The atomic clocks of the Greenwich Time Service

page under construction


Built by the National Physical Laboratory in west London in 1955, the world's first accurate caesium atomic clock set a new standard in timekeeping. Less than ten years later, in 1964, Hewlett-Packard produced the world's first commercial atomic clock to use only solid state comoponents. Denoted HP5060A, it was one of these that the Observatory acquired as its first atomic clock in 1966. By 1974, it had a total of six clocks. They consisted of two HP5060As and four of its successor the HP5061A, though the identity of the fourth clock has been presumed rather than positively identified at this stage..


The naming of the Observatory clocks

The caesium tubes originally produced for the earlier models of the HP clocks tended to have a much short life compared to those of the clocks produced today. In 1968, the tubes of model HP5061A had a guraranteed running life of just 10,000 hours (just under 14 months). When a tube needed replacing, the clock was sent back to the manufacturer and assigned a new name opperating name by the Observatory on its return. Broadly speaking, the clocks were named CsA, CsB, CsC etc, in their order of arrival or return to the Observatory after a tube replacement. CsA for example was later renamed as CsC, then CsK, then Csl.

Information published by the Observatory about the arrival of new clocks and tube replacements tends to be incomplete, scattered, and inconsistent in its detail. The information in the first table below covers the period up to the end of 1978. It has been compiled from the Time and Latitude Bulletins/Greenwich Time Reports with confirmation in some cases coming from the more general Observatory Reports which from 1966 until 1980 were normally issued annually but ocassionally after intervals of nine or 15 months.

From the start of 1977, following the departure of Humphry Smith as officer in charge of the Time Department, the scope of the information published in the Greenwich Time Reports was slighly altered. On the plus side, the rates of all the running clocks were published for the first time. On the downside, no information was published on how existing clocks were renamed after a tube replacement. Although it is possible to deduce that CsB was renamed CsN and that that CsF was renamed CsN, it has not been possible to identify the parenthood of all the clocks from 1979 onwards. The last of the Greenwich Time Reports to be published covered the second quarter of 1982 (Q2).

If the Observatory Reports are to be believed, the Observatory never owned more than six clocks. The Greenwich Time Reports however show that there were several ocassions when details of seven observatory clocks were published as can be seen in the second table below.


Table showing date of acquistion or renaming of the six caesium clocks 1966 – 1978
1 HP5060A1 1966, May 63 CsA B140
1968, Mar (by)
CsC GTR1968Q1,p.9
1974, Apr CsK GTR1974Q3,p.39
1975, Mar 21 CsL GTR1975Q1,p.80
2 HP5060A 1967, Mar 33 CsB B149, GTR1968Q1,p.9 & GTR1968Q1,p.9

1977, Nov 2 (by) CsN GTR1977Q4,p,66
3 HP5061A 1968, Aug 293 CsDc/CsD GTR1968Q4,p.51 & GTR1968Q1,p.9
1972, Mar 30 ? CsG GTR1972Q1,p.264 & GTR1972Q2,p.281
1975, Nov CsM GTR1975Q4,p.139
4 ? 1970, Dec (by)
1973, Aug 23 CsH GTR1973Q3,p.369
5 HP5061A 1972, Jan 14 CsF GTR1972Q2,p.281

1978, Jul (By)5 CsP GTR1978Q3,p.110
6 HP5061A2 1974, Dec6 CsJ GTR1974Q4,p.61

1. Wilkins records it had orginally been scheduled for delviery in August 1965
2. Tube Option 4
3. Described in Bulletin or Report as 'installed in ...'. Returned to Hewlett Packard for a new tube on 19 October 1976 (GTR1976Q4,p235
4. Received on January 1 and put on test on high beam current; on being moved on March 24, there was a change of rate; it was installed in its cellar on May 26 and the C-field adjusted on May 31
5. CsF last mentioned on 1978, Jan 1 (GTR1978Q1,p.82)
6. Date run started rather than date of acquisition

Table showing appoximate dates clocks A – M existed between 1966 & 1976
1966 May 1967 Mar 11 A

