Reprint and Circular Series of the National Research Council (1919)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

360 INTERNATIONAL ASTRONOMICAL UNION 7. The Central Bureau shall take the necessary measures to reach its subscribers in all parts of the world. If necessary, special agreements shal] be made for this purpose with observatories or secondary centers situated in other continents which may serve as relays. 8. The Central Bureau shall make every effort to reduce the expenses of transmission to a minimum; it shall continue to employ, for this purpose, the numeric code that has been in use for several years. NOTES A. Telegrams sent to the Central Bureau are to be addressed: ASTRA BRUXELLES B. Subscribers are requested to inform the Director of the Royal Observatory of Belgium, Uccle, of the SHORTEST TELEGRAPHIC ADDRESS to which messages conveying notices of discoveries should be sent in order to reach them. REPORTS OF COMMITTEES OF THE AMERICAN SECTION OF THE INTERNATIONAL ASTRONOMICAL UNION REPORT OF THE COMMITTEE ON NOTATION, UNITS, AND ECONOMY OF PUBLICATION The Committee on Notation, Units, and Economy of Publication has given careful consideration to these subjects, and, although it has no recommendations it wishes to urge upon the Union, it begs to submit the following remarks in support of the appointment of a similar inter- national committee. I. UNITS The multiplicity of units, applying to the same kind of measurement, that are now used in astronomical literature, and the indefiniteness that attaches to some of them, suggest the need of a careful studv of this sub- ject and international agreement as to what units should, as a rule, be adopted. Astronomical distances, for instance, are already expressed in terms of the kilometer, mile, ‘‘astronomical unit’’ (mean solar distance), light- year, “‘parsec’’ (distance corresponding to a parallax of 1”), siriometer (corresponding to a parallax of 0."2), “the unit’’ (corresponding to a paral- lax of 0.”1), megaparsec = 10° parsecs, and, perhaps, a few other units. Although no other astronomical quantity is measured in so many differ- ent units as the stellar distance there are some whose numerical values are more confusing because the name of the unit employed has at least a double meaning. Thus the ‘‘absolute magnitude’ of a star may refer to its bright- ness in terms of some supposed constant; or to that brightness which the star under discussion would show at a distance of 10 parsecs. Similarly, the unit of heat commonly used in certain astronomical discussions, the calorie, often leaves one in doubt as to which calorie is meant, the small or the large.

INTERNATIONAL ASTRONOMICAL UNION 361 Obviously, then, there is need for general agreement in respect to the units to be used. The following suggestions are offered for discussion: 1. Distance.—So far as practicable let the unit distance be, a, the kilo- meter (in reference to dimensions of sun, planets, etc.). It can even be used in expressing stellar distances without employing larger numbers than the chemist and the physicist are accustomed to in speaking of the number of molecules in a cubic centimeter of gas), b, the “astronomical unit’ (in reference to the solar system chiefly), c, the light-year (especially in popular articles), and d, the parsec (wherever appropriate), or preferably a unit 10 times as great, to be given a separate name. 2. Absolute magnitude.—The meaning given to the expression “absolute magnitude’”’ should be the magnitude at the distance of 10 parsecs, at which the star’s parallax would be 0."10. 3. Unit of heat.—Use the joule for the unit of heat instead of calorie, as the latter 1s not definite unless explained. 4. Unit of wave-length—Use the international angstrom. 5. Unit of pressure —One bar = 10® dynes per square centimeter. This is in keeping with very general use in meteorology. II. NOTATION It is only right that considerable latitude should be allowed the author in the choice of notation, nevertheless some of the existing confusion could and should be avoided. There is no uniformity, for instance, in the nota- tion for the elements of a visual binary system. Also different methods are used in designating the brighter stars—by constellations and Greek letters, and by right ascension and declination. Again, the notation used in celestial mechanics, though difficult to standardize, appears to be more confused than necessary. Suggestions 1. Binary systems.—Use a common notation—Aitken’s, perhaps. 2. System of units —If metric units are used do not add the English equivalents; if English units are used add, generally, the metric equivalents. 3. Wave-length svstem.—Use the abbreviations JS and R S, and not / A and RA. Do not use the diacritical mark over either the ab- breviation A or the word angstrom. 4. Unit of acceleration.—A short name is needed for the C. G. S. unit of acceleration. Additional suggestions in reference to notation will be made in connec- tion with “economy of publication.” III ECONOMY OF PUBLICATION That there is need for economy in amount of publication, in cost of pub- lication, and in manner of publication so as to save the time and effort

362 INTERNATIONAL ASTRONOMICAL UNION of the reader, are all too evident to admit of question. The problem is how can these results be best attained. Suggestions 1. Observatory bulletins —There should be as few bulletins and as little published in them as circumstances will permit. It is better to publish in standard journals and purchase reprints. 2. References——A nearer approach to standardization of references to journals, etc., than now exists should be adopted—librarian’s method perhaps. 3. Standard kinds of work.—So far as practicable all results obtained in any one standard line of work should be presented in the same form. 4. Summaries.—Most articles should end with concise and accurate summaries. . 5. Catalogues.—Catalogues of scientific literature should give concise abstracts. 6. Economy in typesetting.—Manuscript should be kept as free as prac- ticable from all letters, symbols, and combinations that worry the printer or annoy the reader. Greek letters, for instance, are expensive luxuries —a dime per letter being the estimated added cost of publication. Simi- larly, subscripts, superscripts, inferior bars, superior bars, vertical frac- tions, etc., should be avoided as far as practicable. The following recent suggestions by the London Mathematical Society are excellent: Instead of Always print v2, i V13 V2 or 2%, 1/V2 or 27%, V13 or v2 13”2 Vax? + 2bx +c V(ax? + 2bx +c) or (ax? + 2bx _ +c)’ V ; V(a/b) or (a/b)’4 VvV—1 ‘or? nun+ian+ 2 n(n + 1)(” + 2) (\2)?, jz + 1, |2n, 2"|n (n!)?, (n + 1)], (2n)!, 2".n! x, y, r, 9 . x’, y’, r’, 0” (by preference) coe Store Ja, (a + 4), (a +b +0) i rr (a + b)/c, a/(b +c), a/b +c c b+c¢b .,

