On Tue, 11 Oct 2016 12:59:47 -0500, Paul Gilmartin <[email protected]> wrote: . > >It's not clear why the convention is to add CVTLDTO to ETOD but subtract >CVTLSO... > . Why do we have to add CVTLDTO (which could be negative if East of GMT), but subtract CVTLSO? Because CVTLSO represents by how much TAI is greater than Earth time, and ETS probably feeds you TAI values. Atomic clock ticks at an absolutely constant rate of some 9192631770 oscillations per second (Cesium-133 atom oscillating between two energy ground levels), while Earth sidereal day gets slower(*) by 1.4 milliseconds a day per century. This lagging behind accumulates over time and mounts to almost a complete second over approx. 500 days. As earth rotates and midnight is about to be reached, observatories might notice that the correct position of a fixed point on earth, in relation to very distant stars, is still far away due to the slowing of earth, and midnight must be "postponed" a bit to let earth catch up with distant stars. When this is about to happen (once in approximately 500 days), we have to "add" one second to earth clocks, to "postpone" midnight. Postponement of midnight is attained by stepping earth clock from 23:59:59 through 23:59:60, to 00:00:00, rather than the normal sequence 23:59:59 to 00:00:00. Internally, CVTLSO is increased by one and the accumulated sum is subtracted from the TAI fed from ETS. As I've said, TAI is greater than Earth time by the accumulated leap seconds (a negative leap second is possible in principle, but has not been observed so far). Below you can find animation of two clocks. The upper is TAI (the atomic clock). The lower is earth clock. The difference at this moment is 36 (relative to 1957, but only 26 relative to 1972, which is an alternative epoch on some systems). https://www.timeanddate.com/time/leap-seconds-background.html (*) Why does earth slow down? This slowing down is caused because earth is not a rigid body. Rather, it has liquid core, has tectonic plates floating and drifting astray, or being shifted during earthquakes, is covered by seas and oceans that undergo cyclic high and low tides, which convey some of earth's angular momentum to the moon, which in turn gets further away from earth at a rate of 1.5 inch a year.
---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