1 A new
1967 Mar
1968 Mar
13 A B 2 B new
1968 Mar
1968 Aug
6 B C 3 A renamed C
1968 Aug
1970 Dec
29 B C D 3 D new
1970 Dec
1971 Dec
13 B C D E 4 E new
1972 Jan
1972 Mar
3 B C D E F 5 F new
1972 Mar
1973 Aug
18 B C E F G 5 D renamed G
1973 Aug 1974 Apr 9 B C F G H 5 E renamed H
1974 Apr 1974 Dec 9 B
F G H K 5 C renamed K
1974 Dec1 1975 Mar 4 B
F G H J K 6 J new
1975 Mar 1975 Nov 9 B
F G H J L 6 K renamed L
1975 Nov 1976 end 14 B F H J L M 6 G renamed M

1. Date run started rather than date of acquisition


Table showing clocks mentioned in each quarterly
Greenwich Time Report Jan 1977 to Jul 1982
1977 Q1 F H J L M 5
Q2 F H J L M 5
Q3 F H J L M 5
Q4 F H J L M N8 6
1978 Q1 F H J L M N 6
Q2 H J L M N 5
Q3 H1 J L M6 N P9 6
Q4 L M7 N P 4
1979 Q1 L
M N P 4
Q2 L M N P Q4 5
Q3 L
N P Q 4
Q4 L N P Q R5 5
1980 Q1 L N P Q R S2 T3 7
Q2 L N P Q R S T 7
Q3 L N P Q R S T 7
Q4 L N P Q R S 6
1981 Q1 L N P Q R S 6
Q2 L N P Q R S T
Q3 L N P1 Q R S T 7
Q4 L N Q R S T 6
1982 Q1 L N Q R S T 6
Q2 L N Q R T V 6

1. Taken out of service in July
2. From February
3. From March
4. From June
5. From Dec
6. Taken out of servivce in August
7. Brought back into service in October
8. From November. Assumed to be a replacement for CsB
9. Assumed to be a replacement for CsF


The Hewlett-Packard models HP5060A and HP5061A

Hewlett-Packard's HP5060A was the world's first commercially produced atomic clock to use only solid state comoponents. Whilst the valves (vacuum tubes) used in earlier clocks had made them comparatively large and fragile, the solid state components of HP5060A not only made it both more reliable, but also more transportable. Launched in 1964, HP5060A was was surperceeded by model HP5061A in 1967, which had an LED clock and a backup battery available as options. HP5061A was superceed by model HP5061B in 1986.

Model 5071A, which was unveiled in 1991 was a direct descendent of and replacement for the 5060A, 5061A and 5061E caesium standards and had a guranteed tube life in the year 2000 of 10 years - roughly eight times longer than HP5061A had had. By the time of its introduction however, the Observatory's Time Department had been shut down and the Observatory relocated to Cambridge.

Temporary Operating Manual Hewlett-Packard model 5061A

HP5061B Manual

5071A Product overview

Images showing parts of an HP5060A from 

Musuem of HP clocks from


The location of the clocks at Herstmonceux

The Time Department was the last of the Departments to be transferred to the Observatory's new site at Herstmonceux. When they did so, in 1957, they moved into the newly completed West Building, the final decisions on the detailed layout of the Department's accomodation having been delayed for as long as possible in order that the adopted designs would be as adaptable and future proofed as possible. As well as providing individual 'cellars' for the quartz clocks, provision was made for housing the atomic clocks of the future, which according to Geoge Wilkins, included a caesium fountain clock, which at that time was little more than a germ of an idea. In order to accomodate such a clock, the allocated space was two storeys high and located at the southern end of the clock corridor in the sub-basement. In the event, no atomic clock was ever housed there.

Neither the Bulletins nor the Greenwich Time Reports mention where at Herstmonceux the atomic clocks were located. Nor did any of the Observatory Reports until the Report for the period 1 October 1980 - 30 September 1985 which states: 'The clocks are kept in six locked, temperature controlled rooms in an air condition sub-basement and have individually stabilised external DC power supplies.' Some information was however published in 1973 when in his paper Facilities and services in the Time Department of the Royal Greenwich Observatory. Proceedings of the 5th PITI Planning Meeting, NASA Goddard Space Flight Center Smith wrote:

'The time department now operates five caesium standards, which are housed in cellars at sub-basement level. Each standard has an independent earthing system, an individual emergency power supply, and the distribution of time and frequency is by buffered screened balanced lines, On initial installation, each standard is carefully set up according to the manufacturers recommended procedure, the C-field being adjusted to the optimum level. No subsequent frequency adjustment is made by off-setting the C-field. In these strictly controlled conditions, the standards are capable of a stability of mean rate over periods of weeks of better than 0.01 microseconds per day. (1in 10l3). The rates of individual standards can differ by a s much as 0.5 microseconds per day, and occasional inexplicable changes of rate of a few tenths of a microsecond per day can occur.'