INTERNATIONAL ASTRONOMICAL UNION 363 p P + A p/q, p/¢ + 7r/s (in current text) qq “°§ p r or p/ € + ‘) b/(q + 1/s) qgq+-r/s S ] 1 - 1 " —n"n “> = 1/x or x”, 1/x" or x xx q a 2% ptiq _% 1 1 ras 3 srtas yta2/t g¢ 47; nex x? enrs/t e t/4kt bie 4k e Ir 7 7 4 x/m ™y) f?, f" In/a, feof a 0 0 0 0 l i (2/1) f sin (rax/l) sin (sxx/l)dx fj sin ~ sin — dx* 0 0 A few of the above expressions might, perhaps, be written in still better forms. Furthermore, it is quite possible to write an integral with limits in a more condensed and more convenient form than it usually has. Thus Instead of Write f; x2dx S (a,b ]x*dx The first, or usual, form occupies, vertically, the space of two lines. The proposed form occupies but one line. W. J. HUMPHREYS, Chairman, EpwIn B. Frost, A. O. LEUSCHNER. REPORT OF THE COMMITTEE ON ALMANACS The Committee on Almanacs recommends to the American Section that its delegates to the Brussels meeting of July 18, 1919, be instructed to endeavor to secure the adoption by the International Astronomical Union, of the Report of the Committee on Beginning the Day at Mid- night adopted by the American Astronomical Society in August 1918. W. S. EICHELBERGER, Chairman, E. W. Brown, R. H. TucKER REPORT OF THE COMMITTEE ON MERIDIAN ASTRONOMY The experience gained through the application of photography to the determining of stellar positions demonstrates that it is advantageous * It may be noted that an integral with limits always involves a double line; so that the ordinary notation for fractions involves no additional use of ‘‘spaces’’ beyond those required for the integral in any case.

364 INTERNATIONAL ASTRONOMICAL UNION to secure star positions, as far as possible, by photographic methods. This leads to the suggestion of the following meridian program, which is de- signed to provide the necessary foundation for photographic astrometry. The suggested plan, in brief, is to adopt 3000 fundamental stars as the initial basis of the system, to build about 40,000 reference stars upon this and then by photographic means to continue a list of reference stars to fainter magnitudes. The details of the plan follow. I. STANDARD STARS It is suggested that the list of fundamental stars compiled in 1910 by the Committee on Fundamental Stars of the Astrographic Chart, and pub- lished in the supplement to the Connaissance des Temps for 1914, be adopted. II. PHOTOGRAPHIC REFERENCE STARS As a secondary step toward providing a suitable structure upon which to base photographic plate determinations of stellar positions it 1s suggested that a list of stars be constructed, approximately one in each square de- gree, and included as far as possible between visual magnitudes 8.0 and 9.0. The list should favor the stars of the intermediary and reference lists of the Astrographic Chart, but in any event stars which have been previously observed should be selected. III. DIFFERENTIAL MERIDIAN OBSERVATIONS It is recommended that differential observations be undertaken in zones 20° wide, four observations being taken of each star, two in each position of the clamp. All the fundamental stars in each zone should be observed, together with a sufficient number of fundamental stars at varying decli- nations to link the zone with the fundamental system. IV. REVISION OF STANDARD SYSTEMS Because of the rapidity with which the weight of a star position di- minishes, it is recommended that standard systems be revised every fifteen to twenty years, and that the results be incorporated in the ephemerides as they become available. V. QUESTIONNAIRE In order to disseminate information in compact form among the meridian observers, it is recommended that 6 months before each meeting of the International Astronomical Union the following questionnaire be sent to each observatory and that the reports be collated and published as part of the proceedings of the meeting. 1. Brief description of meridian work in hand, and estimated time of conclusion. 2. Instruments, character, size, etc. 3. Equipment:

INTERNATIONAL ASTRONOMICAL UNION 365 (a) Collimators (6) Mires (c) Nadirs (d) Impersonal right-ascension micrometers (e) Personal equation apparatus (f) Reversing prism on ocular—on microscopes (g) Screens or other devices for magnitude (hk) Flexure—how determined (2) Clock—how safeguarded from temperature and barometric changes (j) Thermometer installation—use of whirling thermometers—Assman thermometers —or other devices (k) Printing chronograph (4) Devices for stabilizing mercury reflecting surfaces (m) Methods of determining corrections for irregularities in the forms of the pivots 4. Remarks on any novel devices in use, with comments on the value of the same. 5. Reflected observations of stars. Are they made—how used? 6. Application of photography to meridian work. 7. Method of determining graduation errors. VI. PUBLICATIONS 1. The present practice varies from publication of the minute detail of observations to that of giving only the final results. It is suggested that this matter be considered and that a recommendation be made in order on the one hand to save the great waste of material and effort involved in unnecessarily extended publication and on the other hand to secure the publication of as much detail as the consensus of opinion shall decide 1s actually desirable. 2. Uniformity should be observed in star names, in number of decimals for right-ascension and declination, and in use of declinations instead of north polar distances. 3. Standard equinoxes should be adopted. 4. No proper motions should be applied in observational catalogues to reduce from the epoch of observation to the date of equinox of the cata- logue. VII. SOUTHERN OBSERVATORIES Attention is called to the need of more southern observatories for meridian work in view of the relative weakness of the positions of the southern stars. BENJAMIN Boss, Chairman, FRANK SCHLESINGER, FRANK B. LITTELL REPORT OF THE COMMITTEE ON COOPERATIVE WORK WITH THE SPECTROHELIOGRAPH In several reports presented to the International Union for Codperation in Solar Research, the advantages of photographing the sun frequently with the spectroheliograph at several stations widely separated in longi- tude have been pointed out. Without enumerating these, it is evident that a more nearly continuous record of the rapidly changing phenomena of the sun’s atmosphere than we now possess would provide material for

