Sunday, January 1, 2023

Leap-Second to Make-Way for Leap-Minute? screen-shot from the last Leap-Second inserted in the civil time scale. In this case, displayed is Coordinated Universal Time (UTC), used internationally and by scientists. This time was equivalent to 6:59:60 p.m. Eastern Standard Time (EST) on Saturday Evening, 2016 December 31. (Image Sources: National Institute of Standards and Technology,, By US Government / NIST - Screen Grab from web display of, Public Domain,

By Glenn A. Walsh

Reporting for SpaceWatchtower

For the last 50 years, some calendar years have ended with a Leap-Second, an additional second added to keep clocks in-sync with the slowing rate of the Earth's rotation. However, scientists have decided to end the practice of adding Leap-Seconds, by or before 2035.

What is a Leap-Second ?

As mentioned, a Leap-Second is an additional second inserted into the civil time scale (which had been converted to the more accurate and consistent Atomic Time), to keep our clocks close to the time as determined by the Earth's rotation rate. The Leap-Second is used to keep Atomic Time synchronized with Astronomical Time or Universal Time.

As of 2022 December, Atomic Time has been between 15 and 20 milliseconds ahead of Astronomical Time. One millisecond is one-thousandth of one second.

Actually Leap-Seconds were designed to be either positive or negative, depending on the uncertain nature of the Earth's rotation. However, to-date, only 27 positive Leap-Seconds have been inserted into the civil time scale since 1972, the last one added on 2016 December 31.

Also, the Leap-Second was designed to be added or subtracted from the civil time scale on either June 30 or December 31, or both. One Leap-Second was added both on June 30 and December 31 in the first year it was instituted, 1972. Although most Leap-Seconds have been added on December 31, 11 Leap-Seconds have been inserted on June 30, the last added on 2015 June 30.

Sometimes, when a Leap-Second is added on December 31, the famous New York City Times Square Ball-Drop holds the ball for an additional second, for the Leap-Second. However, this is just for show. The Leap-Second, whether inserted on June 30 or December 31, is always added at 23:59:60 Coordinated Universal Time (UTC - time used internationally and by scientists), which is the equivalent of 6:59:60 p.m. Eastern Standard Time (EST).

The Times Square Ball-Drop harkens back to the early days of marine navigation. Time Balls, mostly at astronomical observatories and other significant marine navigation-related buildings, would drop each day at local Noon, so ships off-shore could accurately set their chronometers. To determine the accurate Longitude at sea. mariners must have the accurate time.

Why Have a Leap-Second in the First Place ?

Astronomers in the 19th century used the stars to tell-time. A special Meridian Transit Telescope would be used to determine the precise time when a star had returned to the same location in the sky, the following day. Samuel Pierpont Langley, a future Secretary of the Smithsonian Institution, sold the precise time to the railroads via telegraph transmission beginning in 1869, when he was Director of Pittsburgh's Allegheny Observatory. This led to the creation of geographic Time Zones in 1883.

However, the Earth's rate of rotation on its axis (which causes day and night), measured by astronomers, is not as consistent as the more accurate time created by our atomic clocks. So, it was decided in 1972 to change the clocks, to more agree with the Earth's rotation, which is measured as Astronomical Time or Universal Time (UT or UT1). Coordinated Universal Time (UTC) is then converted to the civil time scales of the world, such as Eastern Time (ET) and the other 23 world time zones.

This was fine for the last 50 years. When the Earth's rate of rotation slowed to the point where Atomic Time was ahead by 0.9 second, a Leap-Second would be added to remedy the discrepancy. Actually, the Leap-Second is usually added once Atomic Time is ahead by 0.6 second, so the discrepancy does not approach the one second deviation. When a Leap-Second is added, automatically clocks come to be ahead of Astronomical Time by 0.4 second; so, the next Leap-Second would not be added until clocks lose an additional 1.0 second of time.

An organization called the International Earth Rotation and Reference Systems Service (IERS) makes the decision when a Leap-Second needs to be added (or subtracted), six months before implementation. In addition to notifying other scientists and the media of the decision, U.S. National Institute of Standards and Technology short-wave radio stations WWV (Fort Collins, Colorado) and WWVH (Kekaha, Hawaii), which broadcast precise time-of-day signals to the general public, transmit an hourly announcement of the Leap-Second change to take-place.

Why Eliminate Leap-Seconds Now ?