The West Building shortly after its completion in 1957. The Time Department's control room was in the basement, whose windows cab be seen immediately in front of the row of parked cars. The clock cellars were located beneath them in the sub-basement. The towers on the roof supported an aerial for the reception of time signals. From a photograph published in 1958

Plan of the sub-basement level of the West Building showing the location of the clock cellars (marked clock rms). Each cellar was designed to hold just one clock. The smaller rooms were where the quartz clocks had been installed. It is not presently known which rooms were used for the suite of atomic clocks. From a plan published in 1958

Looking southwards along the clock cellar corridor towards the double doors of the cellar originally constructed for a future atomic clock at the far end. The door on the right corresponds to the door of the battery room in the 1958 plan above. Humphry Smith Photographic Archive


The Greenwich Atomic Time scales


The flying clock experiments


Information relating to the Time Service published from 1956 onwards

Following his appointment as Astronomer Royal, Woolley discontinued the annual publication of the Greenwich Observations with effect from the start of his term of office. From 1956 the results were supposed to appear in one or other of two new publications that were to be issued at irregular intervals:– The Royal Greenwich Observatory Annals, containing long research papers or star catalogues of the kind that had hitherto appeared as separate Observatory publications or appendixes to Greenwich Observations; and Royal Greenwich Observatory Bulletins, containing the more routine results obtained by the various departments. The opportunity was taken at the same time, to combine several departmental reports containing ephemeral matter into a new series entitled Royal Greenwich Observatory Circulars.

Following the merger with the Cape Observatory in 1959, Woolley decided that their publications should also be merged into the RGO series of Bulletins and Annals, but without changing the system of numeration or pagination. To reflect this, the word Greenwich was dropped from the titles, the Bulletins from number 21 onwards, becoming known as the Royal Observatory Bulletins. The original title was restored in 1976 for bulletins number 182 onwards, following the transfer of the Cape Observatory to the Combined South African Observatories (now known as the South African Astronomical Observatory). By virtue of their timing, none of the annals were ever published under the title Royal Greenwich Observatory Annals. But although those of the RGO were all published under the title Royal Observatory Annals, the name change was never actually applied to those of the Cape. These were published as they always had been since 1898, under the title Annals of the Cape Observatory.

Although the bulletins were numbered sequentially, each one was originally assigned to one of five different series (A-E). Within any one series, the bulletins are divided into volumes with the page numbering continuing sequentially from one bulletin to the next – the page numbers being prefixed by the series letter.

The fives series of bulletins were:

A          Meridian work (blue covers) – 1 volume
B          Time Service / Time and Latitude Service (yellow covers) – 4 volumes
C          Solar work (grey covers) – 2 volumes
D          Magnetic work (red/brown covers) – 4 volumes
E          Astrophysical and astrometric papers, and miscellaneous other work – 7 volumes

At the beginning of 1969 the division of the Royal Observatory Bulletins into five series was discontinued. The magnetic results became the responsibility of the Geomagnetism Unit, Institute of Geological Sciences. The Time and Latitude Service results continued to be published not as a Bulletin, but as the Greenwich Time Report. The remainder of the material (i.e the old series A, C and E) were merged into a single series starting with Royal Observatory Bulletin No. 154.

None of the publications are available to consult via the SAO/NASA Astrophysics Data System. Nor have they been made available in digitised form elsewhere. This together with the fact that copies are held in relatively few libraries and archives, means that even today (2020), the information they contain is difficult for most people to access.

When the division of the Royal Observatory Bulletins into five series was discontinued at the start of 1969, the Time and Latitude Service results began to be published as Greenwich Time Report. Although the Reports are not mumbered, the pagination continues from one Report to the next, with a third and seemingly final volume being started for the Reports for 1977 onwards (when the page size was also reduced). Like the Bulletins, each Report covered a period of three months except in 1974 and 1980, when the first six months of the year were combined into one issue. Additionally, the 1973 issue for Jul-Sep contained serious errors and a joint issue for the period Jul-Dec issued in its place. The Report for the second quarter of 1982 appears to have been the last to be published.

