The study of polar motion, termed “variation of latitude” at the time of its discovery in the late 19th century by F. Küstner and S. Chandler, is of historical interest for many reasons. From a scientific viewpoint, its discovery must be seen in the context of positional astronomy, geodesy and Earth rotation studies. From an institutional viewpoint, the founding of the International Latitude Service (ILS) in 1899 represents an early case of international cooperation in astronomy, preceding the International Astronomical Union by two decades. In addition to discussing these themes in this review, we analyze in some detail the Gaithersburg (USA) station of the ILS as a case study of the early problems and promise of polar motion studies. With milliarcsecond accuracies, polar motion studies are important today not only for astronomy and geophysics, but also for practical problems like spacecraft navigation and positioning.

Among the immense heritage of Leonhard Euler’s astronomical, mechanical, and mathematical works there has been particularly strong interest in those that deal with principles describing the Earth’s perturbed rotation around its axis. The outstanding meaning of the works is obvious even today, since they give a deep insight into the essence of the phenomena of precessional and nutational displacements of the fundamental coordinate planes and axes constituting astronomical frames of reference.

The discovery by Seth C. Chandler (1891) that the motion of the pole (the reality of which had been established by K.F. Küstner and by the simultaneous latitude observations at Honolulu and Berlin by German astronomers) resulted from two components i.e. a free circular motion with a period of 427 days and a forced elliptical motion with a period of 365.25 days, raised considerable interest in the scientific community of astronomers and geophysicists.

The celebrated Mécanique Céleste of Tisserand (1890) had been published just one year before at a time when doubts still persisted and arguments could be presented in favor of the fixed pole. Starting with Tisserand’s arguments, we describe in this paper the impact of the successive contributions by A. Greenhill, S. Newcomb, Th. Sloudsky, S. Hough, G. Herglotz, A. Love, J. Larmor and H. Poincaré to the solution of the problems raised by the Chandler period.

The lines of reasoning taken by these eminent scientists were rigorously correct so that, after about one hundred years, contemporary researchers, who benefit from a far better knowledge of the inner structure of the Earth and are able to take advantage of modern computing power, do not contradict any of their conclusions and instead refine them with an accuracy which was not imaginable one century ago.

Bessel was not able to detect significant latitude variations in his observation series made between May 1820 and June 1821 at Königsberg Observatory. He left the topic until July 22, 1843, when he suspected that the latitude might have changed since 1820. Between March 1842 and April 1843 Peters, one of Bessel’s students, performed an observation series of the pole’s zenith distance at the Pulkovo Observatory using new observation techniques and reduction methods first developed by Bessel. From this series he deduced a significant value of 0″.079 ± 0″.017 for the amplitude of polar motion, but he was not able to separate the expected 10 month Eulerian period from an annual signal possibly caused (as he supposed) by seasonal temperature variations. Peters’ successors at Pulkovo, Gyldén and Nyrén, continued to observe latitude. Using these observation series Nyrén derived in 1873 significant values for the amplitude of polar motion, but he was not able to determine the phase angles because he assumed the period of polar motion being known. We conclude that Peters probably was the first one to have observed a variation of the latitude, but that he and Nyrén were misled by the Eulerian period when modeling polar motion.

Latitude observations performed at the Paris Observatory from its beginning in 1667 are recalled together with the results mentionned in archives and publications of the time.

Romanian astronomy has an old tradition, which is rather less well-known abroad. More known perhaps are the Dacian sanctuary of Sarmizegetusa Regia, very similar to that of Stonehenge, or the contribution of the monk Dionysius Exiguus (born at Tomis, in Dobrogea) to the initiation of the Christian Era. But one of the most important applications of the astronomical knowledge was coordinate determination. It is interesting to remark that the development of astronomy since the 16th century followed closely the advances of accuracy in estimating the coordinates of the main localities.

Astronomy in Prague has long been associated with Charles University, since its founding in 1348. After a long and complicated history, “The State Observatory” was established in the former Jesuit college of Klementinum. The polar height was measured at this location in the last century, during the periods 1889 – 1892, 1895 – 1899 and 1900 – 1904. The observations were made by Gerstner, Laska, Spitaler and others, under the leadership of Prof. Lad. Weinek. The measurements were performed with a transit instrument. The inspection was made in the Archive of Academy of Sciences, where practically all the observations are still preserved.

