Under One Sky: The IA U Centenary Symp osium
P roceed i n g s I A U S y m po s i u m N o . 349, 2019
C. Sterken, J. He arnshaw & D. V al ls-Gab aud, e ds.
c
 In ternational Astronomical Union 2019
doi: 10.1017/S1743921319000462
On the ev e of the 100th anniv ersary of IA U
Commission 19/A2 “Rotation of the Earth”
Zino vy Malkin 1 , Ric hard Gross 2 , Dennis McCarth y 3 ,
Aleksander Brzezi ´ nski 4 , Nicole Capitaine 5 ,V ´ eronique Dehan t 6 ,
Chengli Huang 7 , Harald Sc h uh 8 , Jan V ondr´ ak 9 and Y arosla v Y atskiv 10
1 Pulk ov o Observ atory RAS, Kazan F ederal Unive rsit y , Russia, email: [email protected]
2 Jet Propulsion Lab oratory , California Institute of T echnology , USA,
email: [email protected]
3 U.S. Na v al Observ atory , con tractor, USA, email: dennis [email protected]
4 W arsaw Univ ersit y of T echnology , P oland, email: [email protected]
5 SYR TE, Observ atoire de P aris, Univ ersit´ e PSL, CNRS, Sorb onne Univ ersit ´ e, F rance,
email: [email protected]
6 Ro y al Observ atory Belgium, Belgium, email: [email protected]
7 Shanghai Astronomical Observ atory CAS, China, email: [email protected]
8 Helmholtz Cen ter P otsdam – GFZ German Researc h Centre for Geosciences, T ec hnisc he
Univ ersit¨ at Berlin, German y , email: [email protected]
9 Astronomical Institute CAS, Czec h Republic, email: [email protected]
10 Main Astronomical Observ atory NASU, Ukraine, email: [email protected]
Abstract. IA U Commission 19 b egan in 1919 with the birth of the IA U at the Brussels
Conference, where Standing Committee 19 on Latitude V ariations w as established as one of
32 standing committees. A t the first IA U General Assembly in 1922, Standing Committee
19 b ecame Commission 19 “V ariation of Latitude”. In the b eginning, the main topic of the
Commission w as the inv estigation of p olar motion. Later, its activities included observ ations
and theory of Earth rotation and connections b et w een Earth orientation v ariations and geo-
ph ysical phenomena. As a result, in 1964 at the XI I IA U General Assem bly , the Commission
w as renamed “Rotation of the Earth”. The in v estigation of Earth orien tation v ariations is pri-
marily based on observ ations of natural and artificial celestial ob jects. Therefore, main tenance of
the in ternational terrestrial and celestial reference frames, as w ell as the co ordinate transforma-
tion b et ween the frames and the impro v emen t of the mo del of precession/nutation, ha v e alw ays
b een among the primary Commission topics. In 1987, the IAU through Commissions 19 and
31 “Time” established, join tly with the International Union of Ge o desy and Ge ophysics ,w h a t
is no w kno wn as the International Earth Rotation and Reference Systems Service. Commission
19 con tin ued to work to dev elop metho ds to improv e the accuracy and understanding of Earth
orien tation v ariations and related reference systems and frames as well as theoretical studies of
Earth rotation. In 2015, Commission 19 w as renew ed as Commission A2 “Rotation of the Earth”
con tin uing Commission 19’s functions and linking the astronomical comm unit y to other scien-
tific organizations suc h as the International Asso ciation of Ge o desy, International VLBI Servic e
for Ge o desy and A str ometry, International GNSS Servic e, International L aser R anging Servic e
and International DORIS Servic e . During its en tire history , IA U Commission 19/A2 has alw a ys
w ork ed in close co op eration with these and other related services to improv e the accuracy and
consistency of the Earth orien tation p arameters and celestial and terrestrial reference frames.
Keyw ords. Earth, astrometry , reference systems and frames, time, history and philosophy of
astronom y , geo desy , geoph ysics
325
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326 Z. Malkin et al.
1. In tro duction
Earth rotation is not uniform. The Earth’s rotational sp eed is not constan t and this fact
leads to v ariations in the length of the day (LOD). The excess LOD, whic h is defined as
the difference b et ween the observ ed LOD and nominal v alue of 86 400 seconds, amoun ts
to milliseconds. The rotation axis also mo v es with resp ect to the Earth’s crust due to the
unequal and v ariable distribution of mass, and this phenomenon is referred to as p olar
motion. Observ ed irregularities in the rotation of the Earth hav e complicated structure
and sho w b oth s ecular and quasi-p erio dic v ariations at time scales ranging from sub-
daily to decadal. These v ariations are affected b y pro cesses acting within the interior
of the Earth, suc h as core-mantle in teraction and glacial isostatic adjustmen t and b y
pro cesses acting at the surface of the Earth, suc h as fluctuations in the transp ort of
mass within the atmosphere and o ceans, called geoph ysical fluids. Studying the Earth’s
time v arying rotation can therefore b e used to gain greater understanding of global-scale
pro cesses.
