Originally the Julian Period was used only to count years , and the Julian calendar was used to express historical dates within years . In his book '' Outlines of Astronomy '' , first published in 1849 , the astronomer John Herschel added the counting of days elapsed from the beginning of the Julian Period : 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 . 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 4713 BC , and the noon of the 1st of January 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 . Astronomers adopted Herschel 's " days of the Julian period " in the late nineteenth century , but used the meridian of Greenwich instead of Alexandria , after the former was adopted as the Prime Meridian after the International Meridian Conference in Washington in 1884 . This has now become the standard system of Julian days numbers
International Meridian Conference
International Meridian Conference
Julian day