I thought it was interesting. Something you never think about...
On Dec. 31 this year, your day will be just a second longer.
Like the more well-known time adjustment, the leap year, a "leap second" is tacked on to clocks every so often to keep them correct.
Earth's trip around the sun - our year with all its seasons - is about 365.2422 days long, which we round to 365 to keep things simpler. But every four years, we add 0.2422 x 4 days (that's about one day) at the end of the month of February (extending it from 28 to 29 days) to fix the calendar.
Likewise, a "leap second" is added on to our clocks every so often to keep them in synch with the somewhat unpredictable nature of our planet's rotation, the roughly 24-hour whirl that brings the sun into the sky each morning.
Historically, time was based on the mean rotation of the Earth relative to celestial bodies and the second was defined from this frame of reference. But the invention of atomic clocks brought about a definition of a second that is independent of the Earth's rotation and based on a regular signal emitted by electrons changing
In 1970, an international agreement established two timescales: one based on the rotation of the Earth and one based on atomic time.
The problem is that the Earth is very gradually slowing down, continually throwing the two timescales out of synch, so every so often, a "leap second" has to be tacked on to the atomic clock.
12 comments:
It gets more complicated than that: we have a leap year every four years, except on the century year (e.g. 1700, 1800, 1900), EXCEPT on the century year divisible by 400 (e.g. 1600 and 2000), when there IS a leap year (i.e. there WAS a Feb. 29, 2000, but there will NOT be a Feb. 29, 2100).
As for this slowing down, this has always confused me ... I understand that Earth's rotation rate varies on the order of milliseconds between the seasons (why this is so is complicated -- see below) and there is the long-term slowing as a result of angular momentum exchange between the Earth-Moon system: the Moon recedes at approx. 3.8 cm/year and the "LOD" (length of day) increases by 1.6 milliseconds per century. (This is actually a net -- lunar tidal drag minus effects from glacial rebound since the last ice age with the removal of so much ice from the Earth's surface.)
This cannot explain the repeated "leap seconds" that get added every several years. (There have been 9 such leap seconds since 1972). The Earth is simply not slowing down that fast. No where near it. Rather, the leap second is needed to account for the difference between solar or astronomical time and the SI second (the standard for UTC). One involves the rotation of the Earth around the Sun and the other involves atomic time standards.
Wikipedia's leap second article is useful.
I will endeavour to ensure that my bonus second is used wisely!
Regulus: See, I'm not all about decadence and debauchery like you sometimes seem to think. I do read!
It does not suprise me that you knew that, it probably flowed out of your head.
OK, maybe you took a quick look on the net to make sure you had the basics right, but I think that was in your head. Amazing.
Hahahaha!! My evil plan worked! I posted an interesting article and enticed you to comment on my blog!!
Fifi: I think that extra second will be added to my *cough* "self-gratfication time" *cough*. I know, too much information.
spend it how ever you like, it's your second.
A whole second to ponder over, use & abuse...
with nanotechnology & focusing on the itsy bitsy bits, amazing how important a second becomes.
Oh, yes, I never did explain the inter-seasonal variations in the Earth's rotation rate (the Length of Day, LOD).
Owing to the fact there is so much more water in the Southern Hemisphere where our dear Fifi lives than in the Northern Hemisphere, where there are also enormous mountain ranges, there is a discrepancy in how fast the prevailing westerlies change between summer and winter each hemisphere. The change is greater in the NH because the inter-seasonal temp. difference is greater there than in the SH.
The prevailing winds act as a torque on the Earth -- yes, the atmosphere actually affects the rotation of the Earth though it is one-millionth the mass or less. Yes, the flea moves the elephant, at least ever so slightly.
So there is a differential torque on the Earth between the NH and SH. The increase in the prevailing westerlies act as a negative torque, slightly slowing down the Earth while their decrease, or conversely, an increase in the trade easterlies or easterly monsoon over the Indian subcontinent, act as a positive torque.
Think of a runner on an east-west oriented race track. As the runner picks up speed (i.e. the increase in the westerlies going into winter), his feet actually push against the Earth acting as a negative torque. Conversely, when he slows down, he actually imparts a slight positive torque to the Earth. Thus, the Earth rotates faster in the NH summer time by a few milliseconds than in the NH winter.
It would balance out except for the asymmetry between the hemispheres I mentioned above.
Clarification: The runner is running east in the same sense that the Earth is rotating, so his increase in forward speed (analogous to the increase in the westerlies going into winter) with his feet pushing back against the ground acts as the negative torque. Slowing down acts as a positive torque since his feet are pushing forward on the Earth in the same sense it it rotating.
Ahhhhhh!!!!!! **brain melts down**
I hope you don't do that in everyday conversaton. (I don't expect you do.) If we do meet for a DC tour, none of that!! My puny brain will end up short circuting!
I find brainy people very sexy.
I do too, that's one of the reasons I like the bloke! That was quiite a bit for my shriveled, tiny brain to try to absorb all at once!
I must have read it 50 times and still only understand it a minimally.
agh.
I'm sure my nose just grew ans inch just from sheer effort of concentrating.
I wish I could spew out facts like that. It would be quite a party piece.
Nose growing? Are you referring to Pinocchio? I'm pretty sure that all flowed out of Regulus' head, with some minor fact checking as a posibility.
I've talked to him on the phone, as wll as emails, he's damned smart. I'm sure if I took the hours it would take me to cross check his accuracy I'd find he was correct.
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