In addition, the gra vitational attraction of the Sun, Mo on and planets acting on the
Earth causes the rotation axis to c hange its orien tation with resp ect to the celestial
reference system. This motion is c haracterized by precession, the slo w con tinuous c hange
in orien tation causing a circular trace of the pro jection of the Earth’s axis in space with
a p erio d of 25 772 years, and n utation, the smaller, more rapid motion of the direction
of the axis in space comp osed of a large n umb er of p erio dic terms. All of these motions
are describ ed b y the Earth rotation angle, the angular co ordinates of the Earth’s p ole
in a terrestrial reference frame, and the angular co ordinates of the celestial p ole in a
celestial reference frame (precession-n utation angles). T ogether, these are referred to as
Earth orien tation parameters (EOP). F or practical purp oses, celestial p ole offsets, whic h
are the difference b et ween actual p osition of the axis and its p osition predicted b y an
adopted mo del of precession-n utation, are computed and pro vided to users instead of the
full precession-n utation angles.
The Earth rotation is studied with resp ect to the terrestrial and celestial co ordinate
systems. Reference systems are critical, not only for astronomers but also for geo desists.
A t its very beginning, IAU Commission 19 on Latitude V ariations w as established sim ul-
taneously with an analogous commission of the International Asso ciation of Ge o desy
(IA G), and b oth organizations supp orted the activities of the International L atitude
Servic e (ILS). During its en tire history IA U Commission 19 has w ork ed in close co op era-
tion with the International Union of Ge o desy and Ge ophysics (IUGG), the IA G, and the
International Earth R otation and R efer enc e Systems Servic e (IERS) and its predeces-
sors, the ILS, IPMS, and the Bur e au International de l’Heur e (BIH), in order to impro v e
the accuracy and consistency of the EOP and terrestrial and celestial reference frames.
Kno wledge of the EOP is needed to relate the terrestrial and celestial reference frames.
The terrestrial reference frame is attac hed to the solid Earth and its orientation with
resp ect to the celestial frame c hanges as the Earth rotates. Knowing the relativ e orien-
tation of the terrestrial and celestial reference frames and ho w it v aries in time allo ws
the p ositions of b oth ground-based and Earth orbiting ob jects to b e known in b oth
frames.
Earth rotation is therefore an in terdisciplinary topic that bridges astronom y , geo desy
and geoph ysics. Precise Earth orientation parameters are needed to p osition and na vigate
ob jects on Earth and in space, and the analysis of Earth rotation v ariations pro vides
imp ortan t information ab out the in teractions among the v arious comp onen ts of the Earth
system. This pap er briefly describ es the organizational history and scien tific activit y of
IA U Commission 19 during its nearly 100-year-long history . It is mainly based on the
IA U Rep orts for the p erio d from 1922 through 2015 ( IA U T ransactions 1922–2015 ).
available at https://www.cambridge.org/core/terms . https://doi.org/10.1017/S1743921319000462
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100th anniversary of IA U Commission 19/A2 327
2. Ov erview of Commission history and activit y
The story of IA U Commission 19 b egins in 1919 at the Brussels Conference where
the IA U, comprised of 32 Standing Committees on differen t branc hes of astronom y , w as
established. One of the 32 w as Standing Committee 19 on L atitude V ariations ch a i r e d by
Hisashi Kim ura, Director of the International Latitude Observ atory , Mizusa w a, Japan. In
1922, at the first IA U General Assembly , all Standing Committees b ecame Commissions
and th us IA U Commission 19 V ariation of L atitude w as established.
During its first y ears, the main topic of the Commission w as the in vestigation of p olar
motion, primarily using observ ations made at the International Latitude Service (ILS)
stations lo cated at the latitude 39 ◦ 08  N, referred to as the North parallel. According
to the first Commission 19 c harter, the main tasks were: con tin uation of observ ations
on the North parallel, compilation of programs of observ ations, unification of reduction
metho ds, dev elopmen t of new instruments and methods for future observ ations, and pro-
viding financial supp ort for these activities. The main scien tific problems discussed at
Commission meetings w ere non-p olar astronomical latitude v ariations, station motion,
atmospheric impact on the results of latitude observ ations, impro v emen t of astronom-
ical mo dels affecting the ILS results suc h as n utation, precession and ab erration, and
impro vemen t of star p ositions used for the computation of latitudes. Many of these tasks
remain of in terest to day , despite revolutionary c hanges in the observ ational techniques
used for Earth rotation studies.
