Its About Time

Now don’t get too silly. The SI definitional unit of time is the second. That forms the base unit. There is nothing that disallows commonly used precise groupings of seconds into convienient derived units like minutes and hours.
If we strictly stuck to the metric system we may be talking about more suitable groupings of seconds like hectosecond and kilosecond. No thanks.


‘It’ has already been addressed :joy:


Something to make us smile?

Why don’t we have metric time?

Because if a minute has ten seconds, an hour has ten minutes, a day has ten hours, a week has ten days, and a month has ten weeks, then we will have a year containing only three months and be left with a New Year Holiday of 65 and a quarter days. Come to think of it - why not?
By Bob Ballinger, Pymble.


We do. Seconds, hours, days etc are metric. They are not decimal however, not all metric or SI units are decimal.


Technically not SI, however yes.The second is the metric unit of time but a number of measures outside the SI system are formally accepted for use with it.

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Time is complicated. Ultimately hours, minutes and seconds are derived from the ‘day’.

Hours from a day using the quadravigesimal number system, and minutes and seconds from the hour using the sexagesimal number system. Decimal, metric, is only used in dividing seconds into smaller parts. Or lawyers who like to divide their billable time into blocks of six minutes in an hour.

But there are four definitions of a day.

ATI, International Atomic Time, is exactly 86,400 seconds per day.

UT1 is the Universal Time derived astronomically from watching the Sun and stars. It varies with season, climate, astronomical cycles, etc.

UTC, Coordinated Universal Time, is the time we use for our clocks and calendars. This time is what most people think of defining a day.

On most days, there are exactly 86,400 seconds in a UTC day.

But for astronomers, there is a sidereal day which is around 4 minutes shorter than a solar day as in UT1.

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And then both the mean solar day and the sidereal day are changing both systematically and non-systematically e.g. No, It’s Not Just You: The Days Are Actually Getting Longer After reading that, it won’t be just the earth that is spinning. :slight_smile:

So a day is a bad unit for anything rigorous (whereas minutes and hours are at least rigorously defined).

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A day is a day is a day. Assumes one is not travelling the globe. In which instance knowing where one is might seem more important. Also where a day is measured from midnight to midnight and one is travelling one might have experienced a shorter or greater number of hours in the day, lost or gained one?

Choose your system carefully, considering modern GPS devices offer a number of choices.

And that is why the base unit of time is now the second. The others are then defined from bottom up using atomic clocks at sea level on earth calculating a second. Note that the atomic clocks on GPS satellites calculate a second differently due to General Relativity effects of gravity and velocity and have to compensate for that.

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Mostly Degrees, minutes and decimal minutes used in my experience. Don’t tend to use seconds when sailing and navigating.
A degree in nautical and air navigation is 60 nautical miles, and a minute is 1 nautical mile.
A second is one sixtieth of a NM and only about 31 metres. Not that useful.

Also GPS devices tend to automatically switch over to metres once a distance gets below 0.1 of a minute.