2009 had to wait
Published on Jan 1, 2009 at 1:31 pm.
4 Comments.
Filed under calendars, time.
It is now the year 2009, as reckoned by the calendar used throughout most of the world. Last night, at midnight, many people were ringing in the new year according to whatever customs they dealt. Byrd and I rang in the year together with just the two of ourselves (and a cat). But, in years past, I’ve been out with friends when the new year came. It just about always works the same way. As midnight approaches, people count down the seconds to the new year. That would have happened last night, too. But, I expect that practically everyone missed the new year by a second. That’s because December 31, 2008, lasted, not for 24 hours, but for 24 hours 1 second. The last minute of the last hour of the last day of the year lasted for 61 seconds, not 6o seconds, as minutes normally do. I doubt that many people thought to put the extra second in, and most probably didn’t even know to do that in the first place.
So, what’s that all about? The problem is in how we keep time. From ancient times, noon was the time when the Sun was highest in the sky, on the meridian (the line running across the sky from north to south passing through the zenith, the point directly overhead). Ante Meridian (AM) is before noon, and Post Meridian (PM) is after noon. The day is then divided into 24 hours. In the oldest days, the day was 12 hours and the night was 12 hours. But, as the seasons change the length of day and night changed, so the daylight hours had to be a different length than the nighttime hours. That is a bit awkward, so eventually the entire day was divided into 24 equal hours. Each hour is divided into 60 minutes, and each minute into 60 seconds. That makes the day 86400 seconds long (December 31, 2008, was 86401 seconds long.).
The problem, though, is that Earth orbits the Sun in an elliptical orbit. It speeds up and slows down. A day, as we normally define it (from noon to noon) is a combination of the motion of the Earth around the Sun and its rotation. That means that the length of the day must change. So, we use the average length. But, the rotation of the Earth is not even constant. So, it is no surprise that a more precise method of keeping time was developed. A couple of years ago, I wrote about the way that we define time today. You may want to read that to get more details if you’d like. We can keep track of time, and key an internationally defined time standard called Universal Time to these exact measurements. But, such an exact way of defining time gives rise to problems.
You see, the rotational rate of Earth is not exact. The gravity of the Moon and planets tug on Earth. This tug exerts a torque on our planet, slowly changing its rotational rate. Also, the Earth is dynamic. Magma moves around on the inside. Tectonic plates move around. Earthquakes, landslides, and slippage along faults shift large masses of material. This changes Earth’s moment of inertia (rotational moment). In my introductory physics class, we learn that the moment of inertia times the rotational rate is the angular momentum of a rotating body. We also learn that angular momentum is conserved. So, if the moment of inertia changes, so does the rotational rate. All of these effects shift when noon occurs. Universal Time is adjusted to try to keep noon as close as possible to the local mean noon at the prime meridian (that’s the average noon time, not the actual solar noon). But, clocks all run at a constant rate. These clocks are keyed to something very close to Universal Time, called Coordinated Universal Time. To differentiate the two, we use UT1 to designate Universal Time and UTC to designate Coordinated Universal Time. Clocks, computers, official time agencies, and the like are keyed to UTC. But, due to the shifts in Earth’s rotational rate, and a gradual slowing of Earth’s rotation due to gravitational interactions, a slight discrepancy arises between UT1 and UTC. As long as that difference is less than about a second, then we just live with it. Almost no one needs to worry about such a tiny shift. But, eventually the error reaches close to one second. When that happens, the two times are synced back closer to one another by adding (or if needed subtracting) a second from UTC. There are several dates within the year where that can happen, but the most common seems to be the last day of the year. When a second needs to be added (generally called a “leap second”, though it is added for different reasons than the leap day of leap years), it is added to the last minute of the day.
That is what happened this past year. There was a need to re-sync UT1 and UTC. So, 11:59 PM on December 31, 2008, lasted for 61 seconds. The day was 86401 seconds long. And, everyone who celebrated 2009 occurring one second after 11:59:59 PM was a second early. Of course, being a second off really didn’t hurt anything. But, it sure gave me something to write about for the first posting of 2009!
And, this has nothing to do with the Zune problems that have been plaguing owners of some Zune devices. That was due to sloppy programming by Microsoft (imagine that!).
-Astroprof
Seeking Solace on January 1, 2009 at 2:06 pm: 1
Happy New Year!
Scott on January 1, 2009 at 4:36 pm: 2
You forgot to mention the complication of time zones. The extra second happened at the end of 2008 UTC, which means that it happened just before 7pm Dec 31 2008 for me in the Eastern Standard Time zone. We celebrated the leap second with our young children by leaping twice at the appropriate time.
Presumably places like TV stations in the US adjust their clocks sometime between UTC midnight and local midnight so that anyone who uses those clocks had an extra second in their 2008.
Astroprof on January 1, 2009 at 5:02 pm: 3
Yes, I forgot to mention that, Scott. Thanks for mentioning it!
Sili on January 2, 2009 at 4:33 pm: 4
I don’t know if people actually took it into account, but it was all over the news here in Denmark.
And the interview from Greenwich they used appeared to be from the BBC.