DAVIS'S REPOKT ON THE NAUTICAL ALMANAC. 



[1853. 



The " Washington Observations" of 1846 have supplied the 

 mean places of what are called the " fundamental stare;" and this 

 volume, tou'ether with subsequent observations at the same 

 instruments, not yet printed, have enabled computers to employ 

 a more exact me:isure of the sun's diameter. 



For this and similar reasons, it has been correctly said that the 

 National Observatory now contributes to the general sum of the 

 requisite materiiils for making an almanac of our own. 



7. Whether any persons except the superiniendent have been 

 paid for services in preparing the "-National Almanac;'''' and 

 if so, how many, and lohat compensation have they rece'ived^ 



A list of the eoraputere and other persons employed in the 

 office of the Nautical Almanac is hereunto annexed ; and also a 

 statement of the number of persons, except the superintentlent, 

 who have been paid for services in preparing the Nautical Alma- 

 nac, and the compensi^tiou they have received up to the hist 

 payment. 



8. When is it expected that a N^autical Almanac will be 

 prepared for publication ? 



The reply to No. 8 is contained in that to No. 4. It is expected 

 tliat the lirst volume will be ready for sale and distribution in 

 about three or four months. 



9. Wiiat improvement, if any, is it expected the American 

 Jfautical Almanac, when published, will have over the 

 English? 



The American Nautical Almanac has made improvements 

 upon the English in the ephemeris of the moon, and that of most 

 of the planets. It has rejected the lunar tables of Burckhardt and 

 Damoiseau, now pronounced obsolete ; and has constructed lunar 

 tables for its own use, which embrace the corrections of Professor 

 Airy, deduced from the lunar observations made at the Royal 

 Observatory, Greenwich, from 1750 to 1830, and the corrections 

 arising from the discovery of Hansen. It is only necessary to turn 

 to the last pid'lished volumes of the W;ishington or Gieenwich 

 observatory to become acquainted with the errore and irregulari- 

 ties that abound in the ephemeris of the moon, very oflen 

 extending to one-third of a minute of ai'C. The determination of 

 the longitude at sea, however, by the method known as " the 

 lunar observation," the only method employed in the common 

 practice of navigators, where chronometers are wanting or are 

 untrustworthy, or require verification or examination of their 

 rates, depends essentially or int)'insically upon the accuracy of the 

 moon's predicted place. Now, this error of one-third of a minute 

 of arc involves an eri'or of ten miles in the determination of a 

 ship's longitude at sea. 



The lunar tables, prepared in the office of the'Nautieal Alma- 

 nac, reduce the average errore in the moon's place, as derived fi-om 

 the obsolete txibles, and given in (he British Astronomical 

 Ephemeris, to one-third of their amount; and a distinguished 

 gentleman ofPliiladel|ihia, Mr. Miere Fisher Longstretli, has since 

 j)ubiished an improvement of the lunar formula, which has pro- 

 babh' reduced this remaining error by two-thirds. Mr.Longstreth's 

 corrections have been emboilied iu the new tables of the almanac, 

 and thus, owing to the genius and laboui's of Pierce, Longstreth, 

 and other distinguished astronomei's, the almanac has it now in 

 its power to predict the moon's jilaco in the heavens with a degree 

 of precision far surpassing an3-tliing heretofore attained elsewhere. 

 And the proof of this is at hand. Whilst the lunar t<ibles were 

 in the coui-so of prcjiaration, the DeiKUtment, iu a letter dated 

 August .5, 1850, authoi'izod the sujierintendent of the Nautical 

 Almanac to publi.sh his predictioTis and elements of the total 

 eclipse of the following year, July 28, 1851, for the express pur- 

 pose of testing the accuracy of the new tables, and of acquiring 



the meaus of other improvements ; aod on the 25th of August, 

 1850, the superintendent, by permission of the Department, 

 communicated the predictions of his office to the American 

 Association for the Advancement of Science, at that time in sessioa 

 at New Haven ; he, at the same time, announced to the mathe- 

 matical and ]ihysical section of that body, the preparation of the 

 new lunar tables, and submitted to its criticism and appcoval the 

 objei;ts in view, and the mode in which they were to be accom- 

 plished. His eommunicatiou is contained in the printed pro- 

 ceedings of the Association at that meeting. 



The event proved highly siitisfactory, by showing conclusively 

 the superiority of theluuixr t:ibles now in use in the office of the 

 American almanac. 



For the prediction at Cambridge the British almanac was in 

 error eighty-five seconds, and the American almanac only twenty 

 seconds. 



At Washington, the British almanac was in error for the 

 beginning of the eclipse seventy-eight seconds, aud for the end 

 sixty -two seconds. The American almanac was in error for the 

 beginning only thirteen seconds, and for the end only one second 

 and a hidf. 



The obsen'ations were made by Mr. Sears C. Walker, at 

 Cambridge, and by Professor Hubbard and Mr. Fergusson (and 

 communicated by Lieut. Maury) at Washington. Where tlie 

 eclipse was total, and where, for this and other reasons, the test 

 was moie rigid aud conclusive, the result was still moie gratify- 

 ing and decisive as to the superiority of our own lunar tables. 

 The same tables of the moon are used in the French and Berlin 

 almanacs as in the British ; the errors, therefore, are the same. 

 The eiroi's exposed in this eclipse may give rise to an eri'or of 

 from fifteen to twenty miles in the determination of the longitude 

 at sea by means of lunar distances, and to an uncertainty of twice 

 that amount. The possibility of such an error, arising from this 

 source, is removed in the American ephemeris. 



It may be mentioned, among the benefits conferred b)' these 

 lunar tables, that tlioy bring into piactical availability a large 

 number of " moon culminations," as they are technically called, 

 obser^'ed by the astronomers of the coast survey on the western 

 coast of the United States, which have been hitherto lost. These 

 obsei-vations are made on the land for the nice and accurate de- 

 termination of geographical longitudes, and in that now difficult 

 and extensive field of labour are of the highest importance: 

 owing, however, to the imperfections in the tables, by means of 

 which the place of the moon iu her orbit is computed, no other 

 observed "moon culminations " can be usefidly ap|ilied than those 

 which have been correspondingly observed elsewhere. That is, 

 these " moon culminations," to be available, must be observed at 

 the same date at two diSerent places. In consequence of this 

 necessity, some six hundi'ed or more of the observations made in 

 Califorcia and Oregon, to be found in the books of the coast sur- 

 vey, have been laid aside " for want of moon's places more relia- 

 ble than the British Nautical Almanac can give us." — [Letter of 

 A. D. Bac^^e, superintendent Lfnitcd States Coast Survey, to tho 

 superintendent of the Nautical Almanac, Nov. 20, 1851.] 



It was said that the ephemeris of the ])lanets has been im- 

 pro\'ed. The epliomeris of the jilanct Jilercurj' will be derived, 

 for the fii-st time, from the now and elegant theory of M. Le 

 Terrier. 



In preparing tho ephemeris of Venus, with that of Mars, the 

 correctness of Lindcnau's elements of the orbits of these planets, 

 deduced from the On'enwich ]ilanetary observations from 1750 

 to 1830, by Mr. Hui;h Breen, have been for the first time intro- 

 duced. But some labour has been bestowed in combining the 

 rough gioupiiigs of Mr. Breen in such a manner a.s to carry for- 

 ward the coriections uiiinterruptcdlv; nil his results have also 



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