366 INTERNATIONAL ASTRONOMICAL UNION the study of many questions, such as the possible connection between solar eruptions and terrestrial magnetic storms. Most of the telescopes hitherto used for this work give a very small solar image, while the spectroheliographs employed with them are in very few cases of sufficient dispersion to permit satisfactory photographs to be taken with the Ha line. As the hydrogen flocculi, for most purposes, are of much greater interest and importance than the calcium flocculi, the object of this report is to indicate the general nature of the instrumental equipment needed to record them in such a way as to render the resulting photographs fairly comparable. The chief instrumental requirements are: 1. A telescope giving a large solar image, not less than 10 cm. and pref- erably from 15 to 20 cm. in diameter. This large scale is needed in order that the minute structure defining the fields of force of the hydrogen flocculi may be clearly recorded. 2. A spectroheliograph of sufficient dispersion to permit the hydrogen flocculi to be photographed with light confined to the central part of the Ha line. A linear dispersion of 1 mm. = 3.5 A will serve very well. It is desirable, but not essential, that this spectroheliograph should be of sufficient aperture to permit the entire solar image to be photographed at once. If this is not feasible, a second objective, giving a smaller solar image (at least 5 cm. in diameter), should be so attached to the telescope that it can be readily substituted for the objective of greater focal length. 3. It is important that spectroheliograms intended for comparative studies of eruptive phenomena and changes in the structure of the hydrogen flocculi should not differ greatly, either in the size of the solar image or in the dispersion of the spectroheliographs with which they are taken. For this reason it is desirable that the optical constants of the instruments used at codperating observatories should be as nearly alike as possible. 4. It is desirable that the spectroheliographs should be adaptable for the photography of the calcium flocculi with Ke and with Ks. It would also be advantageous to design them for use as spectroregistreurs des vitesses (Deslandres) and for the study of the magnetic fields in sun-spots, in case spectrographs especially designed for these purposes are not avail- able. A final word should be said regarding the necessity for good atmospheric conditions, to permit the sun to be photographed on the largest possible number of days and to afford the perfection of definition required to reveal the minute details of the floccull. GEORGE E. HALE. REPORT OF THE COMMITTEE ON ECLIPSES (Abstract) The Committee believes that it might be of assistance to astronomy along the following lines:

Pd INTERNATIONAL ASTRONOMICAL UNION 367 1. To advise as to what eclipse problems are most worthy of attack, and what equipment is needed for successful work. 2. For the eclipse of 1923, to collect well in advance all available in- formation regarding possible eclipse sites, weather probabilities, trans- portation facilities, living conditions, etc. (The Mexican Meteorological Service has already been communicated with in regard to the eclipse of 1923.) 3. To publish such information and eclipse computations well in ad- vance. 4. To advise intending observers as to location so as to secure a wide distribution of stations. - §. To gain information regarding the apparatus that is available at different observatories, and to arrange as a central clearing-house for the loan of apparatus to intending observers who have not adequate facilities. A discussion is given of the various eclipse problems under the headings: Photography of the corona. What is the corona? What causes the corona? Motion of the corona. Polarized light in the corona. Brightness of the corona. Spectrum of the corona. Wave-lengths of coronal lines and rotation of the corona. Flash spectrum. Einstein effect. Times of contact. Other eclipse problems. Movie camera. S. A. MITCHELL, Chairman, E. E. BARNARD, H. D. Curtis. REPORT OF THE COMMITTEE ON STANDARDS OF WAVE-LENGTH 1. It is recommended that the International Astronomical Union appoint at the earliest practicable date, a committee whose functions shall be essentially the same as those of the Committee on Wave-Lengths of the International Union for Co6dperation in Solar Research. 2. The primary standard.—The remarkable sharpness of the red Cad- mium line, the extraordinary skill with which the meter was evaluated in terms of this wave-length, and the final definition of the angstrom, as an arbitrary unit, by the international Solar Union at Paris in 1907, would seem to place the primary standard almost beyond the range of question. However, the recent perfection of a new method for making end-stand- ards of length, the comparative ease with which the optical length of these end-standards can be determined, and the possible superiority of some other

