Geodesy. Latitude Observations with Photographic Zenith Tube At
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Serial No. 7 ^.DEPARTMENT OF COMMERCE ) U. S. COAST AND GEODETIC SURVEY HI E. LESTER JONES, Superintendent GEODESY LATITUDE OBSERVATIONS WITH PHOTOGRAPHIC ZENITH TUBE AT GAITHERSBURG, MD. BY FRAIfK K. ROSS, Ph. I>. IN COOPERATION WITH THE INTERNATIONAL GEODETIC ASSOCIATION SPECIAL PUBLICATIONNINO.I 27 1 WASHINGTON GOVERNMENT PRINTING OP1TOK 1913 Serial No. 7 DEPARTMENT OF COMMERCE U. S. COAST AND GEODETIC SUEVEY E. LESTER JONES, Superintendent GEODESY LATITUDE OBSERVATIONS WITH PHOTOGRAPHIC ZENITH TUBE AT GAITHERSBURG, MD. BY FRAJSTK TG. ROSS, Pli. D. IN COOPERATION WITH THE INTERNATIONAL GEODETIC ASSOCIATION SPECIAL PUBLICATION No. 27 WASHINGTON GOVERNMENT PRINTING OFFICE 1815 ADDITIONAL COPIES OF THIS PUBLICATION MAY BE PROCURED FROM THE SUPERINTENDENT OF DOCUMENTS GOVERNMENT PRINTING OFFICE WASHINGTON, D. C. AT 25 CENTS PER COPY V 137 CONTENTS. Pago. Introduction 7 Some considerations leading to a choice of instrument 9 Description of instrument and adjustments 16 Specimen measurement and reduction sheet 25 The Elizabeth Thompson comparator 27 Reseau 30 Adjustments of the zenith tube 33 Scale value 38 Temperature coefficient (semi-theoretical) 40 Film distortion '. 42 Position of the second Gaussian point 46 Investigation of the accuracy of the carriage motion 46 Program and latitude observations, photographic 49 Star program 49 Results of observations, photographic 51 Group differences and closing sum 70 Constant of aberration 72 Solar parallax 72 Daily mean latitudes, definitive 73 Comparison of photographic variation of latitude curve with the normal curve 76 The Kimura term 78 Discussion of errors 81 Probable errors 81 Temperature conditions 82 Probable errors arranged according to zenith distance 85 Probable error of the daily mean latitude 85 Relative accuracy of observations before and after midnight 86 Systematic and accidental errors in plate measurement 87 Probable error of the photographic impression of stellar images. 90 Probable error depending upon stellar magnitude 91 Refraction oscillations 93 The systematic error N-S 95 Common pairs 97 Results of observations with the visual instrument 104 Discussion of visual latitudes 119 Probable error with visual instrument 121 Night error and abnormal results 122 Daily variation of latitude i 125 Personal equation with zenith telescope 126 Number 'of observations summarized ; 127 Summary of results 127 ILLUSTRATIONS. View of reseau 31 [Together at end of volume.) Plate. Section of zenith tube and building, . A Plan view of head of zenith tube B Sectional view of head of zenith tube ' C Section of magnetic clutch D Plate holder E Focusing rod F Zenith tube, upper part - G Zenith tube, lower part H View of exposure control Tube clock J Elizabeth Thompson comparator K Mercury basin and stand L Microscope and stand for testing motion of carriage M View of observatories - N General view of grounds and buildings O Photographic and visual latitude variations compared with normal curves P Specimen photographic plate Q 3 LETTER OF TRANSMITTAL. Department of Commerce, U. S. Coast and Geodetic Survey, Washington, April 17, 1915. Sir: There is transmitted herewith manuscript of a publication entitled "Latitude Observations with Photographic Zenith Tube, by Frank E. Ross, Ph. D., at Gaithersburg, Md., in Cooperation with the International Geodetic Association," and intended for publi- cation as Special Publication No. 27. The zenith tube was constructed according to plans prepared by Dr. Ross and was used by him at the latitude observatory maintained by the International Geodetic Association at Gaithersburg, Md., which observatory was under his charge. By the use of the zenith tube results of very remarkable precision and of great importance have been obtained. By an agreement approved by the Treasury Department January 23, 1899 (under which department the Coast and Geodetic Survey then was), the Superintendent of the Coast and Geodetic Survey, being a member of the Permanent Commission of the International Geodetic Association, has exercised supervision over the work of two of the three observatories for the observation of the variation of latitude maintained by the Association in the United States, Gaithersburg being one. The observatory at Gaithersburg was closed some months ago, and the final work of Dr. Ross has been the preparation of the report now transmitted. E. Lester Jones, Superintendent. To Hon. William C. Redfield, Secretary of Commerce. LETTER OF SUBMITTAL. To the Superintendent of the United States Coast and Geodetic Survey and Member of the Per- manent Commission of the International Geodetic Association. Sir: I herewith submit to you the following report of the operations conducted at the