Recent Advances in Positional Astronomy

A. KOPFF Astronomisches Rechen-Inst i tut , Heidelberg, Germany

SUMMARY The present contr ibut ion of German astronomers to the foundations of a s tudy of the physical and kinematic structure of the stellar system is described, with special reference to the development of the "Fundamenta lka ta log" and the repetition of the Catalogues of the Astronomische Gesellschaft.

IN my GEORGE DARWIN Lecture to the Royal Astronomical Society, in 1936 ("Star catalogues, especially those of fundamental character"), I concluded with these words: "We must continue to study the motions in the Universe, both near and distant, more exactly and more in detail; and we are glad that we can go back to our star catalogues--often a little despised--as the foundation of our science".

In contrast to the att i tude of resignation which at that time was perhaps justified, the present period has directed its attention again much more to the objects of positional as t ronomy-- the determination of stellar positions on the sphere and of the apparent proper motions of the stars.

The subsequent discussion will be essentially restricted to the contribution of German astronomers to this development. There was a time in the nineteenth century in which they played such an important r61e in our field that NEWCOMB could speak of a "German School of Practical Astronomy". Its founder was Bessel, who on his part built on the great achievements of the Royal Observatory at Greenwich. The last great representative of this School was ARTHUR AUWERS (1838--1915). To him we owe the first modern "Fundamentalkatalog" (FC), the essential basis of which was a new reduction of the obervations of BRADLEY. His work inspired the international enterprise of the "Kataloge der Astronomischen Gesellschaft" (AGK), catalogues which are based on the FC, in which the zone Berlin A (declination ~- 15 ° to -~ 20 °) was observed and reduced by AUWERS in a fashion that can be considered as exemplary, even to-day. Finally, his initiative gave rise to the "Geschichte des Fixsternhimmels" (GFH) ; this title was inspired by the star list in FLAMSTEED'S Historia Coelestis Britannica of 1725 and by the Histoire Cgldste of LALANDE, which was completed a century later. These remarks show the range of work which is to-day an integral part of the German contribution to positional astronomy; this work always goes back to AUWERS.

A summarizing discussion of the papers published in the years 1939-46 has been given by A. KOttLSCHOTTER (1948) in the Fiat Reports. Referring to this we shall here concentrate only on recent further developments.

1. THE FUNDAMENTAL CATALOGUE

The "Neuer Fundamentalkatalog des Berliner Astronomischen Jahrbuchs" (NFK) originated from the FC which AUWERS produced in various steps; it was carried out in collaboration with PETERS (1907); KOPFF (1937, 1938) and his collaborators, in particular HEINEMANN and KAI-IRSTEDT, then extended it to the "Drit ter

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1006 Recent advances in positional astronomy

Fundamentalkatalog des Berliner Astronomischen Jahrbuchs" (FK3). We add some remarks to the discussion in the Fiat Report. Since this improvement of the NFK was needed for the transit circle observations required for the repetition of the AGK, it was advisable to form first an interim system which would give the individual corrections of the stars in the N F K virtually in its own system as A+ and A~S, respectively for the two hemispheres (KoPFF, 1928, 1931). In this interim system therefore the co-ordinate system as fixed by the NFK has been maintained; on this the improvement of the system was then based.

Such a division of the improvement of the N F K proved useful also for fundamental reasons. For the individual correction within a given system we find that particu- larly the old catalogues, with known systems, are valuable because of the long time that has elapsed since their epoch; the same applies to the new differential catalogues.

Only catalogues containing absolute observations are usable for the improvement of the system of a Fundamental Catalogue, that is those which give right ascensions referring to observations of the Sun for the determination of the Vernal Equinox, and whose declinations are derived from absolute measures of zenith distances. The old catalogues, say, those observed before about 1845, could not be used for a systematic improvement of the N F K because of their very faulty systems.

This exclusion of the older catalogues is the main advantage of the system of the FK3 (KoPFF, 1949) as compared with the system of the General Catalogue (GC) by B. Boss (1937). The reliability of the FK3 is also shown by the various investigations of H. It. MORGAN (1948, 1949, 1951) and by a comparison with his catalogue N30 (1952). Various comparisons of the FK3 with the GC are available, amongst them extensive tables by KOPFF (1939a, 1939b).

