## Things I Didn't Know

The length of a day varies slightly year by year.  I would presume this is due to small changes in the Earth's core, which would effect angular momentum.  I wonder if the water cycle on the earth (ie moving water from the ocean say to lakes or high-altitude ice) measurably affects angular momentum.

1. #### Peter:

How does this compare to global temperatures? At first glance there seems to be some correlation.

2. #### ErikTheRed:

The length of the day is affected by the amount of man-made greenhouse gases in the atmosphere. If we don't cut back now, we won't have any daylight left and we'll start living underground like mole-people.

3. #### dearieme:

Mrs Clinton will put a stop to it.

4. #### Larry Sheldon:

I ran across this "length of day" thing some time ago and wondered about it (not enough to remember to look it up).

My guess is that it is either due to the oft-mentioned "wobble", or a know on the math to get the models to produce the right answers.

5. #### Nick:

The main reason is expansion of the oceans and melting of ice (if real)

Just like a rotating skater speeds up and slows down when their arms are pulled in and out repectively, the same is true for the earth when the angular momentum changes.

This effect causes problems for the alarmists, because it puts real limits on what can be claimed for changes in sea levels. The changes in sea levels have to match the long term record of the earth's rotational speed. If the claim doesn't agree with the rotational speed record it must be wrong.

Nick

6. #### Larry Sheldon:

I wish I had gotten my stab line right....danged typos.

"My guess is that it is either due to the oft-mentioned "wobble", or a knoB on the math to get the models to produce the right answers."

The ocean depths (and tidal differences?) makes sense.

7. #### markm:

If you ignore the part before 1910, it does somewhat resemble the temperature graph for the last century - that is, it peaks about 1930, declines slowly into the 1970's, and rises since. But that very high peak in 1892 and steep decline is totally uncorrelated with temperature changes. Where the changes from 1910-present sort of match temperature changes, I think they are leading them by a few years.

If I'm reading the vertical scale correctly, the maximum deviation was around 1910, of almost +4 ms/day. That amounts to a bit over 1 extra second a year, not a big threat to the accuracy of your wristwatch. Note that they arranged the scale with minus being up - that is, the higher it goes on the scale, the shorter the day is, and the faster the earth must be spinning. Nick is correct (just basic physics) that this has to be due to some sort of momentum shift, and the effect since 1972 has been mainly to lower the angular momentum - that is, an average pulling in of mass slightly closer to the center of the earth. Warming and expanding oceans would increase the day length, the opposite of what is happening, but of course it could be happening and be overpowered by greater effects in the opposite direction. Melting of glaciers and snow high in the mountains would shorten the day, but I just don't think there's been enough mass of snow lost to account for the change. Various parts of the earth's crust are also rising and falling, and that is more than enough mass to have these effects. (E.g., Venice and The Netherlands are sinking, while at Thermopylae, what was a narrow strip of beach in the 5th century BC is now a half mile wide. 300 men could no longer hold a line against a huge army there, even with machine guns, let alone with the Spartans' swords and spears.) And of course volcanos and earthquakes can shift quite a lot of mass overnight. Could Mt Helena have caused the 1987 peak? (Note that if it did, the eruption must have removed mass from the peak and moved it somewhere lower.) But I cannot think of any event to explain the greatest peak, 1892.

Since the momentum changes seem to be leading the temperature, it seems pretty unlikely that temperature-driven water redistribution is driving the momentum changes. I see no way that causation could directly run in the opposite direction (tiny changes in the length of the day driving global temperature changes), but it's possible that geological events that shift large masses of crust cause both a change in the rotational speed (immediately), and a delayed change in global temperature.

Finally, if the earth's core rotates at even a slightly different rate than the mantle and crust, then even the tiniest change in the core rotation would require an opposite change in the rest of the world. What I don't see is how such changes would arise, let alone how they would propagate in just a few years without shaking the heck out of the entire surface... But I'm no geologist.

Darn, I'm getting hooked but I don't have time to pursue this.

8. #### bnettles:

Why doesn't anybody mention the moon, it's 5 degree orbital inclination which makes eclipses non-periodic (although predictable), spring tides and neap tides of different heights due to the moon's eliptic variation, other planets' gravitational tugs, etc? That would have a bigger effect than the temperature of the atmosphere. Sheesh!

9. #### bnettles:

Why doesn't anybody mention the moon, it's 5 degree orbital inclination which makes eclipses non-periodic (although predictable), spring tides and neap tides of different heights due to the moon's eliptic variation, other planets' gravitational tugs, etc? That would have a bigger effect than the temperature of the atmosphere. Sheesh!

10. #### Charles D. Quarles:

The variation of day length is mainly due to gravitational tides from the moon, sun, Jupiter and Saturn. There are also effects that are due to the net wind vector in the atmosphere, that is, whether the main component is pole ward (N/S) or equatorial (E/W).