Since Euler in 1765 predicted the possibility of polar motion, the search for an observational confirmation went on. While some indications were found before, the proof without doubt is generally attributed to Küstner. Ironically, the aim of Küstner’s precise observations of 1884–1885 in Berlin was another one, namely an improved value of the constant of aberration. Using the Horrebow-Talcott method, he could conclude that the latitude of Berlin had decreased significantly by 02 in about one year. His findings promoted the start of international cooperation to observe the polar motion. Measurements from Honolulu in 1891 exhibited the effect in counterphase, thus omitting the last chance for a local explanation.

It took Seth Carlo Chandler Jr. just one year to discover the 14 month and annual components of the variation of latitude (polar motion) that had eluded the most talented astronomers for more than a century. He succeeded where those before him had failed because: he used a global set of observations that had been painstakingly collected by astronomers of many nationalities over more than 150 years; he ignored the common wisdom that unexpected annual variations in astronomical observations were always caused by temperature effects on instruments or atmospheric refraction anomalies; and he was not misled by the theory, which predicted that the Eulerian wobble must have a period of approximately 10 months. Chandler was a true amateur only in the sense that he pursued astronomy out of love, while making his living as an actuary. He began his scientific training at the youthful age of 15 under the tutelage of Benjamin Pierce, perhaps the greatest American mathematician of that time. He then became: a private assistant to Benjamin Apthorp Gould, Jr., an aide to Gould while employed by the US Coast Survey, and Assistant Editor to Gould for the Astronomical Journal. It would be impossible to understand and appreciate Chandler’s achievements without recognizing his relationship with Gould, his brilliant mentor and lifelong colleague — the man he once described as his “Magnus Apollo.”

In 1901 Chandler announced that the 14 month motion was not a simple oscillation but was in itself a compound motion consisting of the previously discovered 428 day component, and a much smaller 436 day component, whose reality was “beyond reasonable doubt.” The beating of two such components would result in a rapid change in the phase and amplitude of the Chandler motion at intervals of about 80 years. If Chandler was correct, in about 2010, give or take perhaps five years, we should see the next occurrence of this phenomenon. With the International Earth Rotation Service in operation there should be no difficulty in detecting and quantifying such an event, very nearly one century after Chandler’s death.

Around 1880, and for some time after that, the possibility of revealing a variation in latitude as a consequence of a separation of the instantaneous rotation axis from the Earth’s axis of inertia gave rise to much perplexity due to the complexity of the problem and the existence of non-negligible and hard-to-find systematic errors in observations of a personal and instrumental nature. These errors also depended on effects of refraction and imprecise knowledge of star declinations.

To this must be added the fact that the very idea that the rotational axis and the axis of inertia were distinct and in relative motions raised difficult problems of a physical and theoretical nature. At that time the idea of the Earth’s rigidity was still generally accepted and, even admitting the hypothesis of an Earth endowed with sufficient elasticity and plasticity, the theory of which had been partly examined by G.H. Darwin, it was then almost impossible, just as it still is today, to create a model of the movements of mass inside the Earth which could offer an explanation of possible aperiodic and secular variations. In reality, more than on the existence of periodic variations, the attention and interest of geodesists and astronomers was in those years focused on the problem of the existence or non-existence of secular variations in the Earth’s principal axis.

The paper reviews in what ways the Geodetic Institute Potsdam contributed to the foundation of the International Latitude Service (ILS) and took part in its activities through the work of Helmert, Albrecht, Wanach and Mahnkopf. Based on international scientific cooperation, the results were the rectangular coordinates of the polar motion from 1890.0 to 1922.7 at 0.1 year intervals, in particular those derived from the latitude observations at independent stations from 1890.0 to 1899.8, and those derived from the latitude observations at the ILS stations from 1899.9 to 1922.7.

The history of the International Latitude Service and the International Polar Motion Service is described, with an emphasis on the evolution of the international cooperation for monitoring the Earth’s rotation. Also given is a brief description of the history of the Japanese geodetic and astronomical activities, as well as the future prospects.