In the b eginning of the 20th cen tury , p olar motion wa s the only irregularit y in the
Earth rotation that could b e measured reliably . As it b ecame clear that there w as a
wide sp ectrum of v ariations in the rotational sp eed, routine observ ations of the Earth
rotation b egan to b e made b y determining adjustmen ts to mec hanical clo c ks based on
optical star observ ations. The BIH b ecame resp onsible for the co ordination of these time
observ ations, and in 1955 b egan to mak e its own determination of p olar motion in order
to impro ve the observ ations of astronomical time. In 1962 the ILS w as sup erseded b y the
In ternational Polar Motion Service (IPMS) in recognition of the con tributions to p olar
motion made b y stations other than those of the original ILS. More detail on the history
of the ILS and IPMS can b e found in Y o koya m a , M a n a b e & S a k a i ( 2000 ).
The Commission’s activities expanded to include the effects of p olar motion on the
time observ ations, to manage the increasing num b er of stations p erforming latitude and
time observ ations and to d ev elop the theory of Earth rotation. The connection b e t w een
v ariations in the Earth rotation and v arious geophysical phenomena and in tegrating
geoph ysical and meteorological data in the analysis and prediction of EOP also came to
b e one of the most imp ortan t topics of the Commission’s discussions. As a result, in 1964
at the XI I IA U General Assembly , the Commission was renamed R otation of the Earth
and included sev eral functions of Commission 31 Time relating to the determination of
Univ ersal Time.
Un til the 1970s, the optical astronomical observ ation of stars w as the only tec hnique
a v ailable to determine p olar motion and Universal Time used to describe the Earth r ota-
tion. After that, new observing tec hniques based on observ ations of artificial satellites,
suc h as Doppler satellite trac king, Global P ositioning System (GPS), Satellite and Lunar
Laser Ranging (SLR), and Doppler Orbitograph y and Radiop ositioning In tegrated b y
Satellite (DORIS), as w ell as Lunar Laser Ranging (LLR) and observ ation of extragalac-
tic radio sources with V ery Long Baseline Interferometry (VLBI) w ere dev elop ed that
so on pro ved to b e capable of determining the EOP and realizing reference systems m uch
more accurately . In the 1980s members of Commission 19 were instrumen tal in organiz-
ing Pro ject MERIT to compare the p ossible contributions of the dev eloping tec hniques.
T o co ordinate these efforts, a new service, the IERS (kno wn then as the In ternational
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328 Z. Malkin et al.
Earth Rotation Service) w as established in 1987 b y the IAU and IUGG com bining the
op erations of the IPMS and the Earth Rotation Section of the BIH ( Wilkins 2000 ). In
recognition of the imp ortance of accurate reference systems in studying the Earth rota-
tion the IERS w as renamed the International Earth R otation and R efer enc e Systems
Servic e in 2003. The IERS pla ys an imp ortan t role in the op erational and long-term
determination and dissemination of the EOP and in the establishmen t and maintenance
of the terrestrial and celestial reference systems and frames based on space geo desy
observ ations. By the end of the 1990s, sp ecific tec hnique services were established, suc h
as the International GPS Servic e (IGS, no w International GNSS Service), International
VLBI Servic e for Ge o desy and Astr ometry (IVS), International L aser R anging Servic e
(ILRS) and International DORIS Servic e (IDS). F rom the b eginning of their activities,
Commission 19 has w ork ed in close co op eration with these tec hnique services.
Describing Earth orien tation v ariations requires clear and accurate definitions of the
terrestrial and celestial co ordinate systems and frames in whic h the Earth motion is
giv en. The Commission has alw ays b een activ e in this area and pla ys a leading role in
main taining the International Celestial R efer enc e F r ame (ICRF) and the In ternational
T errestrial Reference F rame (ITRF) in partnership with the IERS and IA G. F or example,
in 1967 the concept of the Conventional International Origin (CIO) was adopted b y the
IA U and IUGG. More recently , commission members hav e con tributed to the mainte-
nance of the celestial reference system and frame. In 1997, the new ICRS definition and
its realization, ICRF, w ere adopted b y the XXI I I General Assem bly of the IA U, follo w ed
b y ICRF2 in 2009. ICRF3, the latest realization of the ICRS, was completed in 2018 and
adopted b y the XXX General Assem bly of the IA U in August, 2018.
