2008 was longer than usual. Really

Welcoming in the new year for 2009 was just a little more complicated than usual.  And actually, the “usual” is already a good deal more complicated than most of us realize as we toast the new year at the stroke of midnight.

Because there is more than one way to measure when the old year ends and a new one begins.

For starters, different cultures start the new year at different times.  The Chinese new year begins with the Year of the Ox on January 26.  The Jewish new year doesn’t begin until September 18, and the Muslim new year – which is 1430 – began December 28.

The date most of us consider the standard as the new year is based on the Gregorian calendar established in 1582 which is based on the time it takes the earth to orbit the sun.  Sort of…

Most of us know that the extra day in leap year every four years is to adjust for the fact that it takes just a little longer for the earth to circumnavigate the sun than a year.  Actually, the leap year doesn’t solve the entire problem because it takes 365.2422 days for the earth to circle the sun, and eventually, leap years would have our time getting ahead of solar time.   So leap years are omitted at the turn of the century of the year is not divisible by 400.  Which is why there wasn’t a leap year is 1900, but there was in the year 2000.

It gets worse.

It doesn’t take the earth exactly 24 hours – which is what we call a day -to spin on its axis.  It actually takes 23 hours 56 minutes and 4 seconds.  But because the earth is moving around the sun at the same time as it is spinning, it does take 24 hours for the sun to get back to the same position relative to earth.  

BUT…

The speed at which the earth is spinning is slowing down ever so slightly.  So every once in a while, an extra leap second has to be added to the end of the year to keep our “human” time in sync with the earth and sun.

This is not the only way to do it.  The Islamic calendar is based on the cycles of the moon, and the Jewish and Chinese calendars are based on both the movement of the moon around earth, and the earth around the sun.

And then there’s the atomic clock…

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About Terry Sissons

Terry Sissons is the author of The Big Bang to Now: All of Time in Six Chunks, and this blog is a dialogue about the universe and what’s happened in the last 13 billions years.
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