The second is already the unit of time in the "Systeme
International" set of units, sometimes called "MKS" -- for meters,
kilograms, and seconds. The base-60 stuff for minutes and hours (and
the 24-hour stuff) dates way back, perhaps to the Babylonians or even
the Sumerians. Seconds are divided up by powers of ten for smaller time
measurements, and typically only every third power of ten is used.
Examples: one millisecond is 10^-3 seconds, and one microsecond is
10^-6 seconds, one nanosecond is 10^-9 seconds, one picosecond is
10^-12 seconds and one femtosecond is 10^-15 seconds (a unit used most
often by laser jocks who make pulses of light which only are a couple
of wavelengths long).
For longer times, physicists normally just use seconds and give a
big number. A day, a week, and a fortnight are unambiguous, if not
particularly metric, time units that are not used in physics very much
except for contact with people and their schedules. A month is an
ambiguous unit of time -- some are short and others are long and so is
not used at all. Even a year is ambiguous -- is it a siderial year, or
the funny calendar year which also gets longer on almost every fourth
year? We just use seconds to be fully unambiguous in physics
calculations.
Since time is useful in arranging our schedules, the number of
seconds in a day has to be given a special name. This won't be a power
of ten, unfortunately, and the only other option is to redefine what a
second is, and it's too late for that.
Tom
(published on 10/22/2007)