Because of the rather small number of stars in the NFK, an extension of the original catalogue of the "Auwers stars" was carried out by including additional stars, "Zusatzsterne" (KOPFF, 1937, 1938), which had to fill the gaps of the NFK; their accuracy was often less than that of the Auwers stars. The observer will make good use of the mean errors, given together with the mean places in the annually published "Apparent Places of Fundamental Stars" of the Nautical Ahnanac Office of the Itoyal Greenwich Observatory.

I t must be emphasized that by this development of AUWERS' FC the "Berliner Jahrbueh" and the "Astronomisch-Geodi~tisches Jahrbuch", published by the Astronomisches Itechen-Institut in Heidelberg, have preserved a high degree of continuity which is an essential factor in positional astronomy. Of course, a further development of the FK3 is necessary; this, however, must be carried out as a unified operation, not as patchwork.

2. THE REVISION AND EXTENSION OF THE FK3

Already considerable observational material is available for the improvement of the FK3. We shall not mention it here in detail, but rather refer to the yearly compila- tions of the Astronomischer Jahresbericht (1951), published by the Astronomisches Rechen-Institut in Heidelberg. Series of observations have been published in Ger- many by PAVEL (1947) and LABITZKE ( 1949, 1951). Furthermore, numerous observa- tions of fundamental stars can be found in the appendix of the AGK2A ("Katalog der Anhaltsterne fiir das Zonenunternehmen der ASTRONOMISCttEbl GESELLSCttAFT", 1943). Also more extensive still unpublished series of observations are mentioned

A. KOPFF 1007

in the Fiat Review (KoHLSCRi)TTER, 1948). The meridian instruments of Munich are the only ones which have at present included the stars of the FK3 into their programme. The transit circles of Hamburg-Bergedorf and Heidelberg have been mobilized for the observation of faint stars (see the concluding paragraph of this section), while the transit instruments of Bonn and Babelsberg (meridian circles of Pistor and Martins) are not in use at present.

The number of stars which require rather large individual corrections is not very great, as can be seen from a comparison with H. R. MORGAn'S catalogue N30 (1948, 1949, 1951, 1952), and refer in particular to the additional stars. In this connection reference should be made to the new reduction of the double star 61 Cygni by NOWACKI (1952).

The system of the FK3 as a whole requires revision. While work on this system was in progress it could already be seen that the southern hemisphere and also both polar caps required further corrections. At this time modern observations extended only to about 1920 and were rather scarce for the southern sky. In the meantime, new and very important catalogues appeared, in particular from the Royal Observa- tory at the Cape, so that now a revision also of the southern sky system appears possible and is to be undertaken by the Astronomisches Rechen-Institut in Heidelberg.

For the right ascensions the observations using quartz clocks are of great impor- tance; here we refer to the paper by v. D. HEIDE (1952).

The revision of the already existing FK3 must be accompanied by an extension, and this in two directions. First, astronomy and geodesy badly need an increase in the number of bright stars in the catalogues, i.e. those down to about 7 m. I t is especially important to close still further the gaps in the stellar distribution of the FK3. A supplement to the FK3 (FK3 Suppl.) is at present in preparation at the Astronomisches Rechen-Institut, Heidelberg (KOPFF, 1952). The recent volume of the Astronomisch-Geodiitisches Jahrbuch (1954) contains only positions and proper motions of these supplementary stars, taken from the GC and N30 catalogues and referred to the system of the FK3, in order to provide the necessary data for most practical requirements as soon as possible. The derivation of final exact positions and proper motions will follow.