368 INTERNATIONAL ASTRONOMICAL UNION line (such as the bright yellow Neon line) over one from solid Cadmium as a source of light combine to urge a reconsideration even of the primary standard. Since a committee, consisting of Messrs. Ames, Anderson, Michelson and Stratton, has been appointed by the National Research Council to consider the practicability of adopting a wave-length of light as the fundamental standard of length, it is recommended that the question of a primary standard of wave-length be held in abeyance—that is, not reopened—until the above-mentioned committee makes its report. In case the International Bureau of Weights and Measures adopts a certain wave-length of light as their standard of length, it would appear to follow, as a matter of course, that this wave-length should be adopted also by the Astronomical Union, and that the numerical value attached to it should be that assigned by the International Bureau of Weights and Measures. As to the possible use of a Neon line, or the sharp component of the green Mercury line, or of a sharp line in the absorption spectrum of Iodine, for the fundamental unit of length, instead of the red Cadmium line, your committee feels that this is a matter belonging entirely to the Com- mittee of the International Bureau of Weights and Measures. 3. Secondary standards.—A series of secondary standards is now being determined at the Bureau of Standards and at Mt. Wilson. The Committee recommends that the Astronomical Union encourage a third series of inde- pendent measures on these lines. It is recommended, therefore, that the International Astronomical Union arrange, if possible, with European spectroscopists for a determination of these secondary standards in Iron, Neon, and other substances which can be worked under definite and easily reproducible conditions. _ The dearth of Secondary Standards in the yellow-green region and in the infra-red appears to your committee an unsolved problem. Its solu- tion will be the discovery of a workable and easily duplicated source of light which will furnish properly distributed lines of sufficient sharpness and in- tensity. 4. Tertiary standards.—Here the greatest need appears to be for lines to fill gaps in the ultra-violet, the yellow-green, and infra-red of Iron. With reference to this need your committee has no recommendation to make except to seek further information as to how the need may be met. For the measurement of tertiary standards independent determinations by the interferometer and by the grating are urgently recommended, on the ground that the errors of these two instruments are largely comple- mentary. The importance of this work is emphasized by the fact that it will be scarcely practicable to print a new “Table of Stronger Lines” for the identification of impurities and similar purposes until the tertiary standards are more complete than at present.

INTERNATIONAL ASTRONOMICAL UNION 369 3. Sources—In order to obtain lines of constant wave-length and con- stant intensity-distribution, it is recommended that the modification of the Pfund arc employed at Mt. Wilson be adopted, namely, the Pfund arc operated between 110 and 250 volts as convenient, with 5 amperes or less, at a length of 12 millimeters, and used over a central zone, at right angles to the axis of the arc, not to exceed 1!/, millimeters in width. 6. Classtfication.—It is recommended that tables of wave-lengths be accompanied by at least two descriptive columns, one indicating the in- herent nature of the line (intensity, sharpness, asymmetry, etc.), the other indicating the character of the measures on the line. There is urgent need of some quantitative description of relative intensity as a function of wave- length. For indicating the intrinsic character of the line, the modification of Gale and Adams’ groups, shortly to be defined at Mt. Wilson, is recom- mended. , For indicating the degree of accuracy, the notation of Burns (Lick Obs. Bulletin No. 247, p. 29 and Bulletin Bureau of Standards, No. 274, p- 251) is recommended. 7. Tables of wave-lengths—The time for an accurate and definitive determination of the wave-lengths of the different chemical elements does not yet seem to have arrived. It is recommended, however, that a 7 able of Stronger Lines in which wave- lengths of all elements shall be arranged in order of magnitude be prepared at the earliest practicable date. It is believed that present needs call for a table containing approximately twice as many lines as Kayser’s Tabelle der Hauptlinten, which includes ten thousand wave-lengths and occupies one hundred octavo pages. 8. Survey.—It is recommended that the conference at Brussels consider the possibility and advisability of making a survey of facilities, equipment, and opportunities for spectroscopic work in the various laboratories of the world, looking toward the avoidance of unnecessary duplication and the encouragement of codperation in research. HENRY CREw, Chairman, HaroLtp D. Bascock, KEIVIN Burns, W. W. CAMPBELL, CHARLES E. St. JOHN. REPORT OF THE COMMITTEE ON THE DETERMINATION OF SOLAR ROTATION BY THE DISPLACEMENT OF LINES Since the Bonn meeting of the International Union for Codperation in solar Research in 1913, results of investigations of the sun’s rotation have been published by a number of observers, namely, J. S. Plaskett, Hubrecht, Schlesinger, DeLury, Evershed and Royds, H. H. Plaskett, and St. John and Ware. _ As the data relative to the linear velocity at the solar equator now cover

