Notes on "Leap Seconds"

Collected by Helen Fryman (

A week or so ago I posted a list of ten "evidences" for a young earth that had been posted on a forum and were responded to by Tim Thompson. I did not get a lot of response here, so I went searching other places. One by one I am trying to collect material to either support the evidence, re-evaluate it, or admit it should be discarded as it is invalid. As many of you know, I feel quite strongly about the need for the honest truth to be available to scientists when they are discussing creation and evolution. The first point that was brought up was regarding leap seconds. Below please find the newsgroup original point as posted, then Tim Thompson's long response, and then responses by Danny Faulkner, Wayne Spencer, and Barry Setterfield. If there is any more that should be added (or deleted) to/from this before it goes up on a web page, I would appreciate knowing. Thank you. The list of points and responses is on Tim's web page at

Helen Fryman

Evidence 1.

Atomic clocks, which have for the last 22 years measured the earth's spin rate to the nearest billionth of a second, have consistently found that the earth is slowing down at a rate of almost one second a year. If the earth were billions of years old, its initial spin rate would have been fantastically rapid-so rapid that major distortion in the shape of the earth would have occurred. a) Arthur Fisher, "The Riddle of the leap Second," Popular Science, Vol. 202, March, 1973, pp. 110-113, 164-166. b) Air Force Cambridge Research Laboratory, Earth Motions and Their Effect on Air Force Systems, November 1975, p. 6. c) Jack Fincher, "And Now, Atomic Clocks," Readers' Digest, Vol. III, November 1977, p. 34.


Response from Tim Thompson:

As explained on the Leap second (1) page of the National Earth Orientation Service (2), the true spin down rate of the earth is 1.5 to 2 milliseconds per day per century. That means that after 100 years, the length of day has systematically increased (on average) 0.0015 to 0.002 seconds. This is also found, for instance, in Kurt Lambecks's book "The Earth's Variable Rotation" (Cambridge University Press, 1980; currently out of print), page 3. This is a long-term secular variation. As Lambeck and numerous others point out, there are variations on the length of day that range from daily to seasonal in scale, so that the true length of day can vary greatly from day to day, over multi-year time scales. The author of this argument has failed to realize that one second as defined by the rotation of the earth is slightly longer than one second as defined by atomic clocks. So the earth-rotation time scale runs about 2 milliseconds per day behind the atomic clock scale (because the two use seconds that are not the same length). The leap second is a convenient device for keeping the two time scales always within 0.9 seconds of each other. It is not a result of the earth slowing down by one second per year.

Text: Leap Seconds

Civil time is occasionally adjusted by one second increments to ensure that the difference between a uniform time scale defined by atomic clocks does not differ from the Earth's rotational time by more than 0.9 seconds. Coordinated Universal Time (UTC), an atomic time, is the basis for our civil time.

In 1956, following several years of work, two astronomers at the U. S. Naval Observatory (USNO) and two astronomers at the National Physical Laboratory (Teddington, England) determined the relationship between the frequency of the Cesium atom (the standard of time) and the rotation of the Earth at a particular epoch. As a result, they defined the second of atomic time as the length of time required for 9 192 631 770 cycles of the Cesium atom at zero magnetic field. The second thus defined was equivalent to the second defined by the fraction 1 / 31 556 925.9747 of the year 1900. The atomic second was set equal, then, to an average second of Earth rotation time near the end of the 19th century.

The National Earth Orientation Service (NEOS) as the Sub-bureau for Rapid Service and Predictions of the International Earth Rotation Service (IERS), located at the U.S. Naval Observatory, monitors the Earth's rotation. Part of its mission involves the determination of a time scale based on the current rate of the rotation of the Earth. UT1 is the non-uniform time based on the Earth's rotation.