The second necessary extension consists in the formation of a fundamental catalogue of faint stars in the system of the FK3. This problem has recently been discussed at the meeting of the INTERNATIONAL ASTRONOMICAL UNION in Rome, during the symposium concerning astrometric problems of faint stars (1952, 1954); see also NEMIRO (1950). A fundamental catalogue of faint stars has already been produced by Russian observatories. I t contains 931 stars, 645 of which are in the region between the North Pole and declination -- 30 °. For many purposes, which have been listed by KOPFF in a lecture at this symposium, this number is too small. An important further contribution has been made on the German side, and has been reported by J. LARINK at this symposium. Both in Bergedorf and in Heidelberg a catalogue of about 3300 faint northern stars (down to magnitude 10 m) has been observed. The reductions are almost complete. This number appears to be sufficient for most purposes, if one takes into account that we are aiming at a "Fundamental- Katalog" to which further stars can be referred. For the repetition of the Zonenun- ternehmen der Astronomischen Gesellschaft (see below, Section 3) the number of faint standard stars on the northern sky was 13,747.

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I t may be expected that the further negotiations within the International Astro- nomical Union will lead to the formation of a general catalogue of faint fundamental stars which will combine the Russian and the German fundamental catalogues.

3. THE CATALOGUES OF THE ASTRONOMISCHE GESELLSCHAFT

In the middle of the nineteenth century ARGELA~DER listed the stars of the northern sky down to the ninth magnitude in charts and catalogues giving approximate positions (N6rdliche Bonner Durchmusterung). SCHiJNFELD extended this work to the South (Si~dliche Bonner Durchmusterung). Both charts and catalogues of the Durchmusterungen have now been published in a revised edition by the Bonn Univer- sity Observatory; the catalogues are now printed in a miniature version (Bonner Durchmusterung, 1949); reference should also be made to BUYER and GRAFF (1950).

The Bonner Durchmusterung has become the starting point of the catalogues of the Astronomische Gesellschaft, an international undertaking which was initiated by AUWERS and contains the positions of stars down to the ninth magnitude from both Durchmusterungen in narrow belts of declination (AGK or AGK1). The observations for the northern sky were completed by the end of the nineteenth century. The original plan already envisaged a repetition of the observations after about half a century in order to derive proper motions.

After the end of the World War 1914-18 Kt3STI~ER and SCHORR took up this idea of repeating the observation of the catalogues of the Astronomische Gesellschaft (AGK2), this time photographically. The plates were obtained around 1930 at the observatories, Bonn, Hamburg-Bergedorf, and Pulkovo. Simultaneously with these photographic observations, a number of transit circles provided a network of reference stars based on the improved NFK. The reduction of these transit circle observations was carried out to a large extent at the Astronomisches Reehen- Insti tut in Berlin-Dahlem under the supervision of J. PETERS, to whose great expertness the rapid progress of this work is mainly due.

I t was also essentially PETERS, with the collaboration of NOWACRI, who compiled the observations of all reference stars (Anhaltsterne) to a general catalogue, abbrevi- ated to AGK2A (ASTRO~NOMISCHE GESELLSCHAFT, 1943). The accuracy of these star positions has been found to be remarkably high (KoPFF, 1942). The synthesis of the FK3 with the catalogue of reference stars and the contemporary photographic plates has furnished a completely homogeneous picture of the northern sky.

The progress of this work was reported first by BAUSCIIINGER (1926) and afterwards regularly by KOPFF (1933) in the Vierteljahrsschrift of the Astronomische Gesellschaft. Here we should also mention the preparatory work of PETERS in the derivation of spherical co-ordinates from photographic plates ; details may be found on p. 5 of the Katalog der Anhaltsterne. A method using Hollerith punched-card machines has been developed and applied to some 12,000 stars of the Bonn plates. Summarizing reports, apart from those in the Fiat Report, have also been given by SCHORR and A. KOHLSCHUTTER (1948) and by HECKMANN (1949). The publication of the catalogue itself has suffered various delays. Hitherto the volumes 1-9 of the Hamburg part of the work have appeared (ASTRONOMISCtIE GESELLSCItAFT, 1953), as well as the northern polar zone of the Observatory Pulkovo (BELJAWSKIJ, 1948). Further volumes of the AGK2 may be expected to appear soon. Mention should also be made of the "t{eststerne" observed by Cou~vois~na (1941, 1949), i.e. stars that could not be measured on the photographic plates ; they have been included in the general catalogue.

A. KOPFF 1009

At about the same time as the observations of the AGK2 were made, the Yale Observatory carried out a photographic repetition of the original zone catalogues; but these catalogues do not have the same strict homogeneity. However, they do contain proper motions derived from a direct comparison of their positions with those of the AGK1.