370 INTERNATIONAL ASTRONOMICAL UNION a period of some twenty years, a review of the results at this time is of interest: LINEAR VELOCITY OF SOLAR ROTATION AT THE EQUATOR OBSERVER VELOCITY | NO. OF LINES REGION DATB Duner......... 0.0 cece cece eee 2.08 2 6301-6302 1900-5 Halm..... 2... ccc ee eee 2.04 2 6301-6302 1904 Adams..... 0.0.0.0 c cee cece eee 2.06 20 4196-4294 1907 Adams........... ccc cece ee eees 2.05 22 4196-4291 1908.5 Storey and Wilson............... 2.08 10 6280-6318 1909 Plaskett, J. S.......... 0.0000 eee 2.01 19 * 5506-5688 1911 Plaskett, J. S.......... 0.000 ee eee 2.02 15 4196-4291 1911 DeLury.... 0.0... 00 cece eee 1.97 19 5506-5688 1911 Hubrecht..............0.000 00s 1.86 40 4299-4400 1911 Plaskett, J.S..............0 000s 2.01 27 4250-5600 1911-12-13 Schlesinger... .........000 cece eee 2.00 20. 4058-4276 1912 Evershed and Royds............. 1.95 3906-5624 1913 Plaskett, H. H.................. 1.98 12 5574-5628 1913 St. John and Ware.............. 1.94 35 4123-4338 1914 Plaskett, H. H.................. 1.95 5 5900 1915 St. John and Ware.............. 1.94 26 5018-5316 1914-18 St. John and Ware.............. 1.95 7 6265-6337 1916-17 In view of the accuracy attainable in spectro-photographic measures the differences between these results are surprising and disappointing and are the outstanding feature in the problem of solar rotation. They are larger than the probable errors of measurement, and either depend upon sys- tematic errors of the measurers or they are of solar or terrestrial origin. The tracing of them to their source is the line along which investigation may be directed with great advantage. Investigations now in progress at Mount Wilson, in which simultaneous observations are made upon the two limbs and the center of the sun, indicate that temporary and local conditions in the sun’s reversing layer are frequently such as to produce differences of ten per cent in the rotation values obtained by comparing east and west limbs directly. The observations show further that high values at one limb are not correlated with high values at the other as they would be if they were due to real changes in the rotation of the reversing layer or to conditions in the terrestrial atmosphere. In the light of these results it is clear that a short series of observations may give misleading results and it even be- comes a question whether the solar rotation can be determined as defi- nitely as has been thought. The means of extended series of observations taken under constant conditions should, however, give results of great value. Such a series at Mount Wilson Observatory is now in its sixth year. The observations show no definite evidence of periodicity in the suns’ rotation from 1914 to 1919, but furnish strong indications of fre-

INTERNATIONAL ASTRONOMICAL UNION 371 quent local disturbances of a definite type in the sun’s reversing layer, a subject still under investigation. The formulae of Faye represent satisfactorily the relation of velocity to latitude as given by the observations of Duner, Adams, Plaskett and DeLury, and Schlesinger, but observations in the higher latitudes should be greatly extended both in number and in range of latitude. The failure of the Faye formula to represent Hubrecht’s observations is probably due to the very limited time covered by them. The only published results relating to a difference in the rotation law in the two hemispheres are those of Hubrecht. His earlier paper (1912) referred to in the report at the Bonn meeting showed an excess of the ve- locities in the southern over the northern hemispheres, but in the later publication (1915) covering the same epoch he comes to the opposite re- sult basing his conclusions upon Tunstall’s measurement of his plates to the exclusion of his own. The question of a difference between the two hemispheres is one of great interest and need not wait for solution until all discrepancies between observers are settled as it is a relative matter for any one observer and instrument. It, therefore, offers an inviting field of investigation at the present time, and is on the working program at Mount Wilson. The different rates of rotation for different levels in the solar atmos- phere shown by the Mount Wilson observations of Adams appear apain in preliminary reductions of the present Mount Wilson series which show relatively high velocities for the strong magnesium triplet in the green, for 4227 and the H and K lines of calcium, but low values for the lines of lanthanum and of the nitrogen (cyanogen) band at 3883. The observa- tions of Schlesinger published since the Bonn meeting agree with those of Plaskett and DeLury, Storey and Wilson, and Hubrecht in showing no positive evidence of differences in rotational velocity It is to be noted that in the Mount Wilson observations lines differing widely in intensity, representing great differences in level, have been employed. As differ- ences in rotational velocity must be directly related to differences in level, such lines, notwithstanding the difficulties of measurement, offer a valuable means of detecting it. The great majority of lines of medium intensity of the common elements originate in not greatly different levels, and for such lines differences in rotation and velocity would be small and corre- spondingly difficult to detect. Conditions in the earth’s atmosphere have been suggested by DeLury as possible causes of divergent results in observations for solar rotation. He finds evidence of the effect of an over-lapping sky spectrum in some of his observations at Ottawa, and attributed to it the large variations in rotational velocity at different times and all differences found for different elements and lines. . That a large proportion of superposed skylight might appreciably re-

372 INTERNATIONAL ASTRONOMICAL UNION duce line displacements at the sun’s limb has been taken into considera- tion by all solar observers. Tests on Mount Wilson show that, under ordinary observing conditions, the integrated light just outside the solar image is not of sufficient intensity to affect the photographic plate during the exposure time employed for solar rotation plates; that it requires con- siderable cloudiness over and around the sun, conditions never approxi- mated in regular work, to cause a measureable decrease in the line dis- placements at the limb; and that this large proportion of superposed sky- light produces no differential effects between strong and weak lines. Recommendations for Program of Work 1. In order to trace to their source the systematic differences that are found in the value of the solar rotation by different observers, it is desirable that further study be given to possible sources of systematic errors and to determining the source of the differences found by the same observer upon plates taken at frequent intervals, differences of the same order of magnitude as those between different observers. 2. Inthe meantime it is desirable to carry on, when possible, continuous series of observations under constant instrumental conditions and by the same observer. 3. As it seems increasingly probable that local conditions in the re- versing layer are frequent causes of divergent results, it is advisable to extend observations over a considerable period of time in investigation of such questions as variation with latitude or differences between the hemispheres. 4. Simultaneous observation on the center and the limb is recommended as a valuable means of checking results and of investigating the hemi- spheres separately. We recommend the appointment of an International Committee on Solar Rotation determined by spectrographic methods, the four points above to be referred as suggestions to this committee. C. E. St. Jon, Chairman, W. S. ADAMS, FRANK SCHLESINGER. REPORT OF THE COMMITTEE ON WIRELESS DETERMINATION OF LONGITUDE The Committee recommends: 1. That the American delegation to the Brussels meeting of the Inter- national Astronomical Union urge the desirability of a Division of the Union which shall promote in the most practicable manner a world-wide program of accurate radio determinations of differences of longitudes, on the basis of one general program to be supported by the inter-allied nations concerned. 2. That the American members of this Division or Committee of the Union should be selected by the American Delegation with reference

