PDA

View Full Version : leap second this year?



spaceboy0
2007-Dec-26, 05:56 AM
are they adding a leap second this year to keep siderial time accurate with solar time?

Hornblower
2007-Dec-26, 02:22 PM
are they adding a leap second this year to keep siderial time accurate with solar time?

The purpose of the leap second is to keep solar time roughly synchronized with atomic time, which runs at a constant rate and ignores our planet's variable rotation rate.

The timekeeping powers that be are considering doing away with the leap seconds, because of the recurring bother of resetting numerous finicky computer systems.

There really is no purely technical need to keep solar time synchronized with atomic time. If it eventually drifts off by more than a half hour, it could be reset administratively in the same way we switch between standard and daylight saving time annually. I don't give a hoot about the future politicians and bureaucrats who might get themselves tied in knots over what I consider to be quibbling with numbers.

crosscountry
2007-Dec-26, 03:19 PM
our planet has a "variable rotation rate"?

or do you mean each solar orbit is not the same length?

Please explain.

01101001
2007-Dec-26, 04:08 PM
our planet has a "variable rotation rate"?

The rate isn't constant.

US Naval Observatory: Leap Seconds (http://tycho.usno.navy.mil/leapsec.html)


The Sub-bureau for Rapid Service and Predictions of Earth Orientation Parameters (http://maia.usno.navy.mil) of the International Earth Rotation Service (http://hpiers.obspm.fr/) (IERS), located at the USNO, monitors the Earth's rotation. Part of its mission involves the determination of a time scale based on the current rate of the rotation of the Earth. UT1 is the non-uniform time based on the Earth's rotation.

The Earth is constantly undergoing a deceleration caused by the braking action of the tides. Through the use of ancient observations of eclipses, it is possible to determine the average deceleration of the Earth to be roughly 1.4 milliseconds per day per century. This deceleration causes the Earth's rotational time to slow with respect to the atomic clock time.

Lots more about leap seconds there.

crosscountry
2007-Dec-26, 08:12 PM
oh yea, slowing due to the tides. Is that variable or constant?

ToSeek
2007-Dec-26, 08:47 PM
oh yea, slowing due to the tides. Is that variable or constant?

It's variable, which is why a decision has (or had) to be made periodically whether or not to stick in a leap second.

Hornblower
2007-Dec-26, 11:34 PM
Here is the leap second history of the past 46 years.
ftp://maia.usno.navy.mil/ser7/tai-utc.dat

Note that there has been only one in the past seven years, compared to an average of roughly one every year and a half from 1972, when the present system was adopted, to 1999. That indicates that our planet, while still slower than it was around 1820, is actually spinning slightly faster than it was in 1999 and earlier. My guess is that this was caused by a redistribution of non-uniform stuff deep inside the planet. It could have been magma under the crust, molten iron in parts of the core, or both. If enough dense stuff settles for whatever reason, conservation of angular momentum could overpower the tidal drag and result in a slightly faster spin.

Variations in the annual monsoon cycle that moves vast amounts of water back and forth between the Himalayas and the Indian Ocean could add to these irregularities.

crosscountry
2007-Dec-27, 03:58 AM
I guess melting of glaciers will redistribute mass too. Wasn't thinking about internal structure either. Should be.

spaceboy0
2007-Dec-27, 04:34 AM
no leap second will be added this year; just listened to WWV which broadcasts the atomic clock signals on shortwave (5, 10, 15 or 20 MHz) and they usually have an announcement within the first few minutes of the top of hour, and right now nothing...

so we can turn our attention to the Rose Bowl and the asteroid headed for Mars

Whirlpool
2007-Dec-27, 05:06 AM
Is it related to Leap Year?

Because next year is a Leap year.

:neutral:

Swift
2007-Dec-27, 02:23 PM
Is it related to Leap Year?

Because next year is a Leap year.

:neutral:
It is related to a Leap Year only in concept, adding an extra unit of time (day, second) so as to adjust our "clocks" (calendars) to the actual planet. Leap seconds aren't necessarily added in a leap year, they are added irregularly as needed, as opposed to leap years, which are every four years.

ToSeek
2007-Dec-27, 03:57 PM
It is related to a Leap Year only in concept, adding an extra unit of time (day, second) so as to adjust our "clocks" (calendars) to the actual planet. Leap seconds aren't necessarily added in a leap year, they are added irregularly as needed, as opposed to leap years, which are every four years.

Except when the year is divisible by 100 (and then except when the year is divisible by 400).

Swift
2007-Dec-27, 06:10 PM
Except when the year is divisible by 100 (and then except when the year is divisible by 400).
I knew that, but I was trying to keep on message. ;)