Volume Year Jan-Mar Apr-Jun Jul-Sep Oct-Dec
1 1956 1 2 3 4
1957 5 6 7 8
1958 9 10 12 13
1959 15 16 19 20
2 1960 22 32 35 39
1961 47 56 59 62
31 1962 71 74 75 76
1963 79 80 83 86
1964 89 94 96 102
4 1965 105 113 116 123
1966 133 138 140 143
1967 148 149 150 151


1. At the start of Volume 3, the series name was changed from Time Service to Time and Latitude Service. Volume 3 also contained the following three Bulletins:

  • Bulletin 68: Time and Latitude Service 1958-1961
  • Bulletin 81: Photographic Zenith Tube: Instrument and method of reduction
  • Bulletin 92: Results obtained with a Danjon Astrolabe at Herstmonceux

The RGO archives (RGO22) contains 50 Reports covering the period January 1968 to March 1981, The original Report for 1973 (Jul-Sep), was withdrawn and reprinted in the next issue (see above). There is no copy of the withdrawn issue. The Royal Astronomical Society has a larger set which covers the complete period to June 1982, and includes the withdrawn issue from 1973.

As well as the Bulletins and the Greenwich TIme Reports, the Observatory alos published circulars and notices. In the 1976/7 Observatory Report, their frequency of publication was stated as follows:

  • Time Circulars A (weekly) - commenced August 1961
  • Time Circulars A (monthly}
  • Time Service Notices (irregularly)

The Observatory Report for 1968 states that: A new series of Circulars [type unspecified] was inaugurated containing the daily readings or the relative phase difference of the atomic standards and received carriers on selected LF and VLF radio emissions.'


Current location of the clocks

When the Herstmonceux site was vacated in 1990, some equipment from the time department was acquired by both the Science Musuem and the British Horological Institute. On the face of it, Object Number: 1990-303Pt1 at the Science Museum appears to include one of the Observatory's two HP5060A atomic clocks. The equipment at the British Horological Institute's Musuem of Timekeeping appears to include the other HP5060A clock. Details are currently being sought from the two museums.


Further reading

Facilities and services in the Time Department of the Royal Greenwich Observatory. Humphry Smith. Proceedings of the 5th PITI Planning Meeting, NASA Goddard Space Flight Center (Dec 1973)

International Coordination of Atomic Time. Humphry Smith, Vistas in Astronomy (1985)

Frequency of Cesium in terms of Ephemeris Time. W Markowitz, R Glenn Hall, L Essen, JVL Parry, Physical Review Letters (1958)

A Summary of Recent Satellite Time Transfers from the U. S. Naval Observatory to the Royal Greenwich Observatory and to Australia. Humphry M Smith, IEEE Transaction on Instrumentaton and Measurement (1974)

The Evolution of TAI and UTC. Humphry M Smith, IETE Journal of Research, 27:10, 407-414 (1981)

Fifty years of commercial caesium clocks. Leonard S Cutler. Metrologia 42 (2005)

Two clocks that changed the World. The birth of atomic timekeeping. Ray Essen. Antiquarian Horology (June 2013)

Time related activities at the Royal Observatory Greenwich. John Pilkington (1986). When Humphry Smith retired as Head of the Time Department in June 1977, John Pilkngton took over his role. In this paper, published as part of the Proceedings of the Eighteenth Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting, Washington, DC, 2-4 Dec 1986, he presents a bleak outlook for the future of the Department describing how a once World renowned pioneering department at the forefront of technology had evolved into a bit-part player in global timekeeping. Trying to remain diplomatic, he appears to have omitted the fact that the Observatory’s six atomic clocks were being left to run down and stop over the next couple of years.

The Science of Timekeeping. Hewlett Packard Application Note 1289 (1997). Alternative link



'On 1966 May 6. a commercial caesium beam atomic standard (Hewlett Packard 5060A) was installed at Herstmonceux to be known as CsA. A frequency of 100 Hz derived form the caesium-controlled crystal oscillator is connected to the beat counters to give comparisons with the other quartz crystal oscillators of the Time Department.


From 1966 July, G.A. has been determined from the mean of the astomic standard CsA, at Herstmonceux, and a similar standard at the National Physical Laboratory, via the line link. ... ' (Bulletin 140).

