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    Re: Question about UT1-UTC
    From: Frank Reed
    Date: 2012 Dec 31, 19:22 -0800

    Sean, you wrote:
    "Looking over the USNO predictions for UT1-UTC in the latest Bulletin-A, I noticed that on Nov. 23 the correction will become negative. Will the rotational pole be aligned with the geographic pole near this time? Or am I way off base?"

    Yes... to your second question. :-)

    If anyone wants to look at the table Sean is referring to, visit this url:
    http://maia.usno.navy.mil/ser7/ser7.dat

    The changing position of the pole is contained in the x and y data. The difference between UT1 and UTC reflects the orientation of the Earth's longitude lines with respect to fixed "longitude" lines in the heavens as a function of time.

    Atomic Time or TAI is a "true" time as nearly as we can make one, in the sense that every second is equal in length, as determined by the laws of physics. If the Earth rotated at a perfectly constant rate, then the time based on rotational orientation, UT1, would be identical to TAI except for some constant of proportionality which would ideally be 1 exactly. But the Earth does not rotate at a constant rate. It is slowing down in the long term, and in the short term its rotation can speed up a bit, too, as angular momentum is tranferred to the atmosphere (and to a lesser extent the oceans and presumably the liquid mantle). Also because the definition of the second was ultimately based on the Earth's rotation rate c.1900, the constant of proportionality is not precisely 1. So time kept by the Earth's orientation gradually drifts from the nearly "true" time kept by atomic clocks (and the latter, by the way, is validated by gravitational phenomena, pulsars, etc.). This drift would amount to some minutes over decades, but UTC was created in the early 1970s as a bridge between the two. At arbitrary dates (but always Dec. 31 or June 30), the time-keeping authorities announce a leap second, positive or negative, that lops off the accumulated difference before it reaches a substantial portion of one second. Since this is done "pre-emptively", the correction to UTC is more than is actually necessary on the date in question so eventually, the difference is whittled down to zero. That's what will happen around November 23, 2013 which is about fifteen months after the last leap second was added, if I remember correctly. On that date, the time defined by the Earth's orientation (UT1) will match the time defined as UTC (and remember, UTC is a compromise between TAI and Earth orientation). After that date the value will increase in magnitude until eventually another leap second is inserted into the calendar. There have been proposals to drop leap seconds and allow this difference to accumulate indefinitely. So far those proposals have been stalled in various committees.

    By the way, back to those x, y values, they are given in arcseconds. Since an arcsecond of latitude is very nearly 100 feet, you can read those columns of data as the offset of the pole in feet by looking at the first two digits after the decimal point. So on that same date, Nov. 23, 2013, when DUT goes to zero, the offset of the pole will be 8 feet in x and 24 feet in y or a net offset of just about 25 feet.

    A great topic for New Year's Eve, I must say. Happy New Year, everyone!

    -FER

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