[Lawtonfogle]

Can someone please explain why this keeps being explained as 'earth's rotation is slowing down'?

Having a scheduled addition of the leap second only corrects an issue resulting from our timing being slightly off (by about 1 second every couple decades). Now, if the second had to be add in changing frequency... the changing in frequency would be a result of the earth's rotation changing... but even still that wouldn't be why the second itself is being added, is it?

[jerf]

"One second every couple of decades" grossly, grossly overestimates our timing inaccuracies. Multiple orders of magnitude.

The problem is in some sense, the opposite... our timing is now so good that we can notice the natural variations in Earth's rotation to astonishing accuracy. Thus, if we want to keep the clock in sync with what the Earth is actually doing, we need to add the leap second.

(There's quite a bit of debate about that "if", and IMHO right now industry after industry seems to be experiencing major, expensive disruptions and repeatedly incurring risk, to save astronomers from having to consult a lookup table that they of course would build into computer programs and pretty much forget about ever after. But I digress.)

The Earth's rotation really does wobble by some amount due to natural shifts in density, temperature changes, and any number of other things. It's why we don't have a schedule of leap seconds for the future. If it were merely a matter of accurate timing we'd have a schedule, because our timing is accurate enough by orders of magnitude to make a schedule for a hypothetical perfectly-steady or perfectly-slowing Earth for millennia in advance. The problem is that we can't predict what the Earth is going to do.

[NickNameNick]

Astronomers use sidereal time instead of solar time anyway, so they're used to making time corrections, the sidereal day is only 23 hrs, 56m, 4 seconds and change. Equivalent to the length of a day if the light from the sun was coming from an infinitely distant source. That way at a given sidereal time, anything far enough away will always be in the same place in the sky.

[NickNameNick]

The change in rotation is not consistent. Earthquakes change the earth's rotational inertia by raising and lowering land near the equator. Sometimes they jog the entire planet forwards or backwards in its rotation by a slight increment. Tidal interactions vary significantly enough to have an effect. Even man made changes are measurable - the three gorges dam in China moved enough water uphill to have a measurable effect.

And simply put, the leap seconds are added so that clock noon, solar noon don't drift too far apart.

[mxyzpt1k]

From the very end of the article...

In the meantime, the Paris-based International Earth Rotation and Reference Systems Service will keep track of the gradual slowing of the planet, caused in part by drag created by the Moon. Millions of years ago days were 22 hours long.

[Lawtonfogle]

I know the speed of rotation changes slowly. But that would be a reason for lengthening a day, not adding a leap second. The leap second is added because our concept of time is slightly off, but it keeps being presented as if we had to add an extra second because of the slowing of the rotation. If that was the case, it would be like those 2 hours that were added in the last few million years. We don't consider them leap years because they aren't a correction but a redefinition (ignore for a moment that humans weren't around to define as a day as 22 hours back then).

Consider the leap year for a second. We add it because our years have 365 days is slightly incorrect (by about .25 days a year). It isn't that the number of rotations per revolution is or isn't changing. That isn't relevant to our use of a 1 day correction every 4 years. If some force caused us to revolve around the sun slightly faster and it took 365 exact, then we could remove the leap year.

Removing the leap year would be a result of our revolution being slightly faster. But the leap year itself is not.

In short, the delta of the leap correction is a result of the change in the system being monitored, but the leap correction itself is because our system of measurement is off.

[jessaustin]

You can see how a 365.248-day year would be inconvenient, right? A 86400.002-second day would be more inconvenient, even though the point of TFA is that a 86401-second day is also somewhat inconvenient.

Seconds are not defined by the rotation of the earth, any more than they're defined by its revolution.

[lmm]

I imagine when the original SI second was specified, there was no need for leap seconds, and if the Earth's rotation had remained exactly the same since then then we would never have needed them?