INTERNATIONAL ASTRONOMICAL UNION 373 to their fitness for planning, directing, and carrying on the American Section of the inter-allied program. — 3. That the selected primary stations be so distributed throughout both the southern and northern hemispheres as to cover, as uniformly as possible, the entire world, and to give suitable checks on the work. 4. That observatories and other related institutions, both govern- ment and private, be encouraged to participate by receiving the funda- mental radio signals and making simultaneous time observations, so as to link up as many positions as possible with the primary stations. 5. That the governments controlling radio stations likely to be of use be asked to give their consent to and promises of assistance in the work. 6. That Greenwich, England, be so included in the plans that the differences of longitude between it and the primary stations will be ac- curately determined. 7. That the determination of the difference of longitude between any pair or pairs of primary stations be made by parties from one country which would select their instruments and work out and be responsible for the result; but that nothing in this recommendation would prevent a duplication of this work by observers from another country, and, where other checks were not provided, that such duplication be encouraged. 8. That in view of the many arrangements and preparations required to secure accurate and complete results, the earliest date practicable for an international determination of differences of longitudes is the winter of 1921-22. J. A. HooGEWERFF, Chairman, W. W. CAMPBELL. Because of his absence, Mr. J. J. Carty did not see the report. REPORT OF THE COMMITTEE ON VARIATION OF LATITUDE The Committee on Variation of Latitude of the American Section of the proposed International Astronomical Union submits the following report, based upon the action taken at a joint meeting of this committee with a similar committee representing the American Section of the pro- posed International Geophysical Union held in Washington on April 29, 1919. There were present at the meeting Messrs. Bowie and Marvin representing the Geophysical Union and Messrs. Schlesinger, Leuschner, and Littell representing the Astronomical Union. Mr. Bowie acted as chairman and Mr. Littell as secretary of the meeting. RECOMMENDATIONS The following recommendations are made by the committee: 1. That the observations for variation of latitude, their reduction, and their publication be entrusted to the International Astronomical Union.

374 INTERNATIONAL ASTRONOMICAL UNION 2. That the continuity of the observations by the present methods at the four international variation of latitude stations, Ukiah, California: Carloforte, Italy; Mizusawa, Japan; and Charjui, Turkestan be main- tained, as far as practicable, and that the matter of utilizing the observa- ' tions made at other stations be considered. 3. That the question of renewing variation of latitude observations in the southern hemisphere be considered. 4. That, inasmuch as no definite plan can be advanced for defraying the expense of the variation of latitude work until some international arrangement is formulated, the several nations maintain the stations within their domains, and that some provisional arrangement be made for caring for the records, reductions, and publications, pending a perma- nent organization of the work. FRANK B. LITYrELL, Chairman, A.O. LEUSCHNER, FRANK SCHLESINGER REPORT OF THE COMMITTEE ON ASTEROIDS AND COMETS Among the most important objects to be served by the discovery, observations, and by researches in general on comets and asteroids, are the following: 1. Testing of gravitation by comparison of theory and observation leading to deviation from the law (e.g., those shown by the moon). 2. Testing of gravitational theories as sufficient to represent their motions for some time in the past and future. 3. Distribution with reference to theories of formation and evolution. 4. Distribution of orbits with reference to theories of the stability of their motions. 5. Positions for photometric research. 6. Photometric research for testing of theories of causes of variation of light. 7. Theories of distribution tested by search for asteroids in regions indicated by theory. 8. Positions of asteroids recorded for future researches, the direction of which is at present unknown. These ultimate aims involve: Observations of position, visual and photographic; Photometric observations, visual and photographic; Methods of observation and reduction; Researches in orbit methods and orbit determinations, preliminary and final, and ephemerides; Methods of special and general perturbations, of varying degree of accuracy according to special requirements; Researches in pure celestial mechanics (e.g., periodic solutions), etc., etc. Complete bibliographies should be prepared for each line of work.

INTERNATIONAL ASTRONOMICAL UNION 375 The Committee is of the opinion that much unnecessary effort and duplication could be avoided and a thorough foundation for future re- searches along the lines indicated above could be furnished by first of all bringing together all published material in a bibliography which would give in abstract form the results of past observations and investigations. Observations of positions, photometric observations, etc., would be listed merely by dates of observation and references. At the present time there is completely lacking a reference work for asteroids such as Galle’s Comet List. The latter also requires to be brought up to date; several fragmen- tary continuations of Galle’s list are in existence. Bauschinger’s Sta- tistik der Kleinen Planeten gives merely statistical information based on adopted asteroid orbits, without giving a complete history of the work done on the individual asteroids, including references to the dates of ob- servations. The research requirements along the various lines indicated above can- not be fully stated until a complete history and bibliography for comets and asteroids are available. CGalle’s list has been of incomparable value in this direction, particularly in the identification of comets. After the history and bibliography have been completed the research requirements along the various lines indicated above should be published. Arrangements should be made by international codperation for the pub- lication of opposition ephemerides. After the plans have been outlined in the rough and published, a census should be made of observatories and astronomers, for the purpose of securing volunteers for specific work of observation or research. The results of this census should be published, so that every astronomer will know to whom to turn for information of any kind regarding asteroids, and so as to avoid the present useless duplications, particularly of observa- tion. The general direction of these plans should be in charge of an international committee composed of the chairmen of the Committees on Comets and Asteroids of the various national sections. As a first step toward the realization of the foregoing plans it is neces- sary to adopt a distinct classification for comets and asteroids. An example for a suitable asteroid classification is attached as Appendix A. This classification is based on the present state of observation and com- putation and on the scientific importance of the object. It at once en- ables the astronomer interested in a particular kind of research to select the object and the material needed. The present designation of asteroids might be supplemented by using the letters a, b, c, etc., for easy recognition of the condition of the avail- able observational material and of its orbit theory, an asterisk being added to designate those of scientific importance. These asteroids might be given a description to indicate the nature of their scientific importance,