observatory of the International Geodetic Association at Gaithersburg, Md., from June, 1911, to October, 1914, with particular reference to the work of the Photographic Zenith Tube and its comparison with the work of the Wanschaff Zenith Telescope. Very respectfully, Frank E. Ross, Observer in Charge. April 15, 1915. LATITUDF OBSERVATIONS WITH PHOTOGRAPHIC ZENITH TUBE AT GAITHERSBURG, MD. By FRANK E. ROSS, In cooperation with the International Geodetic Association. INTRODUCTION. At the conference of the International Geodetic Association at Cambridge, England, in September, 1909, plans were submitted by Mr. O. H. Tittmann, Superintendent of the United States Coast and Geodetic Survey, for the construction and operation of a photographic reflex zenith tube to be used in determining the latitude variation. It was proposed that this instru- ment be installed at Gaithersburg, Md., one of the six latitude observatories of the Association, and operated in conjunction with the Wanschaff zenith telescope without interfering with the program of work mapped out for the latter. It was proposed that the results secured be com- pared with those obtained by the Wanschaff instrument, and its curve of latitude variation further compared with the normal curve resulting from the combined work of all six stations. It was hoped that the data secured would shed light upon the Kimura term in the latitude variation, which is of obscure origin and has remained an unsolved problem; that a value of the constant of aberration be obtained, a by-product of the work, which should be entitled to the greatest weight, possibly shedding light on the remarkable discordances now existing in its various determinations. It was further hoped that the accuracy of the individual obser- vations and freedom from all classes of systematic error would be such that short-period or diurnal terms in the latitude variation might be detected, if any such exist, apart from those minute terms given by theory, which are well known. The Conference thereupon voted a subvention of 10,000 marks for the construction of the instrument and a further subvention of 2,000 marks annually for assistance in measuring the photographs and making the reductions. The following year Mr. Tittmann secured a grant from the trustees of the Elizabeth Thompson Science Fund for the construction of a comparator, or instrument of a design suitable for measuring the photographs. Yearly reports of the kind and amount of work accomplished with the comparator thus secured have been submitted to the chairman, the late Dr. C. S. Minot. The contract for the construction of the photographic zenith tube and the comparator was awarded to William Gaertner & Co., of Chicago, 111. The completed instruments were received at Gaithersburg in May, 1911. The zenith tube was immediately installed in the building which had been prepared for it, and observations commenced early in June. Except for a break of seven weeks in August and September, 1911, during an absence of the writer, observations have been secured with it uninterruptedly from June 9, 1911, to October 23, 1914. Altogether 6,944 stars have been photographed on a total of 450 nights, or an average of 15 per night. For measuring the plates it was necessary to install the comparator in a stone or brick building, in which diurnal temperature changes are small. The wooden buildings at Gaith- ersburg are entirely unsuitable on account of their steep temperature gradient in the daytime, or during the hours when the plates are measured. The main office building of the United States Coast and Geodetic Survey in Washington was found ideal for the purpose. A room with a northern window was here set aside for the purpose and every facility placed at our disposal for the convenient and successful prosecution of the work, 7 8 U. 8. COAST AND GEODETIC SURVEY SPECIAL PUBLICATION NO. 27. and the subse- Practically the entire work of measuring the photographic plates making faithfulness and skill in quent reductions was performed by Miss Edith Jarboe, whose per- forming this arduous labor are dcsorving of special mention. The total number of stellar images measured by her was approximately 42,000, each being measured in two positions of the plate. Thanks are due to Dr. Frank Schlesinger, Director of the Allegheny Observatory, for his encouragement and valued criticism and advice; to Dr. P. G. Nutting and E. D. Tillyer, both of the United States Bureau of Standards, for valued conferences on the optical and photo- graphic problems involved in constructing and operating the instrument. To Mr. Tillyer is duo the credit for the plan adopted of connecting through the intermediary of a magnetic clutch, the fixed driving clock with the gearing of the movable reversible plate carriage track. The details of this clutch were designed