Editing Julian day (section)

===Julian day numbers===
Julian days were first used by [[Christian Ludwig Ideler|Ludwig Ideler]] for the first days of the Nabonassar and Christian eras in his 1825 ''Handbuch der mathematischen und technischen Chronologie''.<ref>Ideler 1825, pp. 102–106</ref>{{refn|The Nabonassar day was elapsed with a typo – it was correctly printed later as 1448638. The Christian day (1721425) was current, not elapsed.}} [[John Herschel|John F. W. Herschel]] then developed them for astronomical use in his 1849 ''Outlines of Astronomy'', after acknowledging that Ideler was his guide.<ref>Herschel, 1849, p. 632 note</ref>

{{blockquote |text=The period thus arising of 7980 Julian years, is called the Julian period, and it has been found so useful, that the most competent authorities have not hesitated to declare that, through its employment, light and order were first introduced into chronology.<ref>Ideler 1825, p. 77</ref> We owe its invention or revival to Joseph Scaliger, who is said to have received it from the Greeks of Constantinople. The first year of the current Julian period, or that of which the number in each of the three subordinate cycles is 1, was the year {{nowrap|4713 BC}}, and the noon of January 1 of that year, for the meridian of Alexandria, is the chronological epoch, to which all historical eras are most readily and intelligibly referred, by computing the number of integer days intervening between that epoch and the noon (for Alexandria) of the day, which is reckoned to be the first of the particular era in question. The meridian of Alexandria is chosen as that to which Ptolemy refers the commencement of the era of Nabonassar, the basis of all his calculations.<ref name="Herschel 1849, p. 634">Herschel 1849, p. 634</ref>}}

At least one mathematical [[astronomer]] adopted Herschel's "days of the Julian period" immediately. [[Benjamin Peirce]] of [[Harvard University]] used over 2,800 Julian days in his ''Tables of the Moon'', begun in 1849 but not published until 1853, to calculate the lunar [[ephemeris|ephemerides]] in the new ''American Ephemeris and Nautical Almanac'' from 1855 to 1888. The days are specified for "Washington mean noon", with Greenwich defined as {{nowrap|18{{sup|h}} 51{{sup|m}} 48{{sup|s}}}} west of Washington (282°57′W, or Washington 77°3′W of Greenwich). A table with 197 Julian days ("Date in Mean Solar Days", one per century mostly) was included for the years –4713 to 2000 with no year&nbsp;0, thus "–" means BC, including decimal fractions for hours, minutes, and seconds.<ref>Peirce 1853</ref> The same table appears in ''Tables of Mercury'' by Joseph Winlock, without any other Julian days.<ref>Winlock 1864</ref>

The national ephemerides started to include a multi-year table of Julian days, under various names, for either every year or every leap year beginning with the French ''Connaissance des Temps'' in 1870 for 2,620&nbsp;years, increasing in 1899 to 3,000&nbsp;years.<ref>''Connaissance des Temps'' 1870, pp. 419–424; 1899, pp. 718–722</ref> The British ''Nautical Almanac'' began in 1879 with 2,000&nbsp;years.<ref>''Nautical Almanac and Astronomical Ephemeris'' 1879, p. 494</ref> The ''Berliner Astronomisches Jahrbuch'' began in 1899 with 2,000&nbsp;years.<ref>''Berliner Astronomisches Jahrbuch'' 1899, pp. 390–391</ref> The ''American Ephemeris'' was the last to add a multi-year table, in 1925 with 2,000&nbsp;years.<ref>''American Ephemeris'' 1925, pp. 746–749</ref> However, it was the first to include any mention of Julian days with one for the year of issue beginning in 1855, as well as later scattered sections with many days in the year of issue. It was also the first to use the name "Julian day number" in 1918. The ''Nautical Almanac'' began in 1866 to include a Julian day for every day in the year of issue. The ''Connaissance des Temps'' began in 1871 to include a Julian day for every day in the year of issue.

The French mathematician and astronomer [[Pierre-Simon Laplace]] first expressed the time of day as a decimal fraction added to calendar dates in his book, {{lang|fr|italic=yes|Traité de Mécanique Céleste}}, in 1823.<ref>Laplace 1823</ref> Other astronomers added fractions of the day to the Julian day number to create Julian Dates, which are typically used by astronomers to date [[astronomy|astronomical]] observations, thus eliminating the complications resulting from using standard calendar periods like eras, years, or months. They were first introduced into [[variable star]] work in 1860 by the English astronomer [[N. R. Pogson|Norman Pogson]], which he stated was at the suggestion of John Herschel.<ref>Pogson 1860</ref> They were popularized for variable stars by [[Edward Charles Pickering]], of the [[Harvard College Observatory]], in 1890.<ref>Furness 1915.</ref>

Julian days begin at noon because when Herschel recommended them, the [[astronomical day]] began at noon. The astronomical day had begun at noon ever since [[Ptolemy]] chose to begin the days for his astronomical observations at noon. He chose noon because the transit of the Sun across the observer's meridian occurs at the same apparent time every day of the year, unlike sunrise or sunset, which vary by several hours. Midnight was not even considered because it could not be accurately determined using [[water clock]]s. Nevertheless, he double-dated most nighttime observations with both [[Egypt (Roman province)|Egyptian]] days beginning at sunrise and [[Babylonia]]n days beginning at sunset.<ref>Ptolemy {{Circa|150}}, p. 12</ref> Medieval Muslim astronomers used days beginning at sunset, so astronomical days beginning at noon did produce a single date for an entire night. Later medieval European astronomers used Roman days beginning at midnight so astronomical days beginning at noon also allow observations during an entire night to use a single date. When all astronomers decided to start their astronomical days at midnight to conform to the beginning of the civil day, on {{nowrap|January 1, 1925}}, it was decided to keep Julian days continuous with previous practice, beginning at noon.

During this period, usage of Julian day numbers as a neutral intermediary when converting a date in one calendar into a date in another calendar also occurred. An isolated use was by Ebenezer Burgess in his 1860 translation of the ''[[Surya Siddhanta]]'' wherein he stated that the beginning of the [[Kali Yuga]] era occurred at midnight at the meridian of [[Ujjain]] at the end of the 588,465th&nbsp;day and the beginning of the 588,466th&nbsp;day (civil reckoning) of the Julian Period, or between {{nowrap|February 17 and 18}} JP&nbsp;1612 or 3102&nbsp;BC.<ref>Burgess 1860</ref><ref>Burgess was furnished these Julian days by US Nautical Alamanac Office.</ref> Robert Schram was notable beginning with his 1882 ''Hilfstafeln für Chronologie''.<ref>Schram 1882</ref> Here he used about 5,370 "days of the Julian Period". He greatly expanded his usage of Julian days in his 1908 ''Kalendariographische und Chronologische Tafeln'' containing over 530,000 Julian days, one for the zeroth day of every month over thousands of years in many calendars. He included over 25,000 negative Julian days, given in a positive form by adding 10,000,000 to each. He called them "day of the Julian Period", "Julian day", or simply "day" in his discussion, but no name was used in the tables.<ref>Schram 1908</ref> Continuing this tradition, in his book "Mapping Time: The Calendar and Its History" British physics educator and programmer Edward Graham Richards uses Julian day numbers to convert dates from one calendar into another using algorithms rather than tables.<ref>Richards 1998, pp. 287–342</ref>