376 INTERNATIONAL ASTRONOMICAL UNION thus: P; = Trojan Group, p2 = Hecuba Group, etc.; v for variable, ! for libration, e for eccentricity, 1 for inclination, etc. The magnitude at unit distance might also be given. An investigator could thus pick out at a glance the asteroids of particular interest in his line of research. After each asteroid has been properly classified and designated, cards should be completed in order of importance. This work should all be done under the auspices of the International Committee, provided such a committee adopts some plan as outlined here. Ephemerides —An international convention will have to be held for providing observations and preliminary orbit determinations for newly discovered planets and for carrying forward the computations. It would seem wise at this time for the international committee to canvas the differ- ent countries as to the share of burden each can undertake. At the out- set probably different countries in the Union would care for the asteroids discovered in their own country, without attempting a complete duplica- tion of the work now in progress at Berlin. In this connection, the names of the observatories and astronomers who have undertaken to secure the necessary and sufficient observations in the case of each planet and to carry forward the computation should be published. It is very essential that an understanding be reached to secure the necessary and sufficient observations of a newly discovered object to secure a satisfactory orbit sufficient to locate the body of the next oppo- sition and to avoid all unnecessary observations and computations. After the International Committee has adopted definite plans in re- gard to the foregoing points national observatories and almanac offices will no doubt be in a position to care for such ephemerides as involve routine computation only. The whole work, however, should be organized under the auspices of the International Committee. As there may be differences of opinion no definite plans are proposed until the different countries are able to state what assistance they can render. It is desired that monographs be published for the benefit of various groups of research men by persons invited to undertake this task by the committee, or by volunteers. Recommendations.—In particular, the Committee makes the following recommendations: 1. That the Executive Board of the National Research Council be requested to secure funds for the preparation of a history and bibliography of comets and asteroids, with the understanding that this work will be undertaken by the United States. 2. That the International Astronomical Union appoint an International Committee on Comets and Asteroids. 3. That the American Delegation to the International Astronomical Union submit to the International Committee the proposals of the Amer- ican Committee on Comets and Asteroids for their approval.

INTERNATIONAL ASTRONOMICAL UNION 377 4. That in particular a preliminary distribution of the necessary ob- servational and computational activities among the countries represented in the Union be agreed upon, on the basis of such offers as may be received from the different countries. 5. That the American Delegation pledge that it will arrange for the care of the observational and computational work incident to comets and © asteroids discovered in the United States. A. O. LEUSCHNER, Chairman, E. W. Brown, G. H. PETERS. APPENDIX A MEMORANDUM ON CLASSIFICATION OF PLANETS Class a.—Asteroids for which accurate definitive orbits and general perturbations are available. 1. Ordinary asteroids which have no immediate scientific interest. Observations of these should be suspended, but approximate ephemerides should be provided for identification purposes, 1f accidentally observed. 2. Important asteroids of further scientific interest should be observed in accordance with prearranged codperative plans with the object of securing sufficient observations for the scientific purposes in hand, and avoiding unnecessary multiplication of observations. Ephemerides should be published, the accuracy of the ephemer- ides in each case to depend on the research requirements for that case. Class b.—Asteroids for which accurate definitive elements are available, but no general perturbations. 1. Ordinary asteroids. Same‘as a (1). Approximate general perturba- tions should be computed as a later part of an asteroid program, ' as opportunity presents itself, preferably only the principal terms in groups. . 2. Important asteroids. Observations to be dispensed with until general perturbations have been computed, degree of accuracy to depend on particular scientific requirements in each case, then same as a (2). Class c.—Asteroids for which satisfactory elements are not available, but which have been sufficiently observed for the determination of satis- factory osculating elements. 1. Ordinary asteroids. Same as a (1). Elements to be improved, then the same as b (1). Asteroids for which osculating elements should be derived, with the use of special perturbations, accuracy in each case to depend on the special research requirements; then same as 6 (2), then a (2). Class d.—Asteroids for which sufficient observations are not available for satisfactory orbit determinations. These should be observed accord-