The Tashkent Astronomical Observatory (Ulugh Beg Astronomical Institute of the Uzbek Academy of Sciences since 1966) was involved to the International Latitude Service (ILS) in September 1899, when the regular latitude measurements were started at Tschardjui (ϕ = +39°08′11″,λ = −63°29′). Because of the Civil War in Central Asia, the latitude observations were ended at Tschardjui in May, 1919. A new latitude station was established at Kitab (Uzbekistan) at latitude ϕ = +39°08′40″ and longitude λ= −66°53′, in November 1930. The Kitab Latitude Station was equipped with two visual zenith telescopes (1929, 1957) and the photographic zenith tube removed from the Pulkovo observatory in 1978. The Kitab station provided the ILS with more than 250000 instantaneous latitude measurements. Since the early 90s Kitab has hosted the ground-based beacons of GPS, PRARE and DORIS satellite tracking systems, providing the IERS data center with precise coordinates at sub daily frequency.

The history of the Carloforte ILS Station is also the history of all the people who have spent part of their life working there. Table 1 shows the names of these people from the beginning of the service till its end.

Running our eyes through the pages of the Carloforte ILS Station Observing books, we can read, beyond the cold figures that refer to the star transits, a lot of notes and comments that allow us to understand the high degree of humanity, scientific reliability and self-abnegation of the observers. Many of them, after their experience at Carloforte, had continued their activity as researchers in other fields of the Astronomy; just as an example, we may cite Bianchi, Volta, Martin, Righini, that respectively became Directors of the Astronomical Observatories of Brera, Turin, Trieste, Arcetri. We also may cite Camera, Cecchini, Nicolini, that played important roles in the Central Bureau of ILS when this one was awarded to Italy. A few on the list were just temporary observers, and their history outside of the Observatory remains unknown for us.

At the beginning of the 20th century, the development of radio time signals offered the possibility of unifying the measurements of Universal Time (UT). A new service, the Bureau International de l’Heure, hosted by the Paris Observatory, was entrusted with this task. The BIH began its operation in 1912, although its statutes were officially settled only in 1919. This paper recalls the activities of the BIH on time determination and various connected fields: Earth rotation, reference frames, atomic time. With the importance of atomic time and the emergence of new techniques for the measurement of Earth rotation, a new organization was required. It was prepared by the MERIT and COTES programs (1978) and by the bodies of the Metre Convention, with the active participation of the BIH. The BIH was dissolved at the end of 1987, its work being shared between the Bureau International des Poids et Mesures and the new International Earth Rotation Service.

Evgeniy Pavlovich Fedorov (1909–1986) was President of Commission 19 of the IAU in 1955–1961. In this period the future of the ILS was discussed very actively and the plan for future study of the polar motion was elaborated. E. Fedorov was the organizer and the leader of these preparatory works.

He was editor of the collection of papers “On the present state and the future of latitude investigations” (1960, Moscow). In this collection well-known scientists discussed their opinions on the state latitude observations and their improvement. Among them where: H. Spencer Jones, P. Melchior, T. Hattori, I. Fleckenstein, N. Sekiguchi, W. Munk, R. Vicente, A. Young, E. Fedorov, M. Torao, N. Stoyko. In that time Fedorov wrote on the problem of the ILS. The inception of the ILS was preceded by a period of thorough discussion of various aspects of latitude work. By the end of the 19th century polar motion had become a problem of vital importance in astronomy and geophysics. Many efforts were made at that time to find the best means for obtaining the most accurate polar coordinates. As a result a plan of international cooperation for the study of polar motion was worked out.

Project MERIT was an international programme to Monitor Earth Rotation and Intercompare the Techniques of observation and analysis. It was conceived by a working group that was set up by the International Astronomical Union in 1978 and was carried through with additional support from the International Union of Geodesy and Geophysics. The first objective was to encourage the development of the use of new techniques, such as laser ranging and radio interferometry, for the regular determination of universal time and polar motion. A successful ‘short campaign’ of observations by six techniques was carried out during the period 1980 August to October. Operational and analysis centres were set up for each technique and a coordinating centre was established at the Bureau International de l’Heure (BIH). The results were reported and discussed at the first MERIT Workshop in 1981.

The preparations for the ‘main campaign’, which was held from 1983 September 1 to 1984 October 31, and the plans for the activities that were to follow it were reviewed at the second MERIT Workshop in 1983. Important additional features of the campaign included the use of ‘MERIT Standards’ for the reduction and analysis of the data, the use of electronic techniques for the distribution of data, the comparison of the results with the changes in the angular momentum of the atmosphere and special emphasis on the improvement of the terrestrial reference frame. Proposals for a new International Earth Rotation Service were prepared at the third MERIT Workshop in 1986. The MERIT programme was continued from 1984 until the new service formally started on 1988 January 1. Over the decade the accuracy of the Earth-rotation parameters improved considerably.