Why must a leap second be added to the year now and then in order to keep Earth's rotation and Coordinated Universal Time in synch?

Answer 1

Because of Calendar Year DifferencesThe current calendar that we use has one extra day (February 29) in 97 years out of every cycle of 400 years, giving 146097 days in every 400 years, an average of 365.2425 days per year. Astronomical measurements show that the day is not exactly that long and occasionally extra bits are added to keep track of the Earth's real behavior. A Highly Detailed AnswerVery few people are aware that there even are leap seconds, let alone understand why they are necessary. Leap days are widely understood to be necessary in order to keep the calendar from drifting, causing a kind of seasonal "precession". It's easy enough to follow how and why they are used, and to see how elegant and accurate our simple "leap day" (in a leap year) solution is. The leap second is another matter altogether.

The earth's rotation on its axis is generally well understood, and it is happening at a reasonably constant rate. We do know that the rotation is slowing very gradually, and this is independent of our orbital velocity. We also know that various kinds of events can from time to time influence the earth's rate of rotation. The huge tsunami of a few years ago actually changed our rotational speed slightly.

"Coordinated Universal Time" as we currently know and use it was determined from measurements taken quite a number of decades ago [research the history of Universal Time].

When the "SI second" used in Coordinated Universal Time was set at its current value, the total absolute length of time of exactly one reference year was divided by the number of "solar days" in the year, taking into account the fractional part of a day that the first responder mentions. That fractional part was easy enough to determine, since the angular positions of the sun relative to the earth could be compared for the moment the reference year started, and the moment the reference year ended. This gives us, in effect, the "average solar day" on earth. All this data gave us our current "second" of Coordinated Universal Time.

Why go to all that trouble? Why not just measure a "solar day" from noon to noon? Because solar days are slightly variable in length. The explanation would take too long to include here. The quick and dirty answer: The earth's orbit around the sun is elliptical, but its rotational rate is reasonably constant.

So, now we have Coordinated Universal Time well defined and in operation. However, the earth's rotation continues to decelerate very very slowly. You may be able to see that this would cause a kind of "precession" of the clock, relative to earth's rotation! Left unchecked, Coordinated Universal Time as originally defined would cause clock time to literally drift around the diurnal cycle. This "out-of-synch" problem would wreak havoc in our daily routines long before it would have any appreciable affect on our reckoning of the seasons.

So, we have a need to add a second to the coordinated universal time clock, to keep everything "in synch". This happens every few years, although measurements are checked often, and adjustments to the clock could happen as often as twice a year if necessary. Seconds can also be subtracted if it ever appears necessary. Subtractions have not yet occurred. These adjustments are made "transparent" to the general population, because the change is too slight to be of any concern to the average person. Everyone who needs to know is aware of the change so that communications and other systems continue to run flawlessly and in mutual agreement.

Here is a mystery about leap seconds. You may conclude (and it seems intuitively reasonable) that the occasional addition of leap seconds is a direct reflection of the slowing down of the earth-- that the earth has just slowed its rotational rate by another second per year, making this leap second necessary, and the next leap second will be necessary because of an additional slowing down of one second per year. People have allowed themselves to believe this, and to make calculations about the length of days in pre-historic times based on this theorized rate of deceleration.

But this is not so; the truth is what is called "counter-intuitive". Imagine that you have a clock at home that runs at a very constant rate (it doesn't speed up or slow down). But when compared to the Coordinated Universal Time Clock, which runs not only at a constant rate, but with perfect accuracy, you discover that your clock loses exactly one second per year.

Easy enough, you simply add one second (a "leap second") to the Coordinated Universal Time Clock during the last minute of the day on Dec 31, and your clock is now "in synch" with the Coordinated Universal Time Clock. Because your clock runs at a constant rate, it will again lose one second by the end of the year, and you'll need to add another second to the end of the last day of the year. The occasional addition of seconds to the other clock does not, by itself, indicate that your clock is slowing down. Your clock, and the Coordinated Universal Time Clock, both run at constant rates, but at slightly different rates.

The same is essentially true for earth's rotational period and for Coordinated Universal Time. While it is true that earth is slowing in rotational speed very, very slowly, it is NOT slowing down by a rate that can be calculated from regular additions of leap seconds.

A Shorter Answer, again because of Atomic Clock Accuracy:Our main measurement of time is now Coordinated Universal Time (known for language reasons as UTC, not CUT) based on very accurate "atomic clocks".

For these clocks, a "second" is defined in a way that no longer depends on the (slightly variable) rotation of the Earth. Unfortunately, the "atomic second "

isn't quite the same as the "second" based on the Earth's rotation.

The "SI second", used by atomic clocks, was defined in a way that fixed it to the Earth's rotation period of about a century ago.

That's the main reason that a second (called a "leap second") has to be added, from time to time, to keep the two time measurements synchronised.

Our 24 hour day was based on the Earth's rotation relative to the Sun. It's called the "mean solar day" and has 86,400 (24 x 60 x 60) "mean solar seconds" in it.

The day based on atomic clocks also has 86,400 seconds, but these "atomic seconds" are very slightly shorter than mean solar seconds.

The advantage of atomic clock time is that it doesn't depend on the Earth's rotation.

The disadvantage is we have two days that must get out of "synch", requiring leap seconds.

The explanation of the Earth's irregular rotation is complicated. We know the overall trend is that the rotation is slowing and that's because of the effect of the Moon's gravity.

So, leap seconds are added because of the slowing of Earth's rotation.

The reason that so many are required is the discrepancy that has built up between the day length based on the "SI second" and the day length based on the Earth's rotation.

This slowing down results in the mean solar day length increasing by about

0.002 seconds per Century.

That's not much, but the effect is "cumulative", year after year.

So, this slow down can result in the need to add leap seconds regularly, even though the actual day length only increases by about 0.002 seconds in a

hundred years!

See "Sources and related links" below for an authoritative source.

Note:

Leap seconds have nothing to do with leap days (February 29th). Leap days are to keep the calendar in synch with the Earth's orbital period.

From a Quantum Mechanics Perspective:Quantum Mechanics infers that there is nothing in the universe that is perfect. The laws of probability, when taken down to an infinitely small level, cannot establish the exact location of any particle in the universe nor its exact motion. And so, in order for us to measure the motion of any object, celestial or terrestrial, there is always going to be a mandatory 'rounding off' to the nearest decimal point. If we add up all the seconds in a year, eventually we will need to add, or even subtract one, in order for our measurement system to accurately label the position of our earth as it revolves around the sun, year after year, both on the large scale of the cosmos, and the very small scale of our seconds.

And so, even the most accurate atomic clocks with our calendar measurement system, cannot be as exact as the quantum clock of the universe, and that itself does not even have a "perfect" unit of measurement.

A note on time scales. Universal Coordinated Time (UTC) is the most widely used time scale, but it is not the only one. There is also International Atomic Time (TAI) and GPS (Global Positioning System) time. Both are distinguished from UTC in that they do NOT have leap seconds.

This is important to GPS because a leap second would cause a glitch that would require all the receivers to resynchronize with the satellites, and that could be a problem for a plane approaching an airport! In 1980 GPS time and UTC were the same, but because of the leap seconds added to UTC, GPS time is now 16 seconds ahead of UTC. The GPS satellites broadcast this difference so the receivers can make the correction and automatically display UTC to their users. TAI was 10 seconds ahead of UTC in 1972, so it is now 35 seconds ahead of UTC. TAI is therefore always 19 seconds ahead of GPS time.

There are proposals to stop adding leap seconds to UTC, but one of the counterarguments is that anyone needing a time scale without leap seconds can just use TAI or GPS time.