378 INTERNATIONAL ASTRONOMICAL UNION ing to codperative plan to secure necessary and sufficient observations to bring them through the preceding classes into a (1) or a (2). Class e.—Lost asteroids. These should be made the object of special orbit investigations to ascertain range of solution, and most probable orbit. Class f.—New discoveries. A fairly definite observational programme for necessary and sufficient observations during discovery opposition should be agreed upon, the discoverer to be responsible for the execution of the observational programme in codperation with one or two other observa- tories, the orbit determinations to proceed hand in hand with the observa- tions, so that a fairly accurate orbit may be available at end of discovery opposition to serve for incorporation of asteroid in one of the preceding classes. Accidental discoveries to be reported to central agency or agencies for assignment of observational programme and orbit computations. Particularly interesting cases, such as 1911 MT, or object Wolf 1918, etc., to be generally distributed by telegraph to stimulate observation and investigation. Class g.—Asteroids observed insufficiently even for preliminary orbit determinations. SUPPLEMENTARY RECOMMENDATIONS OF COMMITTEE ON COMETS AND ASTEROIDS Recommended for Reference to International Committee on Comets and Asteroids 1. That mean places of asteroids and comets visually observed with an. equatorial be published instead of appareat places. At present the orbit computer makes no essential use of the apparent place reduction of the comparison star. It is practically universal to employ the mean: place, corrected or uncorrected for annual aberration according to method, ob- tained by forming apparent place of object minus apparent place reduc- tion of comparison star. This mean place may also be obtained by ap- plying Aa and Aé directly to mean place of comparison star. In case the orbit computer wishes to take account of the differential apparent place reduction, he would, if apparent place were discontinued, be limited to the use of the differential formulae, which, however, are not longer than a single apparent place reduction. Thus it is seen that the proposed change would give the orbit computer directly what he uses; would in no way in- crease his labor; would save the observer an apparent place reduction for every observation; and visual observations would be presented in the same way that photographic now are. 2. That each observatory adopt its own distinctive provisional designa- tion for asteroids not identified up to the moment of publication with a previously discovered one. Washington has chosen W and Johannesburg

INTERNATIONAL ASTRONOMICAL UNION 379 has chosen 7, followed by a number, but it might be anything distinctive. It is advisable that this always be preceded with the year of discovery, thus, 1917 W15, 1912 716, etc. This will enable an asteroid to be re- ferred to by a distinctive designation in current discussion and in indexes, thus facilitating such discussion and the looking up of references. 3. That this provisional designation of asteroids should be retained until undoubted identification with an old asteroid or until identified at a second opposition, when it may be given the permanent number designation. This would prevent holes in the consecutive numbering such as (132) Aethra, etc. 4. That the term ‘‘astrographic,”’ as first used in Greenwich Observa- tions 1910, be more generally adopted to denote the position of an object obtained from a photographic plate, by using in the reduction the mean places of comparison stars at the beginning of the year. Neglecting the differential apparent place reduction, it is identical with the position ob- tained by applying Aa and Aé directly to the mean place of the comparison star, as mentioned in (1). 5. That observers be urged to publish observation times in decimal of day, G. M. T. (Greenwich Civil Time after 1925, if the present agitation results in discontinuance of astronomical mean time). For this G. M. T. decimal of day is the form which most computers use, since it is most convenient, because the national ephemerides use G. M. T. 6. That the ephemerides give the rectangular solar coédrdinates at the beginning of the year, since they are most made use of by orbit computers who almost universally use mean place at beginning of year, and also to give these coérdinates in the usual manner so that they may be readily differenced. The Berliner Jahrbuch has done this since 1868. The re- duction to some other epoch ‘such as 1900 or 1925 would also be useful. In addition to the codrdinates the velocities of the codérdinates per one- tenth of a day should also be given to facilitate interpolation and orbit computation by Laplacean methods. 7. That 1900.0 be adopted for the publication of positions of asteroids and for orbit computation. Many times more reference stars for both photographic and visual purposes will be referred to 1900.0 when the astrographic program is completed than to any other equinox. Perhaps more photographic positions of asteroids are now published than visual and probably in a few years they will greatly outnumber the visual. Since it is easier for the photographic observer to derive positions in the equinox of the catalog, it would seem that in the long run, more computation will be saved by this coéperation of observer and computer than by the adoption of any other equinox or equinoxes. A. O. LEUSCHNER, Chairman, E. W. Brown, G. H. PETERS.

380 INTERNATIONAL ASTRONOMICAL UNION REPORT OF THE COMMITTEE ON STELLAR PARALLAXES At the present time determinations of stellar parallax by the trigono- metric method form part of the regular work at the following observatories : APERTURE CHARACTER POCAL LENGTH Allegheny................. 30 inches Photographic Refractor 46 ft. Dearborn...............-. 18.5 Visual Refractor 28 Greenwich................ 26 Photographic Refractor 23 McCormick............... 26 Visual Refractor 32 Mount Wilson............. 60 Reflector 80 Swarthmore............... 24 Visual Refractor 36 Yerkes. .............0205- 40 Visual Refractor 62 At these seven institutions material sufficient for the determination of three hundred parallaxes is obtained each year. The average probable error of one determination is under .01”. Determination of stellar distances by the spectroscopic method are being carried out at Mount Wilson with the 60-inch reflector. When the 100-inch telescope is in regular operation, a larger part of the time of the 60-inch telescope will probably be devoted to obtaining spectrograms for this purpose, so that we may expect a rapid increase in our knowledge from this source. This committee recommends that other institutions be urged to undertake this work, their programs to include a large number of objects already observed by the spectroscopic method. Attention is called to the fact that existing spectrograms (obtained, for example, in radial velocity investigations) could profitably be examined from the point of view of absolute magnitudes and stellar distances, at least for the spec- tral types Fo to M. The paucity of reliable parallaxes determined by the trigonometric method has hitherto been a serious obstacle to the effective application of the spectroscopic method, since the latter depends principally upon the trigonometric results for the determination of relationships between line intensities and absolute magnitudes for the various spectral types. The war has delayed the publication of much material that will overcome this obstacle to a considerable extent, and it is likely that in a few years ample material of this kind will be at hand. One of the most important services that observers employing the trigonometric method can render at the present time is to arrange their programmes with the needs of the spectro- scopic method in mind. | This committee makes the following recommendations in addition to those already mentioned: 1. That observers employing either method should record the reason for each star being placed upon their observing lists, in order that their results may be properly used in statistical discussions.