DEC 31 Would Be 1 Second Longer

On Dec. 31 this year, your day will be just a second longer.





Like the more well-known time adjustment, the leap year, a "leap
second" is tacked on to clocks every so often to keep them correct.



Earth's trip around the sun — our year with all its seasons — is about
365.2422 days long, which we round to 365 to keep things simpler. But
every four years, we add 0.2422 x 4 days (that's about one day) at the
end of the month of February (extending it from 28 to 29 days) to fix
the calendar. Likewise, a "leap second" is added on to our clocks every
so often to keep them in synch with the somewhat unpredictable nature
of our planet's rotation, the roughly 24-hour whirl that brings the sun
into the sky each morning.



Historically, time was based on the mean rotation of the Earth relative
to celestial bodies and the second was defined from this frame of
reference. But the invention of atomic clocks brought about a
definition of a second that is independent of the Earth's rotation and
based on a regular signal emitted by electrons changing energy state
within an atom.