Comment by brownbat | Hacker News Reader

Comment by brownbat | Hacker News Reader

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The rotation of the Earth is a terrible timekeeper. We used to have 6 hour days. Now it wobbles. Major earthquakes throw it off. On the other hand, ignoring this leads to weird results. In Rome, the priests used to control the Calendar, and they had to decide how many leap days to put into the cycle. Senators served through the end of the year, and they had influence over the priests. By the time Julius Caesar implemented a new calendar, this sort of political drift in the calendar had January in the middle of summer.

I guess people don't like mid-summer January because it makes historical comparisons more difficult. Not impossible, just awkward.

It's a horrible messy dilemma. Leap seconds don't seem as dramatic as the Roman intercalary days, but there's the same battle between synchronicity and simplicity (where simplicity = something short of insane perturbations).

Maybe none of this matters for radio broadcast time signals. Maybe you just want a fixed counter everyone's synchronizing to, and people should convert to the Earth's messy system at the last possible moment.

Fun story about the awesome 13 month Calendar that we could have adopted (except for Hitler): http://99percentinvisible.org/episode/the-calendar/

http://www.livescience.com/38083-earth-core-day-length-patte...

http://www.scientificamerican.com/article/earth-rotation-sum...

http://news.nationalgeographic.com/news/2011/03/110316-japan...

https://www.uwgb.edu/dutchs/PLATETEC/RotationQk2004.HTM

https://en.wikipedia.org/wiki/Julian_calendar#Motivation

Keep in mind that leap days serve a different purpose than leap seconds. Leap days exist because the number of rotations of the earth per one revolution around the sun is not a whole number. Leap seconds exist because the duration of one rotation is not constant.

wouldn't dropping leap seconds result in noon during the night after a while?

I have no idea how often leap seconds occur, but if we were to say that there should be one leap second every year and we instead ignored them, it would take 43,200 years before that happened.

It would take 60 years of skipping yearly leap seconds before we saw even a 1 minute offset.

Wikipedia says that we've had 26 leap seconds since 1972, or about 0.6 seconds per year.

Some leap seconds are negative, so the total change since 1972 is less than 26 seconds.

TAI (weighted average of atomic clocks, correcting for relativistic effects due to altitude) is the basis for UTC, and is currently 35 seconds ahead of UTC.

Prior to 1972, UTC and TAI used different definitions of a second (UTC used a definition based on positions of celestial bodies while TAI used a definition based on the hyperfine transition of cesium, and the two were periodically manually "stepped" back into sync). As of 1972 they use the same definition of the second (TAI's definition, which is also the SI definition), and a final "step" was applied to cause the difference in the two times to be an integer number of TAI seconds. That created an offset of 10 seconds, which has since grown with the addition of 25 (and soon to be 26) positive and zero negative leap seconds to UTC.

The rotation of the Earth is slowing down, and this slowing down is expected to accelerate in the future. So in several thousand years, we will need multiple leap seconds per day to keep up. Whether we should take this into account now is the question, though.

> [...] it would take 43,200 years before that happened.

That's how long for noon to end up at midnight. Noon would occur at night after only around 22000 years (I'm being fuzzy because people may not all agree on what night means).

Yes, after thousands of years. What percentage of humans will still be living on Earth by the time that happens?

I expect less than 100%

bold guess, since it's already < 100%

Where else are they living? People visit the ISS, but they don't really live there.

don't live there? they receive mail, store clothes, and coordinate bathroom time...

buddy, you're only visiting earth for a precious handful of years

They're on the Earth, but they're not living.

That Wikipedia article on the Julian calendar is both fascinating and a great example of Wikipedia at its best in terms of attention to detail.

Do earthquakes also change how long it takes for the earth to rotate around the sun? Because it seems like the earth's orbit should be more or less constant (ignoring relativistic effects).

Earth's orbit is probably variable, but not because of earthquakes; rather, when the gravitational pulls of other Solar System bodies are consisdered, there are forces other than the Sun's gravity acting on the Earth. The most notable of these is the moon, but in terms of tangible orbital disruption, other planets (particularly Jupiter, which is massive enough to pull the Sun-Jupiter barycenter outside the sun, meaning that Jupiter orbits the sun much in the same way that Charon orbits Pluto) can and do disturb the orbits of their fellow planets, often pulling them whichever way.

Earthquakes are also not the only means by which the Earth's rotation can be affected. The moon's gravity creates all sorts of tidal forces on Earth; such forces don't just create tidal waves, but can influence everything from Earth's rotation to continental drift.

Of course, hypothetically, a sufficiently-violent volcanic eruption could end up working like a giant rocket engine and thus push the Earth in some direction or other (in much the same way as normal rocket engines do; in this case, magma/lava/steam would be the propellant(s)). That's the only way I can think of off the top of my head for earthquakes to affect Earth's orbit, and should that happen, I highly doubt UTC being synchronized with Earth would be particularly relevant at that point, since something of that magnitude would probably significantly damage (if not destroy) Earth, and would almost certainly exterminate anything unfortunate enough to still be living on the planet.

No it does not. Earthquake changes the internal structure of the earth, which offset its rotational rate (as angular momentum must be conserved and the moment of inertia changed).

It does not change the angular momentum of any system, nor does it change the moment of inertial of the sun-earth system.

I don't believe so - earthquakes should only affect the number of days (rotations) that occur during an orbit.

> the awesome 13 month Calendar that we could have adopted

That calendar would have never worked. You would have to keep changing when the weekend is each year as the real Saturday/Sunday would shift by a day each year relative to the calendar. Some people would not care others would, and it would be a mess.

And don't just flippantly suggest "so don't shift", the 7 day pattern of weeks has been kept by humanity for thousands of years by cultures all over the world. It's not something so easily dropped.

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