An essential task still before us is the derivation of new proper motions which are systematically better than those given in the Yale catalogues. To do so we shall require a careful investigation of the systematic errors in the AGK1 catalogues. The possibilities have been discussed on an earlier occasion by GONDOLATSCH (1948). Recently HECKMANN and his collaborators (HECKMANN, DIECKVOSS, KOX, 1948; DIECKVOSS, 1949) have developed and partly carried out a method which makes use of the photographic Carte du Ciel as an intermediate step. Further possibilities have been discussed by BROUWER (1954) at the 1952 Rome Symposium on astrometric problems of faint stars.

In performing this task one will always be concerned with the derivation of the systematic errors of the AGK1, using the AGK2 in some sort of connection with other catalogues.

A valuable link will here be formed by KiJSTNER'S catalogue of 10,663 stars, among which are numerous faint ones (K~STNER, 1908). A catalogue observed by KOPFF before the First World War with the Heidelberg transit instrument, which had already been partly reduced, was lost at the end of the Second World War. Mention should also be made of a short investigation by KOPFF (1914) of the magni- tude equation of declination observations taken with the transit circle.

Plans are now discussed for obtaining proper motions by a second photographic repetition of the AG-Catalogue (AGK3) in the near future.

4. THE "GESCHICttTE DES FIXSTERNHIMMELS"

AUWERS was also the originator of the Geschichte des Fixsternhimmels (GFH), which was undertaken by the Preussische Akademie der Wissenschaften at Berlin and carried out by H. PAETSCH and afterwards by J. HAAs. The GFI t aims at the inclu- sion of all meridian observations obtained between 1750 and 1900 and reduced to a common equinox. The observations of the northern hemisphere were published some time ago; but the stock was lost during the war.

The southern sky has been continued by J. HAAS (1954), now at Bonn. Jus t recently the last section of volume 10 (Right Ascension 9 h) has been published (Geschichte des Fixsternhimmels, 1954). The reduction of the part of the GFH which had remained in Berlin has been taken in hand by J. DICK in Potsdam-Babelsberg; beginning with volume 24 (right ascension 23h), the first part of this volume has appeared ( Geschichte des Fixsternhimmels, 1952).

The Hamburg Observatory published in 1928 the Index der Stern6rter 1900-1925 (SCKOR~ and KRUSE, 1928). This work forms a continuation of the Geschichte des Fixsternhimmels for the years 1900-25 in a simplified form. I t is now intended to extend this Index for the period 1925-50.

Opinions differ as to the value of the old meridian circle observations which have now again become generally accessible in the GFH. From the experience in the production of the FK3 one can certainly say that, for the production of a fundamental catalogue, observations from before 1845 are unusable for an improvement of the system. On the other hand, for the determination of the proper motions within a given

1010 Recent advances in positional astronomy

fundamental system, it is particularly the old observations which are of the greatest value.

5. OTHER INVESTIGATIONS

It would exceed the scope of this article if we would try to attempt to discuss in detail the numerous papers in the field of positional astronomy which deal with special problems. These include mainly investigations of proper motions in limited regions, which are partly still unpublished, like those of HECKMANN and his colla- borators for star clusters (Hyades, clusters near ~ Persei), by A. K6Nm of Heidelberg (for the clusters near ~ Persei), or those carried out at the Bonn Observatory (for clusters photographed previously by Ki?STNER). In this field belongs also a photo- graphic programme taken up by DIECKVOSS, which aims at an investigation of the motions within a 20 °-wide belt of the Milky Way, reaching down to stars of about 14 m. We also mention in this connection an investigation by DICK (1949) of the standard co-ordinates and proper motions in the Perseus region for the epoch 1950.

Finally, we emphasize that positional astronomy, apart from its practical impor- tance for time service and geographical survey, provides the essential foundation for a study of the dynamics of the stellar system. In addition to the literature already mentioned (KoHLSCHOTTER, 1948), we quote in this connection also the reports by FRICKE (1948) and by HECKMANN (1948).