The Earth is constantly undergoing a deceleration caused by the braking action of the tides. Through the use of ancient observations of eclipses, it is possible to determine the deceleration of the Earth to be roughly 1.5-2 milliseconds per day per century. This is an effect which causes the Earth's rotational time to slow with respect to the atomic clock time. Since it has been nearly 1 century since the defining epoch (i.e. the ninety year difference between 1990 and 1900), the difference is roughly 2 milliseconds per day. Other factors also affect the Earth, some in unpredictable ways, so that it is necessary to monitor the Earth's rotation continuously.

In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds, a leap second is added to the atomic time to decrease the difference between the two. This leap second can be either positive or negative depending on the Earth's rotation. Since the first leap second in 1972, all leap seconds have been positive (click here for a list of all announced leap seconds). This reflects the general slowing trend of the Earth due to tidal braking.

Confusion sometimes arises over the misconception that the regular insertion of leap seconds every few years indicates that the Earth should stop rotating within a few millennia. The confusion arises because some mistake leap seconds as a measure of the rate at which the Earth is slowing. The one-second increments are, however, indications of the accumulated difference in time between the two systems. As an example, the situation is similar to what would happen if a person owned a watch that lost two seconds per day. If it were set to a perfect clock today, the watch would be found to be slow by two seconds tomorrow. At the end of a month, the watch will be roughly a minute in error (thirty days of the two second error accumulated each day). The person would then find it convenient to reset the watch by one minute to have the correct time again.

This scenario is analogous to that encountered with the leap second. The difference is that instead of setting the clock that is running slow, we choose to set the clock that is keeping a uniform, precise time. The reason for this is that we can change the time on an atomic clock while it is not possible to alter the Earth's rotational speed to match the atomic clocks. Currently the Earth runs slow at roughly 2 milliseconds per day. After 500 days, the difference between the Earth rotation time and the atomic time would be one second. Instead of allowing this to happen a leap second is inserted to bring the two times closer together. The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS). According to international agreements, first preference is given to the opportunities at the end of December and June, and second preference to those at the end of March and September. Since the system was introduced in 1972, only dates in June and December have been used.

The official United States time is determined by the Master Clock at the U. S. Naval Observatory (USNO). The Observatory is charged with the responsibility for precise time determination and management of time dissemination. Modern electronic systems, such as electronic navigation or communication systems, depend increasingly on precise time and time interval (PTTI). Examples are the ground-based LORAN-C navigation system and the satellite-based Global Positioning System (GPS). Navigation systems are the most critical application for precise time. The newest satellite navigation system, the Global Positioning System (GPS), is used for navigating ships, planes, missiles, trucks and cars anywhere on Earth. These systems are all based on the travel time of electromagnetic signals: an accuracy of 10 nanoseconds (10 one-billionths of a second) corresponds to a position accuracy of about 10 feet.

Precise time measurements are needed for the synchronization of clocks at two or more stations. Such synchronization is necessary, for example, for high speed communications systems. Power companies use precise time to control power distribution grids and reduce power loss. Radio and television stations require precise time (the time of day) and precise frequencies in order to broadcast programs. Many programs are transmitted from coast to coast to affiliate stations around the country. Without precise timing the stations would not be able to synchronize the transmission of these programs to local audiences. All of these systems are referenced to the USNO Master Clock.

Very precise time is kept by using atomic clocks. The principle of operation of the atomic clock is based on measuring the microwave resonance frequency (9,192,631,770 cycles per seconds of the cesium atom. At the Observatory, the atomic time scale (AT) is determined by averaging 60 to 70 atomic clocks placed in separate, environmentally controlled vaults. Atomic Time is a very uniform measure of time (one tenth of one billionth of a second per day).

The USNO must maintain and continually improve its clock system so that it can stay one step ahead of the demands made on its accuracy, stability and reliability. The present Master Clock of the USNO is based on a system of some 60 independently operating cesium atomic clocks and 7 to 10 hydrogen maser atomic clocks. These clocks are distributed over 20 environmentally controlled clock vaults, to insure their stability. By automatic inter- comparison of all clocks every 100 seconds a time scale is computed which is not only reliable but also extremely stable. Its rate does not change by more than about 100 picoseconds (.000000001 seconds) per day from day to day.