Remember the “Y2K” scare of 1999? Moving from “99” to “00”, on computers which represented the year with only two digits, was predicted to confuse and compromise some computer applications. Preparations and preventive measures were taken and few problems occurred. Regrettably, two-digit year expressions can still be found in some applications.

In the 21st century, Leap-Seconds have proven to be quite disruptive to computers, particularly with applications requiring time-stamping or time-critical process control. This includes cellular telephone networks and GPS (Global Positioning System), as well as aviation and satellite navigation systems – any system that requires precise time-keeping. System failures and anomalies can be caused by the insertion of a Leap-Second.

Scientists have been considering the elimination of the Leap-Second since 2015. On 2022 November 18 the final decision was made at a general conference in France of the International Bureau of Weights and Measures (BIPM), which is responsible for international time-keeping.

The United States was one of the nations which voted to eliminate the Leap-Second. This is no surprise considering how dependent our nation has become to computer and other high-technology systems.

What Are Alternatives to the Leap-Second ?

At this conference, it was decided that by or before 2035, insertion of Leap-Seconds would end. When, exactly this would take place, and what would be the alternative, has not yet been determined. Another international conference in 2026 would need to iron-out those details.

It is anticipated that, without Leap-Seconds, the difference between Atomic Time and Astronomical Time would continue to increase, but not by an alarming rate. It is predicted that it would take 50-to-100 years before such a difference would add-up to one minute. It could take up-to five millenia for the difference to become as large as one hour!

So, what are possible alternatives? One idea is to add a Leap-Minute, when needed, every 50-to-100 years. Another alternative is similar, but instead of calling it a Leap-Minute, simply “smearing” the last minute of a particular year into two minutes. Again, so far no specific alternative has been endorsed by scientists.

Of course, this assumes that the Earth's rotation rate will continue slowing. But what if the Earth surprises us? If the Earth's rotation rate should suddenly speed-up, no change may be necessary or a negative Leap-Minute (subtracting a minute at the end of a year) could become necessary.

Scientists still have much to learn about the rotation rate of celestial bodies, including our own planet. Subtle changes could affect a planet's rotation rate. In the case of the Earth, the Moon's gravity which affects ocean tides could change, as the Moon very, very gradually moves farther from our planet each year.

The change in the distribution of the Earth's mass from shifts in our planet's molten interior, perhaps caused by greater volcanic activity, could affect the Earth's rotation rate. Some scientists suggest that Climate Change, with warmer air and melted water moving closer to the poles, may cause the Earth to spin faster!

Once Leap-Seconds no longer occur (by or before 2035), astronomers will need to adjust times, by a specific amount for the time period of their observations, to ensure that their telescopes and other instruments can find the star, planet, or other celestial body they wish to study. No doubt, astronomical almanacs will need to begin reporting on the time corrections needed by these scientists, as well as by amateur astronomers.

At any rate, there is no Leap-Second planned for the end of 2022. And, there may never be a Leap-Second again! We will just have to keep watching our time.

Internet Links to Additional Information ---

Leap-Second ---

(National Institute of Standards and Technology): Link >>>

(Wikipedia): Link >>>

International Atomic Time (TAI): Link >>>

Astronomical Time >>>

Universal Time (UT / UT1): Link >>>

Coordinated Universal Time (UTC):

Link >>>

Astronomical Time Balls: Link >>>

Samuel Pierpont Langley: Link >>>

Related Blog-Posts ---

"Leap-Year to be Even Longer w/ Added Leap-Second!" Mon., 2016 July 11.

"Slowing Earth Rotation Rate Necessitates June 'Leap Second'." Tue., 2015 Jan. 27.

Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friends of the Zeiss          

               Sunday, 2023 January 1.

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Glenn A. Walsh, Informal Science Educator & Communicator                                                               (For more than 50 years! - Since Monday Morning, 1972 June 12):
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Formerly Astronomical Observatory Coordinator & Planetarium Lecturer, original Buhl Planetarium & Institute of Popular Science (a.k.a. Buhl Science Center), America's fifth major planetarium and Pittsburgh's science & technology museum from 1939 to 1991.
Formerly Trustee, Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of Carnegie, Pennsylvania, the fourth of only five libraries where both construction and endowment funded by famous industrialist & philanthropist Andrew Carnegie.
Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh: Link >>>  Buhl Observatory: Link >>>
* Adler Planetarium, Chicago: Link >>>
* Astronomer, Educator, Optician John A. Brashear: Link >>>
* Andrew Carnegie & Carnegie Libraries: Link >>>

* Other Walsh-Authored Blog & Web-Sites: Link >>>


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