General Science

Leap Second Day

posted Saturday, 30 June 2012

Today is longer than yesterday and tomorrow by one second.

The hyperfine transition is the basis of International Atomic Time (TAI). By definition, the outermost electron in an ordinary cesium 133 atom cycles through this transition 9,192,631,770 times in one second.


International Atomic Time (abbreviated TAI after the French Temps Atomique International) began at midnight GMT on the first day of 1958 and has continued advancing forward at the rate of one second every 9,192,631,770 periods of the hyperfine transition in 133Cs. TAI is maintained by the Bureau International des Poids et Mesures (BIPM) in Paris, which periodically averages the time kept by various atomic clocks around the world. The BIPM then disseminates correction factors needed to synchronize these clocks with the master clock in the Observatoire de Paris.

Coordinated Universal Time (abbreviated UTC) is the basis of legal time throughout the world. All local civil times differ from UTC by either a whole number of hours or an odd number of half hours, but never by any other amount. One second of Coordinated Universal Time is the same as one second of International Atomic Time, but UTC and TAI are slightly out of step. TAI marches forward uniformly, while UTC is adjusted from time to time to keep it synchronized with the earth’s rotation.

The earth is not an effective timekeeper. For most of the last two hundred years the mean solar day has been slightly longer than the 86,400 s currently defined by the International System. Universal Time UT, or more specifically UT1, is in effect the mean solar time. It is continuous (i.e. there are no leap seconds) but has a variable rate because of the Earth’s non-uniform rotation period. It is needed for computing the sidereal time, an essential part of pointing a telescope at a celestial source.



When UT1 lags too far behind UTC, a leap second is inserted at the end of the day before January 1 or July 1 as appropriate. When this happens, 23:59:59 is followed by the unusual time of 23:59:60 before turning over to 00:00:00 and starting the next day. In the unlikely event that UT1 were to lead UTC (that is, if the earth’s rate of rotation were to increase) the provision exists for the insertion of a negative leap second. Were this to ever occur, 23:59:58 of one day would be followed by 00:00:00 of the next, skipping 23:59:59 altogether. In any case the absolute difference between UTC and UT1 must never exceed 0.9 s.


year month offset   year month offset   year month offset
1961 January 0   1986   23   2002   32
1971 * 10   1987   23   2003   32
1972 July 11   1988 January 24   2004   32
1973 January 12   1989   24   2005   32
1974 January 13   1990 January 25   2006 January 33
1975 January 14   1991 January 26   2007   33
1976 January 15   1992 July 27   2008   33
1977 January 16   1993 July 28   2009 January 34
1978 January 17   1994 July 29   2010   34
1979 January 18   1995   29   2011   34
1980 January 19   1996 January 30   2012 July 35
1981 July 20   1997 July 31   2013   ?
1982 July 21   1998   31   2014   ?
1983 July 22   1999 January 32   2015   ?
1984   22   2000   32   2016   ?
1985 July 23   2001   32   2017   ?
Leap seconds and cumulative adjustments to UTC
Adjustments from 1961 to 1971 followed a more complicated protocol and were omitted. Only the final offset is shown.

Are leap seconds even necessary?

  • In seven or eight centuries the difference between TAI and UT1 will be about an hour.
  • By the year 5000, day and night will have reversed; that is, 12 noon will occur in the middle of the night and 12 midnight will synch up with the midday sun.

What would be so wrong with that? Does it really matter what number we assign to a position of the sun in the sky?

In 1582 Pope Gregory XIII managed to extract ten days from the calendar. On 4 October 1582 the Catholic world went to sleep. When they woke up it was 15 October 1582. By 1752 the protestant nation of England and her American colonies also accepted the change. (They needed to add 11 days to catch up.) In 1873 Japan made the switch. (They needed 12 days.) Then Russia in 1917 and China in 1949. (13 days.) The Greek Orthodox Church is possibly the only European agency that has not accepted this change (although the nation of Greece made the switch in 1923).

I think the big thing is that everyone agrees what time (or day) it is. Not that the time is any particular number. Time is a social construct, remember.

Mount Everest Summited (1953)

posted Tuesday, 29 May 2012

Mount Everest was conquered as Edmund Hillary of New Zealand and Tenzing Norgay of Nepal became the first climbers to reach the summit on 29 May 1953.

North Pole discovered? (1909)

posted Friday, 6 April 2012

A team of 5 men lead by American naval engineer Robert Peary arrived at the North Pole on 6 April 1909. This claim is subject to dispute.

Charles Hall separates metallic aluminum from rocks (1889)

posted Monday, 2 April 2012

Aluminum statue of Charles Martin Hall

US Patent 400,664 was awarded to 26 year old Charles Martin Hall of Ohio on this day in 1889. His "Process of Reducing Aluminium from Its Fluoride Salts by Electrolysis" described a new method for the extraction of aluminum metal from ore. Hall’s process reduced the cost of producing aluminum to one half of one per cent of its former value. All humans should travel to his aluminum statue in Ohio and kiss his aluminum toes in perpetual gratitude.

Deepest ocean dive (1960)

posted Monday, 23 January 2012

23 January 1960: The bathyscaphe Trieste carrying carrying Jacques Piccard and Don Walsh visited the Challenger Deep — the deepest part of the ocean — 10,911 meters below sea level. No humans have ever repeated this feat.

Robert Byrd first to fly over South Pole (1929)

posted Tuesday, 29 November 2011

29 November 1929: Robert Byrd first to fly over South Pole

Royal Society Founded (1660)

posted Monday, 28 November 2011

The origins of the Royal Society lie in an “invisible college” of natural philosophers who began meeting in the 1640s to discuss the ideas of Francis Bacon. Its official foundation date is 28 November 1660, when 12 of them met at Gresham College after a lecture by Christopher Wren and decided to found “a Colledge for the Promoting of Physico-Mathematicall Experimentall Learning.” This group included Christopher Wren, Robert Boyle, John Wilkins, Robert Moray, and William Brouncker.

Evolution Day (Origin of Species published in 1859)

posted Thursday, 24 November 2011

Evolution Day is the anniversary of the first publication of The Origin of Species on 24 November 1859.

Charles Darwin’s On the origin of Species by means of natural selection, or the preservation of favoured races in the struggle for life first appeared on 24 November 1859 and was sold out that very same day. The publisher had been skeptical and had printed only 1250 copies. "Next time write about pigeons, a subject everybody is interested in" he had told Darwin. The word "evolution" is used for the first time only in the sixth edition of the book. The term "descent with modification" is the forerunner of evolution. Even today On the Origin is still being published.

Time zones adopted in the US (1883)

posted Tuesday, 11 October 2011

The first five standardized time zones of the US and Canada went into effect on this day in 1883. The agreement reached by the General Time Convention (later known as the American Railway Association) was designed to simplify railway scheduling.

This year’s winners

posted Tuesday, 4 October 2011

This year’s winners.

year laureate(s) achievement

Saul Perlmutter
Brian P. Schmidt
Adam G. Riess
Received a Nobel Prize for "for the discovery of the accelerating expansion of the universe through observations of distant supernovae".

Philippe Perrin
Cyril Perrot
Dominique Deviterne
Bruno Ragaru
Herman Kingma
Received an Ig Nobel Prize for "for determining why discus throwers become dizzy, and why hammer throwers don’t".