Comment by brudgers | Hacker News Reader

Comment by brudgers | Hacker News Reader

Personally, a leap hour seems about the right level of granularity, but I suppose an approximately once per century leap minute would be ok. But leap seconds are a waste of time.

If ignored since their introduction in 1972, official time would be out of sync with the Earth's rotation by 26 seconds. There's been a lot of running around and gnashing of teeth to correct 1.845668179868485e-7 error that isn't even an error most of the time. The 26 seconds of error fleshes out as 7.389 meters at the equator when using celestial navigation and for anything that accurate we use radio signals not a sextant.

The Earth's rotation and revolution are useful abstractions at a certain level of granularity. But the reason we have atomic clocks is because those abstractions break down when we need precise measurements of reality. Reality is real because the rotation and revolution of the Earth are not cosmological constants. Leap seconds are an expression of the superstition that humans control the arrow of time: as if one thing happening after another would cease to be the case without them.

We use leap seconds, instead of minutes of hours exactly because mostly everybody can ignore them.

Unless you are doing something very interesting, that relies on some absolute standardized time (ie. not relative measurements) and requires great precision, you'll throw away clocks faster than they will accumulate errors. That's a feature, not a bug.

An error of 7 meters would probably mean the FAA would refuse to certify GPS for aircraft use. Even now it's only usable in many circumstances with ground based corrections (eg WAAS or GBAS). A minute of error would probably make it unusable for enroute navigation.

I'm sure someone will point it out, but GPS doesn't use leap seconds.

"Adding leap seconds is a waste of time" I see what you did there. And agree!

Interesting. I didn't know that. I think half of the point remains (what sounds like a small error can be critical) but I guess GPS isn't a great example, then, as safety critical systems tend to be smarter by design anyway.

The safety critical systems don't reset their clocks because safety critical systems maintain independence in order to maintain safety. Safety critical systems are a place where the abstraction of a Terran Year as a cosmological constant breaks down.

GPS is indeed happily unaware of leap seconds, it started counting seconds on January 6th, 1980 (UTC) {1 week is 604800s, weeks count 0..1023, there already was one rollover}.

http://imgur.com/a/uq6D0

A few minutes ago, when the screenshot was taken, this was second #501074 in week #1928.

As a courtesy to users, GPS transmits the current number of leap-seconds inserted in UTC (currently: 17) and the fact that a leap-second will be inserted the next occasion (end of this year: "LEAP PENDING!"). That's of course very helpful when you are using GPS disciplined clocks for timekeeping.

The error would be in celestial navigation not GPS. The frame of reference for a satellite orbiting the earth is the same as the edge of the landing strip.

Now, I'm not a proponent of leap seconds. But I'm a bit curious to where the 7.389 m comes from?

The earth rotates about 12 km along the equator in 26 s.

Is the error of ~7 m for determining your position north-south wise? East-west it should be about 12 km then I think.

I exaggerated, latitude can be determined without a clock from celestial objects. Longitude requires both a synchronized clock and an equation of time (because noon GMT varies). [1] The equation of time is published. Rather than adjusting all the clocks in the world, the equations could be modified to account for additional variation.

https://en.wikipedia.org/wiki/Equation_of_time

I'm not sure that this answers the question. Where did the ~7m answer come from, if the Earth rotates 12km in 26s?

I over estimated the effect of an absolute clock shift upon celestial navigation. The actual number is 0 meters because the absolute clock shift affects both the reference clock and the referencing clock equally when determining longitude.

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