I~EFEREN CES

Astronomisch-Geoddtisches Jahrbuch (Astron. Reehen-Institut Heidelberg). G. Braun, Karlsruhe. Pub- lished 1949-54.

ASTRONOMISCHE GESELLSCHAFT; "Katalog der Anhaltsterne fiir das Zonenunternehmen der Astro- nomischen Gesellsehaft". Edited by A. KOI'FF. Ver6ff. Kopernikus-Institut (Astron. Rechen-Institut). Berlin-Dahlem, 1943, No. 55.

ASTRONOMISCHE GESELLSCHAFT; "Zweiter Katalog d. Astron. Ges. f. d. Aquinoktiunl 1950". Bd. 1-9. Decl. -~ 90 ° bis + 25 °. Hamburg-Bergedorf, 1953.

Astronomischer Jahresberieht (Astron. Rechen-Institut). Published up to vol. 54 (literature of 1954); W. DE GRUYTER, Berlin. For unpublished work see the reports of the German observatories in Mitteilungen der Astronornischer~ Gesellschaft (published volumes: 1949-51, G6ttingen).

BAUSCHINGER, J . ; Vierteljahrsschrift d. Astron. Gesellschaft since 1926, 61, 297. BELJAWSKIJ, S. I.; Publ. Pulkowo, 1948 (Ser. 2), No. 60. BEYER, M. and GRAFF, K.; Stery~atlas, 3rd Edition (contains on twenty-seven charts the stars down to

9 m, as well as the bright star clusters and nebulae down to -- 23 ° declination). F. D(immler, Bonn, ] 950. Bonner Durchmusterung. Revised edition. Ferd. Diinmfler, Bonn, 1949. Boss, B.; General Catalogue of 33,342 Stars for the Epoch 1950. Vol. I-V. Washington, 1937. BROUWER, D,; International Astronomical Union, Rome Meeting, 1952, Transaction.s, Vol. VIII.

Cambridge, 1954. COURVOISIE]% L.; Vero'ft. Sternwarte Berlin-Babelsberg, 1941, 12, Heft 5. COU:aVOISIER, L.; Vero'ft. Steruwarte Berlin-Babelsberg, 1949, 12, Heft 6. DICK, J. ; Astron. Naehr., 1949, 277, 73. DIEc~voss, W.; Die Himmelswelt, 1949, 56, 140. FRICKE, W.; Fiat Review of Germa~ Science (Naturforschung und Medizin in l)eutschland 1939-46);

vol. 20, No, 13. Wiesbaden, 1948. Gesehichte des Fixsternhimmels, Abt. II, Band 8; edited by Deutsche Akademie d. Wiss. Berlin. Vol.

9, 10; edited by J. HAAS. G. Braun, Karlsruhe, 1947-54. Geschichte des Fixsternhimmels, Abt. II, Band 24. I. Lieferung. Edited by Deutsche Akad. Wiss.

Berlin (Astron. Rechen-Institut). Akademie-Verlag, Berlin, 1952. GONnOLATSCH, Fr.; Astron. Naehr., 1948, 276, 221. HAAS, J. ; Gesehichte des Fixsternhimmels, 1954 (see Preface and Introduction, Abt. iI , vol. 10, Lieferung 3). HECI~MA~N, O. ; Fiat Review of German Science (Naturforschung und Medizin in Deutschland 1939-46),

Vol. 20, No. 14. Wiesbaden, 1948. HEC~MANN, O.; Forschungen und Fortschritte, 1949, 25, 282. HECKMANN, O., DIECKVOSS, W. and Kox, H.; S.B. Dtsch. Akad. Wiss. Berlin; Mathem.-Naturw.

Klasse, 1948, No. 7. v. D. HEIDE, J. ; Astron. Naehr., 1952, 281, 31. INTERNATIONAL ASTRONOMICAL UNION (Rome Meeting), Draft Reports, 1952.

F. SCHMEIDLER 1011

INTERNATIONAL ASTRONOMICAL UNION (Rome Meeting, 1952), Transactions, Vol. VIII. Cambridge University Press, 1954.

KOHLSCH~TTER, A. ; Fiat Review of German Science. (Naturforschung u. Medizin in Deutschland 1939-46.) Bd. 20, No. 4. Wiesbaden, 1948.