On the basis of this computed time scale, a clock reference system is steered to produce clock signals which serve as the USNO Master Clock. The clock reference system is driven by a hydrogen maser atomic clock. Hydrogen masers are extremely stable clocks over short time periods (less than one week). They provide the stability and reliability needed to maintain the accuracy of the Master Clock System.

Very Long Baseline Interferometry (VLBI) is used to determine Universal Time (UT) based on the rotation of the Earth about its axis. VLBI is an advanced technique used for observing with radio telescopes. The information gained using VLBI can be used to generate images of distant radio sources, measure the rotation rate of the Earth, the motions of the Earth in space, or even measure how the plates are moving on the surface of the Earth. Measuring the Earth's motion is critical for navigation. The most accurate navigation systems rely on measurements of satellites which are not tied to the Earth. These systems can provide a position accurate to a few feet, but the position of the Earth relative to the satellite must also be known to avoid errors of hundreds of feet. The U.S. Naval Observatory has been in the forefront of timekeeping since the early 1800's. In 1845, the Observatory offered its first time service to the public: a time ball was dropped at noon. Beginning in 1865 time signals were sent daily by telegraph to Western Union and others. In 1904, a U.S. Navy station broadcast the first worldwide radio time signals based on a clock provided and controlled by the Observatory.

A time of day announcement can be obtained by calling 202-762-1401 locally in the Washington area. For long distance callers the number is 900-410-TIME. The latter number is a commercial service for which the telephone company charges 50 cents for the first minute and 45 cents for each additional minute. Australia, Hong Kong, and Bermuda can also access this service at international direct dialing rates. You can also get time for your computer by calling 202-762-1594. Use 1200 baud, no parity, 8 bit ASCII.


Astronomer Danny Faulkner:

The earth's rotation is slowing at the rate of 0.0017 seconds per century. What that means is that a hundred years ago the day was 0.0017 seconds shorter than today. This is called a secular change because it changes at a fixed rate for a very long time. Superimposed on top of this is the periodic change, much larger variations that change in magnitude and sign over much shorter time scales than the secular change does. The periodic and secular time variations have nothing to do with each other. The atomic clock measurements that necessitate leap seconds every 18 months or so measure the periodic variations. The secular change is much more difficult to detect and must be deduced by other means. Much of the criticism offered by our critic here is correct. Many people apparently don't know that there is a difference between the periodic and secular changes. Extrapolating the secular change into the past does not produce a problem for an old earth. 4.6 GYr ago the day would have been a little more than 2 hours at this rate. Rearrangement of material would have probably blunted this a little. So while this pushes it to the limit, an old earth could survive this. But one cannot extrapolate this into the past. The change in rotation is caused by a tidal interaction between the earth and moon, with the other effect that the moon spirals away. When the moon was much closer in the past the effect would have been much greater. So the secular change would have been much larger early on. This means that this tidal evolution could not have been going on for 4.6 GYr. The moon would have been in contact with the earth less than 1.4 GYr ago. The rotation of the earth about that time would have been very short as well. So while our critic here nibbled at how this was presented (and it may have been botched by some of us), this is still a huge problem for an earth more than 1.4 GYr old.

Wayne Spencer:

Earth's slowing spin, lengthening day. This is related to the tides, I think this has not been adequately examined by creationists to say anything one way or the other. I think it is worth looking at though. A similar but more complicated process is at work at Io, Jupiter's moon that I think has promise for being a plausible young age indicator.