Date of installation also included in QJRAS p.249


'A second caesium, standard CsB, was installed at Hertmonceux on 1967 March 3 and brought into regular use on April 1. CsA was withdrawn for maintainance from 1967 April 1 to June 6.' (Bulletin 149, p.B183). Clock model: Hewlett Packard 5060A (Greenwich Time Report, 1968 Jan-Mar, p9)

Date of installation also included in QJRAS p.176


Clock model: Hewlett Packard 5060A (Bulletin) at Observatory by March 1968 (Greenwich Time Report, 1968 Jan-Mar, p.9)

QJRAS at odds withthis Installed in August 1968 and brought into opperational use in November (QJRAS p.125)

CsC seems to be CsA with a new tube.



'A third [fourth?] casesium standard CsDc was installed on August 29 [1968]. This is a Hewlett Packard Model No. 5061A and is running on UTC rate. CsD derived from CsDc by applying the difference in rate between atomic time and UTC i.e. 2.592 msec/day, was brought into opperational use on December 1 [1968]. GA2 was determined from CsB during Octoner and November and from the mean of CSB and CsD in December (Greenwich Time Report, 1968 Oct-Dec, p51)

Loran C receiver installed in December 1968

QJRAS report ofr 1970, p.285: 'There are now four caesium standards at the RGO onw of which has been continually in use since March 1967 on low beam current.



Installed by Feb 1971

'During February  GA2 was determined from the mean of CsB CsD and CsE with rate corrections applied as follows ...' (Greenwich Time Report, 1971 Jan-Mar, p188)

No mention of a fifth clock in the QTEAS reports for 1971 or 1972.



Greenwich Time Report, 1972 Apr-Jun, p.281 states: ' CsG whiich had been installed in it cellar on March 30 running on high beam current ...
CsF, a Hewlett Packard c

CSF mentioned in Report 1972 Jan-Mar which also states (p.264): 'CsD was brought into use on February 13, following a change of rate of 0.07 micro-seconds per day losing, and was taken out of use on March 27 when a new beam tube was fitted, This standard ins now known as CsG

Report 1972 Apr-Jun p.281: 'CsF a Hewlett Packard caesium standard model number 5061A, was received on January 1 and put on test on high beam current; on being moved on March 24, there was a change of rate; kt was installed in its cellar on May 26 and the C-field adjusted on May 31




Greenwich Time Report, 1972 Jan-Mar, p.264 CsD was brought into use on February 13, following a change of rate of 0.07 microseconds per day losing, and was taken out of use on March 27 when a new beam tube was fitted. This standard is now known as CsG.



Report 1973 Jul-Sep Greenwich p.369: 'On August 23 the standard formally known as CsE was returned from the makers with a new tube and will in future be known as CsH. ...'


Mentions B, F, G, H. Erratum. p.369 of Report 1973 Jul-Dec Greenwich atomic time scale Table reading for CsD read CsF



Greenwich Time Report, 1974 Oct-Dec p.61 states: 'CsJ, a Hewlett Packard caesiu, standard model number, 5061A - option 4, was brought into operational use on December.
A fault developed in CsK on November 28 and the standard was returned to the makers for investigation.

Greenwich Time Report, 1975 Jan-Mar p.79 mentions clocks B, F, G & H, but states J had only just started its run, implying it was a new clock (see above)



Greenwich Time Report, 1974 Jul-Sep p.39 mentions clocks B, F and K and states 'CsH was taken out of GA2 for investigation at the begining of July.
'CsK, a Hewlett Packard caesium standard model 5060A, formerly CsC, was supplied with a new beam tube and brought into operational use in April and into GA2 on July 1.'



'A Hewlett Packard caesium standard, model number 5060A, originally installed in 1966 May, was returned from Geneva on 1975 March 19 with its fourth new caesium tube. The standard las known as CsK is now renamed CsL and was switched on on March 21' ((Greenwich Time Report, 1975 Jan-Mar, p80)

'CsG was taken out of use on June 4 and returned to Hewlett Packard for a new caesium beam tube. CsL became operational on April 9' (Greenwich Time Report, 1975Apr-Jun)



Greenwich Time Report, 1975 Oct-Dec p.139:

'CsM, previously known as CsG, was returned from Hewlett Packard in November with a new tube, option 004.'

Greenwich Time Report, 1976 Jan-Mar p.159 mentions clocks as does the Report for the next three months (p.193): CsF, CsH, CsJ, CsL, CsM

Greenwich Time Report, 1976 Oct-Dec p.235 mentions CsB, CsH, CsJ, CsL, CsM and that CsB was returned to Hewlett Packard for a new tube on October 19 and that CsH stopped on October 8 because of a battery failure.


Annual report ending Sept 1976 mentions 6 clocks