[LEAPSECS] drawing the lines

Tom Van Baak tvb at LeapSecond.com
Mon Apr 29 16:25:57 EDT 2013

> To quote a friend: "If UTC is time, and if UT1 is angle, then what is (UT1-UTC)?

> How can one subtract angle from time and have a meaningful result, without both

> being instantiations of the same concept?"

No problem. If UT1 is angle, just divide by 2pi to get units of time. We do the same with any pendulum, quartz, or atomic clock. From planets to tuning forks, timekeeping is simply the sum of integer cycles plus fraction of a cycle, with optional units scaling.

I reject the notion that UT1 is "angle" and UTC is "time". Both are angle, both are time. There's angle of revolution, angle of rotation, angle of swing, angle of sinewave. Timekeeping is the counting of cycles (phase). Each clock has a different cycle rate (frequency).

The key point is that if you want to *compare* two clocks it's convenient to compare their "time", not their raw phase angle or cycle counts. To compare a 9.192 billion cycle per second cesium resonance with a 10 million cycle per second quartz oscillator with a one cycle per 86400 second day earth rotation you scale angle to a common time unit (seconds).

> Leap seconds are a means to an end, which is to root civil timekeeping in mean

> solar time - that is, in the synodic day. By all means lets discuss alternate ways

> to implement this, but simply declaring that Universal Time no longer exists as a

> concept is a non-starter.

"Leap seconds are a means to an end" -- I agree.

"Leap seconds were to root civil timekeeping in mean solar time" -- I don't think so.

I've been reading old UTC papers again and my impression is that there was far more concern for *navigation* than for civil timekeeping or astronomy. The 100 ms time steps and the 0.7 second UTC tolerance and the code or voice radio signals were there for mobile navigators (using tube or transistor short wave radios and sextants), not fixed astronomical observatories with postal, telex, or telephone service, or for local town clocks.

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