On June 30th we had
a leap second added to the year. A leap second? That's right,
a second. We're all familiar with the leap day we add to the year
every 4 years (next year will be one). That day is added to keep our
calendars aligned with the stars. When someone asks how many days in
a year, we automatically say 365. And that's pretty close.
But it actually
takes us about 365.25 days. That extra quarter of a day gets added
together every four years into a leap day. (Actually it's not really
a quarter of a day so the rules are actually more complicated but
most of us won't see years that don't follow this basic rule.) And
why do we do this? To keep the seasons at about the same point in the
calendar. Otherwise given enough time the seasons would get out of
line and we'd have spring in winter.
Well, a leap second
is similar. In terms of seconds, there should be 86,400 seconds in a
day. That is the amount of time it takes the Earth to rotate once on
its axis, or one day. But you see, that isn't true. Right now the
Earth takes 86,400.002 seconds to rotate once. Or an extra two
one-thousandths of a second over the “proper” number. And again,
given time that extra fraction of a second can build up and get
astronomical events out of sync with when they should be. So we add
an extra second every now and then.
That's right,
unlike leap days that occur every four years (sort of) we never
really know when a leap second will be added. And that's because
there are many things that affect how fast the Earth is rotating.
Most of the change
is due to the Moon. As you know the Moon is a cause of tides in the
ocean. Well those same tidal forces drag on the Earth and is actually
slowing down how fast the Earth rotates. But this value isn't
constant. In the past the Moon was closer to the Earth and the rate
was different. Even now the Moon is still moving away from the Earth,
so the rate changes.
But there are other
things that affect the rate the Earth spins. And some of those are
geological. Movement of the tectonic plates affects how fast the
Earth rotates. And earthquakes can make big changes all at once. In
2004, the earthquake that generated the Indonesian tsunami shortened
the day by over two and a half microseconds.
Since there are
random events, like earthquakes, that affect the speed at which the
world turns, we never know when a leap second will be added. They
aren't that common since there have been only 26 between 1972 (when
they started) and now. I hope you made the best of yours.
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