The in vestigation of Earth orien tation v ariations is primarily based on observ ations of
natural and artificial celestial b o dies, such as stars, extragalactic radio sources, and arti-
ficial Earth satellites. Therefore, the accuracy of the co ordinate transformation b etw een
terrestrial and celestial reference systems is of primary imp ortance. F or this reason,
impro ving the mo del of precession/nutation has alw a ys b een among the Commission’s
discussion topics. During its first decades, latitude observ ations w ere used to impro v e
the n utation c onstan t. In the 1970s, IA U Commission 19 and the IERS join tly promoted
the new IA U 1980 nutation theory . In the 2000s, the IAU2000/2006 mo del of preces-
sion/n utation w as adopted ( IA U Resolutions 1922–2018 ). This mo del currently pro vides
the b est accuracy for relating the con v entional reference frames, namely ICRF and ITRF.
T o da y , the Commission’s ob jectiv es include:
• Encouraging and dev eloping co op eration and collab oration in observ ation and theo-
retical studies of Earth orien tation (the motions of the p ole in the terrestrial and celestial
reference systems and the rotation ab out the p ole).
• Linking the astronomical comm unit y to the official organizations pro viding the ITRS,
I T R Fa n dE O P :I A G ,I E R S ,I V S ,I G S ,I L R Sa n dI D S .
• Dev eloping metho ds for impro ving the accuracy and understanding of Earth
orien tation and related reference systems/frames.
• Ensuring agreemen t and contin uit y of the reference frames used for determination
of EOP with other astronomical reference frames and their densification.
• Pro viding means of comparing observ ational and analysis metho ds and results to
ensure accuracy of data and mo dels.
F ollo wing the reorganization of the IA U adopted at its XXIX General Assem bly in
2015, Commission 19 w as ab olished and Commission A2 with the same name R otation
of the Earth was established.
The Commission has b een c haired by man y distinguished scien t ists. Figure 1 presen ts
all the Presiden ts of IAU Commission 19/A2 during its history .
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100th anniversary of IA U Commission 19/A2 329
Figure 1. Presiden ts of IA U Commission 19/A2.
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330 Z. Malkin et al.
IA U Commission 19/A2 has organized, co-organized and participated in man y IA U and
in ter-union working groups relev an t to its field of interest. Eac h w orking group serves as
a platform for discussions and in teractions of sp ecialists on the most imp ortan t topics of
the Commission. During the last 20 y ears these are:
• Join t IAU/IA G W orking Group “Theory of Earth rotation and v alidation” (2015–).
• Join t IAU/IA G W orking Group “Theory of Earth rotation” (2012–2015).
• IA U W orking Group “Third Realization of ICRF” (2012–2018).
• IA U W orking Group “Second realization of ICRF” (2003–2009).
• IA U W orking Group “Nomenclature for F undamen tal Astronom y” (2003–2006)”.
• IA U W orking Group “Numerical Standards for F undamen tal Astronomy” (2006–).
• IA U W orking Group “Definition of Co ordinated Univ ersal Time” (2001–2006).
• IA U W orking Group “Precession–n utation” (2000–2003).
• IA U W orking Group “Reference Systems” (1997–2000).
• Join t IAU/IUGG W orking Group “Nonrigid-Earth Nutation Theory” (1994–2000).
IA U Commission 19/A2 also organized or co-organized with other IA U Commissions
and other comm unities and institutions many IA U sp onsored and co-sp onsored scien tific
meetings, suc h as
• Journ ´ ees “Syst ` emes de R ´ ef´ erence et de la Rotation T errestre” (2017).
• Join t IAU/IA G/IERS Symp osium “Geo desy , Astronomy & Geoph ysics in Earth
Rotation” (2016).
• 23 Journ ´ ees “Syst` emes de R ´ ef´ erence Spatio-T emp orels” (1988–2014).
• Join t D iscussion 7 at the XXVI I I IA U General Assembly (2012).
• Join t IAU/GGOS W orkshop “Observing and Understanding Earth Rotation” (2010).
• Join t D iscussion 16 at the XXVI GA (2006).
• Join t Discussion 16 at the XXV IAU General Assem bly (2003).
• Join t D iscussion 2 at the XXIV GA (2000).
• IA UC 180: T o w ards mo dels and constan ts for sub-microarcsecond astrometry (2000).
