[bad_user]

Our sun will turn into a red giant and destroy earth long before earth has a chance to stop its rotation, as we're talking about an increase in milliseconds per century. Since 1820 the earth's rotation slowed down by about 2.5 milliseconds.

[valleyer]

That sounds like it contradicts with the fact that we've had dozens of leap seconds in the last several decades. Explain?

[jug]

The Earth's rotation speed can be influenced by many things and can both be sped up and slowed down. For example, the Sumatran earthquake in 2004 shortened the rotation period by 3 milliseconds. In theory negative leap seconds could be added but none have yet.

This gives rise to problems for computers since they are irregular and unpredictable, and can't be made part of algorithms. In extent, to properly calculate a precise time difference (actual elapsed time) over several years, you actually need to consult a table of added, historic leap seconds and take these into account too...

[bad_user]

Simple. What's happening is that in the past we were measuring seconds relative to the duration of a day, but since then we've switched to atomic clocks.

So the day is not 24 hours or 86400 seconds, but rather 86400.002 seconds on average. That's about 2 milliseconds or more of deviation per day and for a whole year that's about 0.7 or 0.9 secs worth of deviation. Yet we still pretend that the day has precisely 24 hours, hence the need for leap seconds.

Given that since 40 years ago since leap seconds were adopted about 25 leap seconds have been scheduled, that sounds about right.

[logn]

How do you reconcile that with this chart: https://upload.wikimedia.org/wikipedia/commons/5/5b/Deviatio... via https://en.wikipedia.org/wiki/Earth_rotation#Changes_in_rota...

Cumulative deviation since 1972 will be 26 seconds.

[cnvogel]

Part of if might be a constant offset/day.

26 seconds over 43 years (1972-2015) is 0.60 seconds/year. If this matches the average offset from 1820...1972 (which would add up to about 92 seconds), it means that our average is wrong by ~0.6 seconds, but the time length doesn't gain additional seconds due to "spinning down" (caused by tidal interaction with the moon, or whatever other effects there might be).

That's actually also what the graph suggests: It's havnig a short-term daviation of +/- 1ms over the course of (judging by eye) weeks, and a long-term deviation of +/- 2ms over the course of decades. The average deviation of 0.6seconds/year (number of leap seconds inserted) translates to an average of 1.65ms/day from 1972-now.