds per day, per century. They got it right in the body of the article.

 

"with every passing century, the day on Earth lengthens by two milliseconds"

 

And then they got it wrong again

 

"The split second gained since the first world war may not seem much,"

 

We've added 26 leap seconds since we started doing them in 1972

 

We've known about this for decades.

[DrKrettin]

 

 

It's wrong. The day lengthens by about 2 milliseconds per day, per century. They got it right in the body of the article.

 

"with every passing century, the day on Earth lengthens by two milliseconds"

 

And then they got it wrong again

 

"The split second gained since the first world war may not seem much,"

 

We've added 26 leap seconds since we started doing them in 1972

 

We've known about this for decades.

 

 

 

 

Are you by any chance the reader who has commented in the Guardian with a very precise post to this effect? That Guardian headline is a disgrace.

[swansont]

 

 

Are you by any chance the reader who has commented in the Guardian with a very precise post to this effect? That Guardian headline is a disgrace.

 

 

No, that wasn't me.

[Janus]

I think this is something that confuses a lot of people. They confuse the effect of the Solar day increasing by 2ms/day/century with the accumulative effect this has in terms of keeping our clocks in sync with the Earth.

 

Yes, 2ms is not a noticeably long time. You aren't going to notice that the day is now 2 ms longer now than it was 100 yrs ago. But if we assume that 100 years ago that the Earth rotated in exactly 24 hrs, that means today it takes 24 hrs +2ms to rotate. So after one day our clocks and the earth will be out of sync by 2ms. After two days they will be out of sync by 4 ms, etc. in 500 days they will be out of sync by 1 second. This is where the leap second comes in. It's a shift in our time keeping in order to temporally bring the Earth and clocks back into sync again.

 

Of course, since the rotation of the Earth is still slowing, in 50 years, our clocks and the Earth will be 3ms out of sync per day and we will have to add leap seconds at a rate a bit faster than 1 per year.(This isn't exact, for a closer estimate you would have to work out as to exactly when the Earth took exactly 864000 seconds to complete a rotation and then work things out from that. In addition, there are other factors that can alter the Earth's rotation. Major earthquakes can make changes too; it is estimated that the 2011 Japanese Earthquake sped up the Earth's rotation by ~1.8 microseconds per day.)

[swansont]

Right. Even if the earth stopped slowing down, we would still add leap seconds, because it's running slow. The trend of a few decades had been one of speeding up, and a stretch a while ago where some days were longer than 86400 sec — as a result there was a long span where no leap seconds were added.

 

http://clarkplanetarium.org/wp-content/uploads/Length-of-Day-Graph.jpg

 

(for reference, that's Modified Julian Date on the lower axis. MJD 54000 was Sep 22, 2006)

 

edit: here's a better graph

 

https://www.ucolick.org/~sla/leapsecs/amsci.png