KOPFF. A.; S.B. Heidelberg Akad. Wiss. (Mathem.-Naturw. Klasse.) Abt. A, 1914, 6. Abh. KOPF~ A.; Astron. Nachr., 1928, 231, 345. KOPFF A.; Astron. Nachr., 1931, 242, 313. KOPFF A. ; Vierteljahrsschrift d. Astron. Gesellschaft, since 1933, 68, 413. KOPFF A.; M.N. , 1936, 96, 714. KOPFF A.; I. Tell: Ver6ff. Astron. Rechen-Instituts Berlin-Dahlem. 1937, No. 54. KOPFF A.; II. Toil: Abhandl. Preuss. Akad. Wiss., 1938, Phys.-Mathcm. Klasse, No. 3. KOPFF A.; Astron. Nachr., 1939a, 289, 160. KOPFF A.; Abhandl. Preuss. Akad. Wiss., Mathem.-Naturw. Klasse, No. 18, 1939b. KOPFF A.; Abhandl. Preuss. Akad. Wiss., 1942, Mathem.-Naturw. Klasse, No. 5. KOPFF A.; M.N. , 1949, 109, 580. KOPFF A.; Astron. Nachr., 1952, 281, 29. KgTSTNER, Fr. ; Verb'ft. d. Kgl. Sternwarte Bonn, 1908, No. 10. LABITZKE, P.; A b h a ~ l . Bayr. Akad. Wiss., Mathem.-Naturw. Klasse, N.F., 1949, Heft 57 (Verbff.

Sternwarte Mi~nchen, 8, No. 13). LABITZKE, P. ; Astron. Nachr., 1951, 279, 113. MORGAN, H. R.; Astron. J. , 1948, 54, 1. MORGAN, H. R.; Astron. J. , 1949, 54, 145. MORGAN, H. R.; Astron. J. , 1951, 56, 97. MORGAN, H. R.; Catalogue N30, Catalogue of 5268 Standard Stars, 1950.0, based on the Normal System

-N30. Astronom. Papers of the Amer. Ephemeris and Nautical Almanac, vol. XII I , Part III , Wash- ington, 1952.

NEMIRO, A. A.; Colloques Internationaux du Centre National de la Recherche Scientifique. XXV: Constantes Fondamentales de l'Astronomie, Paris, 1950 (reprinted from Bull. Astron., 15, Fasc. 3 and 4).

NOWACKI, H.; Ver6ff. Astron. Rechen-Inst., Heidelberg, 1952, No. 4. PAVEL, Fr.; Astron. Nachr., 1947, 275, 23. PETERS, J. ; Ver6ff. d. Kgl. Astron. Rechen-Inst. Berlin, 1907, No. 33. SCHORR, R. and KOHLSCH~TTER, A.; Die Himmelswelt, 1948, 55, 146. SCHORR, 1~. and KRUSE, W.; Index der SternTrter 1900-1925. Vol. 1: The Northern Sky; vol. 2: The

Southern Sky. Published by the Hamburger Sternwarte in Bergedorf, 1928.

On the Increase of the Accuracy of Fundamental Declinations

F . SCHMEIDLER

Universit~ts-Sternwarte, Miinchen, Germany

SUMMARY As the determination of absolute declinations can be considered as the most fundamental problem of positional astronomy, ways are discussed how the accidental and systematic errors can be reduced. To improve the accuracy of the circle-reading seems to be the most promising way of reducing accidental errors. In order to eliminate systematic errors: (1) careful consideration of flexure; (2) observations of minor planets; and (3) observations from both hemispheres with the same instrument by the same observer are suggested.

1. INTRODUCTORY REMARKS

THE problem of determining star places in the sky has in principle a different aspect for the co-ordinates of right ascension and declination. The equator is a natural zero-point for declination which can be realized by measuring latitude, whereas the zero-point for right ascensions is a matter of arbitrary definition. It is thus possible to obtain absolute declinations by direct measurements, if these are properly arranged ; the measurement of right ascensions, however, gives primarily the mere difference of the right ascensions of two or more particular stars. To determine absolute right