Astronomer Barry Setterfield:

This problem can be looked at in several ways. Firstly, it is often considered that the idea of a lengthening day is supported by the paleontological evidence. However, there are other options that should also be examined before this interpretation is unhesitatingly accepted. As Wesson [Cosmology and Geophysics, pp. 126-140, 155, (Adam Hilger Ltd., Bristol)] explains, this could instead be evidence of "G variability, global expansion, or other cosmological effects." Consequently, this aspect of the issue is by no means settled by the paleontological evidence.

Coral growth today is seen to fairly accurately reflect the alternation between day and night. For this reason, many try to use coral growth as a means of determining day length in the past. Some studies of Upper Cretaceous corals reveal a year of about 370 days, while the earlier Mid Devonian specimens suggest something like a 400 day year [See for example Berry and Barker NATURE Vol. 217, pp. 938-939, or Wells in NATURE Vol. 197, pp. 948-950]. These researchers, and others like them, could only conceive this meant the earth's day was shorter back then and has increased in length since. Bowden [personal communication] maintains, however, that results of this type of work are due far more to the selectivity of the researcher in choosing what he was counting rather than from the data itself. Furthermore, Wesson explains that other interpretations of these data could be used as evidence to support "G variability, global expansion, or other cosmological effects." [Cosmology and Geophysics, pp. 126-140, 155, (Adam Hilger Ltd., Bristol)]. Thus, extrapolating into the past via this means might not be either accurate or dependable in determining the earth's actual long-term rotational spin-down rate. For this reason, we have limited data to work with on the leap second issue, and must concentrate on what is more available to us.

Secondly, the whole leap second scenario is also open to several different interpretations. Essentially, what is showing up is a discrepancy between the atomic clock and the dynamical or orbital clock. The dynamical clock, based on the earth's motion around the sun, is running at a different rate to the atomic clock. The point of comparison which is used would be, for example, the number of seconds elapsed on each clock between the passage of a star directly overhead at midnight on December 31st on two successive years. While the difference is often interpreted as the earth's rate of rotation changing, it need not be. The star clock need not necessarily be a marker of the earth's rotation: it is also a marker of our orbital period. If it were merely rotational changes, the factors affecting this should give both positive and negative leap seconds. Instead, the addition of leap seconds has been decidedly one-sided. This suggests that the cause of the trouble may not be the earth's rotation; rather it may lay in the difference in run-rate between the orbital (dynamical clock) and the atomic clock.

Interestingly, some comments about this very issue have been made. Firstly, Kovalevsky [METROLOGIA, Vol 1:4, 169-180 (1965)] has pointed out that if the dynamical (orbital) clock rate was different from the atomic clock rate, then Planck's constant as well as atomic frequencies would drift. Such changes have been noted. Firstly, the measured value of Planck's constant has shown a systematic increase this century (Atomic Constants, Light and Time, Norman Setterfield, SRI International, 1987). Secondly, over the decade up to 1984 Van Flandern examined data from lunar laser ranging using atomic clocks and compared them with dynamical data. He concluded that: 'The number of atomic seconds in a dynamical interval is becoming fewer. Presumably, if the result has any generality to it, this means that atomic phenomena are slowing with respect to dynamical phenomena.' [Precision Measurements and Fundamental Constants II, NBS (US) SPECIAL PUBLICATION 617, Taylor Phillips, editors, pp. 625-627 (1984)]. Van Flandern then went on to establish similar results with planetary radar and laser ranging using atomic clocks in association with the US Naval Observatory in Washington.

The Van Flandern data establishes the general principle being outlined here. The NBS data set may do likewise. From January 1972 to January 1998, the National Bureau of Standards found it necessary to add 31 "leap-seconds" since the atomic clock was varying compared with the orbital standard [IERS Bulletin C 14, Paris, 8 July 1997]. Although various reasons have been given for this one-sided variation, it is possible that a changing rate of ticking for the atomic clock may be partly responsible. It may therefore be unwise to insist that the leap seconds are showing a variation in the earth's rotation rate, as other factors may well be at work.

November 16, 1998, November 19, 1998. December 10, 2001.

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