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Alan
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### Leap second teaser

"Thanks to hundreds of records of lunar and solar eclipses carved in clay tablets and written into dynastic histories, modern scientists have determined that the speed at which Earth spins on its axis has slowed by 1.8 milliseconds per day ...

If humanity had been measuring time with an atomic clock that started running back in 700 BC, today that clock would read 7 p.m. when the sun is directly overhead rather than noon"

Ignoring the awkward phrasing, does the second statement essentially follow from the first, or did the reporter make a gross arithmetic blunder?

From:
http://www.latimes.com/science/sciencen ... story.html

Posts: 23951
Joined: September 20th, 2002, 8:30 pm

### Re: Leap second teaser

If on January 1st, 699 BC, the Earth lost exactly 1.8 msec/day of rotational velocity, it would amount to 0.0018*365.25*2717 = 1786 seconds or about 1/2 hour.

If the loss of rotational speed between then and now was linear (it's NOT!) then the loss would about 15 minutes.

Only if the rate of loss were 1.8 msec/day/century would the answer be 7 PM.

Perhaps there was a typo in the units if the reporter or eager-beaver editor removed the "/century" part of the term.

Alan
Topic Author
Posts: 9975
Joined: December 19th, 2001, 4:01 am
Location: California
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### Re: Leap second teaser

Well done. Sorry, the eager-beaver editor was apparently me. Unless the article has been edited since I posted the excerpt, the reporter said "1.8 milliseconds per day over the course of a century ...", so all looks well and you spotted my inadvertent omission.

Posts: 23951
Joined: September 20th, 2002, 8:30 pm

### Re: Leap second teaser

Technically, the reporter should have said "over the course of each/every century" but even that falsely implies a steady linear effect.

Here's the short-term pattern of deviations in day length over that past half century:

As you can see, days are actually slightly shorter now then they were in 1972.

outrun
Posts: 4573
Joined: April 29th, 2016, 1:40 pm

### Re: Leap second teaser

Should correlate a bit with this:

I know someone who would decide if he would go skiing in the alps based on these deviations.

Posts: 23951
Joined: September 20th, 2002, 8:30 pm

### Re: Leap second teaser

Indeed! Melting glaciers and snow fields cause the rotation rate to accelerate. Water reservoirs, volcanic flows, and rocket launches cause it to decelerate.

When the Dutch pull a bicycle out of the canal, the days get longer!

Cuchulainn
Posts: 61185
Joined: July 16th, 2004, 7:38 am
Location: Amsterdam
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### Re: Leap second teaser

When the Dutch pull a bicycle out of the canal, the days get longer!
Correlation or causation?
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Jean Piaget

Cuchulainn
Posts: 61185
Joined: July 16th, 2004, 7:38 am
Location: Amsterdam
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### Re: Leap second teaser

When the Dutch pull a bicycle out of the canal, the days get longer!
Correlation or causation?

The world after Jan 1 1970 can be summarised by std::time_t.
http://www.datasimfinancial.com
http://www.datasim.nl

Every Time We Teach a Child Something, We Keep Him from Inventing It Himself
Jean Piaget

Posts: 23951
Joined: September 20th, 2002, 8:30 pm

### Re: Leap second teaser

When the Dutch pull a bicycle out of the canal, the days get longer!
Correlation or causation?

The world after Jan 1 1970 can be summarised by std::time_t.
Hmmmm... It looks like std::time_t may not know about leap seconds and will get further and further behind as actual time goes on.

It won't be long before we'll haveJulian and Gregorian versions of std::time_t.

And summarizing the world with an integer!??!?!?!?!??!?!

Cuchulainn
Posts: 61185
Joined: July 16th, 2004, 7:38 am
Location: Amsterdam
Contact:

### Re: Leap second teaser

When the Dutch pull a bicycle out of the canal, the days get longer!
Correlation or causation?

The world after Jan 1 1970 can be summarised by std::time_t.
Hmmmm... It looks like std::time_t may not know about leap seconds and will get further and further behind as actual time goes on.

It won't be long before we'll haveJulian and Gregorian versions of std::time_t.

And summarizing the world with an integer!??!?!?!?!??!?!
No,, 148.413!
http://www.datasimfinancial.com
http://www.datasim.nl

Every Time We Teach a Child Something, We Keep Him from Inventing It Himself
Jean Piaget

Posts: 23951
Joined: September 20th, 2002, 8:30 pm

### Re: Leap second teaser

Correlation or causation?

The world after Jan 1 1970 can be summarised by std::time_t.
Hmmmm... It looks like std::time_t may not know about leap seconds and will get further and further behind as actual time goes on.

It won't be long before we'll haveJulian and Gregorian versions of std::time_t.

And summarizing the world with an integer!??!?!?!?!??!?!
No,, 148.413!
That's better!

The diligent student should be able to figure the world out with just paper and pencil.

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