What Is a Leap Second? | National Geographic

as you probably know we measure the
length of a day by how long it takes for
the earth to rotate once on its axis
which is about 24 hours
but sometimes the days get a little
longer because we add what’s called a
leap second but what is a leap second
and why do we need it
this is leap second 101 a leap second
helps make sure that the clocks we use
to measure time and the actual rotation
of the earth stay as closely in sync as
possible we do this because throughout
the year the speed of the Earth’s
rotation changes ever so slightly it can
be sped up or slowed down by natural
geological events like earthquakes and
volcanoes so when our measurement of
time is thrown off by Earth’s rotation
slowing down we have to account for it
somehow and adding a leap second is one
way to do it when the International
earth rotation and reference system
service yes this is a real thing
predicts that the difference in Earth’s
rotation and our clocks will be off by
0.9 seconds a leap second is added to
the day the leap second is announced
almost six months in advance so that
everyone has time to prepare by
adjusting our clocks like this every few
years we are able to more closely match
the world standard measurement of time
with the natural changes of the Earth’s
rotation
this slight change can actually be a
really big deal if the computer systems
that we all rely on aren’t properly
programmed to handle it when a leap
second was added in 2012 it caused some
webpages to crash a few GPS receivers
failed and airline passengers were left
stranded because entire airline booking
systems failed since 1971 more than two
dozen leap seconds have been added but
that doesn’t mean the days are now
longer it just means that the day on
which the leap second was added was one
second longer many of us might miss it
because it’s so quick but this and all
the other small shifts from leap seconds
will keep us on track and on time on a
planetary level