Leapin’ Leap Seconds!

Seth Jarvis

Astronomy can sometimes really mess things up.

Doubt me? 

OK, how many seconds in a day?

Do the math: 24 hours in a day x 60 minutes in an hour x 60 seconds in a minute = 86,400 seconds per day.


Except on June 30th, which will have 86,401 seconds.

Specifically, the minute between 5:59 PM Mountain Daylight Time and 6:00 PM Mountain Daylight Time will have 61 seconds.

What’s going on?

Blame it on that old devil Moon (not the great old Sinatra song by that name, I mean the actual Moon orbiting Earth).

OK, it’s not 100% the Moon’s fault, but it’s mostly the Moon’s fault.  You see, Earth’s gravity holds the Moon tightly in orbit around our planet, but the Moon is, after all, pretty big. We pull on it, it pulls back on us. The Moon is (mostly) responsible for ocean tides. That requires a lot of energy.

All that sloshing of the oceans, plus the flexing that those tides exert on Earth’s crust, causes Earth to lose angular momentum.  That’s geek-speak for “makes Earth’s rotation slow down a little.”

The length of a day changes for lots of reasons, but in general, it's slowly slowing down.

The tidal tug-of-war between Earth and Moon has been going on for billions of years.  A long time ago Earth spun a lot faster, and the Moon was a lot closer.  Just 600 million years ago a day on Earth was less than 22 hours long and there were 400 days in a year.

Today, we navigate our airplanes and run our banking system on hyper-accurate clocks.  Thousandths of a second are important.  But if we measure our days, hours, minutes and seconds according to the length a day based on Earth’s rotation, we’ll find that as Earth slows down by itsy-bitsy amounts due to tidal friction from astronomical and geological influences that our hyper-accurate clocks will get badly out of snyc with what astronomers observe.

The solution?  Leap-seconds.  Just as we have to modify our calendars a tiny bit every now and then with a 29th day in February, a “leap-day,” we similarly modify our clocks occasionally by throwing in a 61-second minute.  A “leap-second.”

The last time a leap-second was added to our clocks was December 31, 2008. They usually are added once every few years.  A  total of 25 leap-seconds have been slipped into our clocks since the precision of atomic clocks became high enough to require the first leap second in 1972.

I don’t know if this will work or not, but I’ve got a GPS unit that shows me the time, including seconds.  I’ll be watching it on June 30 at 5:59 PM MDT to see if the leap second appears on the display.


It worked!  Here’s a screen capture of the official US Government Time website on my computer. In front of my computer screen I have placed my iPad set to show UTC (Coordinated Universal Time) time as averaged among several Network Time Protocol servers.

The last minute of June 30th was 61 seconds long.

Capturing the 61-second minute of June 30th, 2012.


My GPS just recorded the time like this:

05:59:56, 5:59:57, 5:59:58, 5:59:59, 5:59:59, 6:00:00, 6:00:01, etc.  It counted the 59th second twice.

Our clocks now measure time more accurately than the rate of Earth’s rotation and they sometimes get out of sync with each other.

Since we can’t make an occasional teeny-tiny fine-tuning adjustment to Earth’s rotation, we have to make occasional teeny-tiny fine-tuning adjustments to our clocks.  Hence, the necessity of the “leap-second.”

How cool is that?


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