• IA UC 178: Polar Motion: Historical and Scien tific Problems (1999).
• IA US 156: Developmen ts in Astrometry and their Impacts on Astroph ysics and
Geo dynamics (1992).
• IA UC 127: Reference Systems (1990).
• IA US 141: Inertial Co ordinate System on the Sky (1989).
• IA US 128: Earth’s Rotation and Reference F rames for Geo desy and Geo dynamics
(1986).
• IA UC 63 : High-Precision Earth Rotation and Earth-Mo on Dynamics (1981).
• IA UC 56 : Reference Co ordinate Systems for Earth Dynamics (1980).
• IA US 82 : Time and the Earth’s Rotation (1978).
• IA US 78 : Nutation and the Earth’s Rotation (1977).
• IA UC 26 : On Reference Co ordinate Systems for Earth Dynamics (1974).
• IA US 48 : Rotation of the Earth (1971).
• IA US 13 : The F uture of the In ternational Latitude Service (1960).
• IA US 11 : The Rotation of the Earth and A tomic Time Standards (1958).
As a result of these scien tific discussions and w orking group activities, the Commission
initiated or supp orted tens of recommendations and IA U Resolutions related to its field
of in terest and resp onsibility . These resolutions relate to the organization of Earth orien-
tation observ ations, computation of the EOP , and dev elopmen t of astronomical mo dels,
reference systems and frames, and time scales. Through the IA U resolutions prop osed
or supp orted b y the Commission, suc h organizations as the ILS Cen tral Bureau, IPMS
and IERS w ere established or maintained, new precession-n utation mo dels were a dopted,
three ICRF realizations w ere endorsed, new dynamical time scales along with barycen tric
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100th anniversary of IA U Commission 19/A2 331
and geo cen tric space-time co ordinates w ere defined and the concept of Non-Rotating
Origin (NR O) was in tro duced.
3. Conclusion and outlo ok
IA U Commission 19/A2 has a long and proud history of activ e in v olvemen t in the
IA U. Starting with inv estigations of latitude v ariations, mostly based on astronomical
observ ations taken at stations on the 39 ◦ 08  North parallel. No w, at its 100th anniv ersary
the Commission has extended its activities to man y topics related to the in vestigation of
all asp ects of the theory and observ ations of Earth rotation and the establishmen t and
main tenance of the terrestrial and celestial reference systems and frames as w ell as time
scales. Summarizing the Commission’s activities during the nearly 100-y ear length of its
history , w e can sa y that:
• IA U Commission 19/A2 has play ed and con tin ues to play an imp ortan t role in the
IA U’s activity of coordinating international coop erativ e efforts to impro v e our kno wledge
of the Earth rotation and to establish and main tain the in ternational terrestrial and
celestial reference systems and frames.
• IA U Commission 19/A2 initiated or supp orted man y imp ortant resolutions related to
the theory of precession-n utation, terrestrial and celestial reference systems and frames,
time scales, and other topics of general scien tific and practical in terest.
• IA U Commission 19/A2 co-organized and collab orates with sev eral international
s e r v i c e ss u c ha st h eI E R S ,I G S ,I V S ,I L R Sa n dI D S .
• IA U Commission 19/A2 organized or co-organized many w orking groups including
in ter-commission and in ter-union ones.
• IA U Commission 19/A2 works in close coop eration with other international organi-
zations suc h as the IUGG and IAG and th us pro vides the necessary link b et ween these
organizations and the IA U.
The Commission will con tin ue to work closely with the services, commissions, in ter-
commission committees and Glob al Ge o detic Observing System (GGOS) of the IA G as
it has in the past. In this manner, the astronom y communit y is k ept abreast of progress
made b y the geo detic comm unity in observing and understanding the Earth’s rotation,
and vice v ersa.
References
IA U Resolutions, www.iau.org/administration/resolutions/general assem blies
IA U Rep orts 1922–2015, T r ansactions of the IA U , vv. I–XXIX
Y oko y ama, K., Manab e, S. & Sak ai, S. 2000, History of the International Polar Motion
Servic e/International L atitude Servic e ,i n : Pr o c. IA UC 178 “Polar Motion: Historic al and
Scientific Pr oblems” , eds. S. Dic k, D. McCarth y , B. Luzum, 147
Wilkins, G. 2000, Pr oje ct MERIT and the F ormation of the International Earth R otation Servic e ,
in: Pr o c. IA UC 178 “Polar Motion: Historic al and Scientific Pr oblems” ,e d s .S .D i c k ,
D. McCarth y , B. Luzum, 187
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