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The Future of Leap Seconds
John Lee
National Measurement System Directorate
Department of Trade & Industry
Time & Frequency Club
15 April 2005
What is a leap second?
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A leap second is
a one-second adjustment to Co-ordinated Universal Time
(UTC)
made on the advice of the International Earth Rotation
Service (IERS)
to ensure that UTC is kept within 0.9 seconds of mean
solar time of the Greenwich meridian.
Key timescales
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International Atomic Time (TAI):
Calculated by BIPM. Based on SI seconds. Continuous scale since
1958, with NO leap seconds.
Mean Solar Time = UT1 = GMT:
Used as the basis of civil time in the UK and other countries:
derived from the apparent movement of the sun.
Co-ordinated Universal Time (UTC):
A compromise timescale using SI seconds and linked closely to
UT1 by occasional insertion of leap seconds. Currently 32
seconds offset from TAI. Used as the basis of civil time in many
countries.
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Definition of the second
SI definition: The second is the duration of 9,192,631,770 periods
of the radiation corresponding to the transition between the two
hyperfine levels of the ground state of the caesium 133 atom.
This value was fixed in 1958 by Essen and Parry (NPL ) and
Markowitz and Hall (USNO) to be equal to the second of
Ephemeris Time used by astronomers.
The length of the SI second is therefore equivalent to 1 / 86,400 of
a mean solar day in about 1820.
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The Length of Day varies
Deceleration of the Earth’s rotation leads to an increase in the
length of day. Deceleration is caused by tidal friction. Short term
variations caused by movement of Earth crust and core.
Length of day increases at a rate of 1.7 millisecond / century
(average observed rate over 2,500 years).
This gives a difference (Delta T) between UT1 and uniform time
which accumulates at the rate of 31 seconds / century2
If LoD is 2.5 ms longer than 86 400 SI seconds (as was typical
during the 1970s), then the accumulated difference after 400
days is 1 second, and a leap second is needed about once a
year.
List of 22 leap seconds
1
30 June
1972
2
31 December
1972
3
31 December
1973
4
31 December
1974
5
31 December
1975
6
31 December
1976
7
31 December
1977
8
31 December
1978
9
31 December
1979
10
30 June
1981
11
30 June
1982
12
30 June
1983
13
30 June
1985
14
31 December
1987
15
31 December
1989
16
31 December
1990
17
30 June
1992
18
30 June
1993
19
30 June
1994
20
31 December
1995
21
30 June
1997
22
31 December
1998
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Length of Day: 500 BC to present
Reproduced with permission of Steve Allen, Lick Observatory, University of California
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Length of Day: AD 1750 to present
Reproduced with permission of Steve Allen, Lick Observatory, University of California
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Length of Day: 1950 to present
Reproduced with permission of Steve Allen, Lick Observatory, University of California
Future of leap seconds
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A decision is likely to be made in 2005 on the future of leap
seconds.
If a US proposal is adopted, the nature of UTC will change
significantly; and
Within a few years the UK’s civil timescale will no longer be GMT.
Is this a problem?
International developments
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ITU-R (International Telecommunications Union –
Radiocommunications Sector)
ITU-R is considering the future of the UTC timescale, and states
that:
“The occasional insertion of leap seconds into UTC creates
serious difficulties for many operational navigational and
telecommunication systems today.”
Is this true?
Questions from ITU-R
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Three questions raised formally by ITU-R:
• What are the requirements for globally accepted timescales for
use both in navigation / telecommunication systems, and for civil
time keeping?
• What are the present and future requirements for the tolerance
limit between UTC and UT1?
• Does the current leap second procedure satisfy user needs or
should an alternative procedure be developed?
Proposal by US Government
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The US Government has proposed to ITU-R that:
After 21 December 2007, no further leap seconds will be inserted
in UTC;
The difference between UTC and UT1 should not exceed +/- one
hour;
Implications:
The new timescale is to keep the name UTC;
A leap hour will not be needed for at least 500 years.
Legal time in the UK
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Interpretation Act 1978:
Section 9 – References to time of day.
“Subject to section 3 of the Summer Time Act 1972 ……..
whenever an expression of time occurs in an Act, the time referred
to shall, unless it is otherwise specifically stated, be held to be
Greenwich mean time”.
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Predicted difference between UTC and UT1
Assuming deceleration of the Earth’s rotation to be 31 s/cy2
and no leap seconds after 2007
Year
2008
1 second
2011
5 seconds
2015
10 seconds
2031
30 seconds
2053
1 minute
2183
5 minutes
2605
30 minutes
2914
1 hour
5558
12 hours
7103
24 hours
Why abolish leap seconds?
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• Leap seconds are irregular, so cannot be programmed in same
way as leap years.
• Manual changes need to be made to systems, creating scope for
error.
• Most IT systems cannot recognise leap seconds, creating
problems for accurate time stamping.
• Leap seconds will become more frequent, causing more of this
type of disruption.
• Synchronisation of systems fails during a leap second.
• Systems cannot be tested as event is infrequent.
• An eventual leap hour would cause no more problem than
daylight saving time.
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Arguments against US proposal
• Civil time has always matched the apparent movement of the
sun.
• Operators of IT and other systems should have them modified to
deal properly with leap seconds.
• Systems requiring a timescale without leap seconds should use
TAI or GPS time.
• The existing form of UTC is a good compromise which gives
users SI seconds plus time of day closely linked to UT1.
• A leap hour could be as difficult to implement as the year
number transition from 1999 to 2000.
• Some systems require the difference between UTC and UT1 to be
less than one second, and modifications will be expensive.
• A new timescale without leap seconds should be given a new
name.
No consensus yet?
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A survey of international scientific bodies by IERS in 2002
showed:
• most scientific users are neutral;
• a significant minority is strongly against the change; and
• a small minority is strongly in favour of the change.
The DTI’s view
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Concern about possible loss of GMT as the civil timescale.
DTI has not finally decided its position, and the matter is still
being considered within Government.
Not yet convinced of the need for change: but we could
reconsider if compelling evidence of serious difficulties is
forthcoming.
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Comments to:
[email protected]