
#REDIRECT Julian day
The Julian day or Julian day number (JDN) is the number of days that have elapsed since 12 noon Greenwich Mean Time (UT or TT) on Monday, January 1, 5th millennium BC in the proleptic Julian calendar . That day is counted as Julian day zero. The Julian day system was intended to provide astronomers with a single system of dates that could be used when working with different calendars and to unify different historical chronologies. ==Julian Date== The Julian Date (JD) is the Julian day number plus the Decimal time#Fractional Days that has elapsed since noon. Historical Julian Dates were recorded relative to GMT or Ephemeris Time, but the International Astronomical Union now recommends that Julian Dates be specified in Terrestrial Time, and that when necessary to specify Julian Dates using a different time scale, that the time scale used be indicated when required, such as JD(UT1). The fraction of the day is found by converting the number of hours, minutes, and seconds after noon into the equivalent decimal fraction. The term ''Julian date'' is also used to refer to: * Julian calendar dates * ISO 8601#Ordinal dates (day-of-year) The use of ''Julian date'' to refer to the day-of-year (ordinal date) is usually considered to be incorrect. == Alternatives == * The Heliocentric Julian Day (HJD) is the same as the Julian day, but adjusted to the frame of reference of the Sun, and thus can differ from the Julian day by as much as 8.3 minutes, that being the time it takes the Sun's light to reach Earth. The Julian day is sometimes referred to as the Geocentric Julian Day (GJD) in order to distinguish it from HJD. * Another version of the Julian day, introduced by Peter Meyer, is the chronological Julian Day (CJD), in which the starting point is set at midnight at the beginning of January 1, 5th millennium BC (proleptic Julian calendar) local time rather than noon UT. Chronographers found the ''Julian day'' concept useful, but they didn't like noon as the starting time. So CJD = JD + 0.5 (in the Greenwich time zone, anyway). Note that JD may use Universal Time (UT) or Terrestrial Time (TT), and so it is the same for all time zones and is independent of Summer Time or Daylight-Saving Time (DST). On the other hand, CJD is not, so it changes with different time zones and takes into account the different local DSTs. Users of CJD sometimes refer to the Julian day as astronomical Julian Day (AJD) to distinguish it from CJD. Because the starting point is so long ago, numbers in the Julian day can be quite large and cumbersome. A more recent starting point is sometimes used, for instance by dropping the leading digits, in order to fit into limited computer memory with an adequate amount of precision. * The Modified Julian Day (MJD), introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik 1, is defined in terms of the Julian day as follows: ::MJD = JD - 2400000.5 :The offset of 0.5 means that MJD started at midnight at the beginning of November 17, 1858, and that every Modified Julian Day begins and ends at midnight UT or TT. * The Reduced Julian Day (RJD) is also used by astronomers and counts days from the same day as MJD, but from noon UT or TT, and thus is defined as: : RJD = JD - 2400000 * The Truncated Julian Day (TJD) was introduced by NASA for the space program. TJD began at May 24, 1968. Since TJD exceeded four digits on October 10, 1995, some now count TJD from this date in order to maintain a four-digit number. It can be defined as: ::TJD = JD - 2440000.5 :or ::TJD = (JD - 0.5) mod 10000 * The Dublin Julian Day (DJD) is a count of days from midnight at the beginning of January 1, 1900. The source of the name is unknown. It is used in computer programs, such as Lotus 1-2-3 and Microsoft Excel. In these particular programs, this date is counted as day 1, instead of day 0, because the year 1900 was [http://support.microsoft.com/default.aspx?scid=kb;en-us;214326 erroneously] treated as a leap year. * The Lilian day number defines day 1 as October 15, 1582, which was the first day of the Gregorian Calendar. It was named for Aloysius Lilius, the principal author of the Gregorian Calendar. * The ANSI Date defines January 1, 1601 as day 1, and is used as the origin of COBOL integer dates. This epoch is the beginning of the previous 400-year cycle of leap years in the Gregorian Calendar, which ended with the year 2000. * Rata Die is the epoch used in ''Calendrical Calculations'' by Edward M. Reingold and Nachum Dershowitz, where day 1 is January 1, 1, that is, the first day of the Christian Era or Common Era in the proleptic Gregorian Calendar. ==History== The ''Julian day number'' is based on the ''Julian Period'' proposed by Joseph Justus Scaliger in 1583, at the time of the Gregorian calendar reform, but it is the multiple of three calendar cycles used with the Julian calendar: : 15 (Indiction cycle) × 19 (Metonic cycle) × 28 (Solar cycle) = 7980 years Its epoch falls at the last time when all three cycles were in their first year together — Scaliger chose this because it pre-dated all historical dates. ''Note:'' although many references say that the "Julian" in "Julian day" refers to Scaliger's father, Julius Caesar Scaliger, in the introduction to Book V of his ''Opus de Emendatione Tempore'' (''Work on the Emendation of Time'') he states: "''Iulianum vocauimus: quia ad annum Iulianum dumtaxat accomodata est''" which translates more or less as "''We call this Julian merely because it is accommodated to the Julian year''". This "Julian" refers to Julius Caesar, who introduced the Julian calendar in 46 BC. In his book ''Outlines of Astronomy'', first published in 1849, the astronomer John Herschel wrote:
==Solar Cycle== The 28-year solar cycle mentioned in the "history" section is clearly not the 11-year solar cycle linked to in that section. Where should the link be pointing? --User:Carnildo 23:27, 27 Oct 2004 (UTC) :The appropriate article does not seem to exist yet in Wikipedia. It should describe the 28-year cycle of the days of the week on which any specific date in the Julian calendar recurs—the product of a quadrennium (a four-year period with one leap day) and a week (of seven days) (4 × 7 = 28). Another possibility would be an article on the Julian Period (also not yet in Wikipedia) within which its three cycles would be described. — User:Joe Kress 06:51, Oct 28, 2004 (UTC) == any Greenwich time == "any Greenwich time" does not parse. Is there more than one Greenwich time? I think they would be Greenwich Mean Time, Greenwich Apparent Time, Greenwich Sidereal Time, etc. Universal Time and Terrestrial Time are time scales derived from GMT, but I don't think that they are called "Greenwich times". The [http://www.iers.org/iers/earth/resolutions/UAI_b1.html IAU] states: :it is recommended that JD be specified as SI seconds in Terrestrial Time (TT) where the length of day is 86,400 SI seconds. :In some cases it may be necessary to specify Julian Date using a different time scale...The time scale used should be indicated when required such as JD(UT1). I also do not see it mentioned, but Herschel's proposal for Julian Days (or "days of the Julian Period") had the day starting at noon in Alexandria, Egypt, since Greenwich was not yet accepted by all countries as the Prime Meridian, although he used GMT elsewhere: :The last 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 B.C., 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 he 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. I will make the edit when I have time.User:Nike 07:59, 18 Dec 2004 (UTC) ==Alternative name== I know that this is no place for original research, which is why I'm putting this on the talk page. I think the subject of this article is extremely important, and am a bit distressed at how confusing the current name is. Wouldn't it be a lot clearer for us to call this something like "Scaliger-Herschel Date" or so? Scaliger set the epoch, but Herschel established the pattern of its modern use, in which it serves to record the times at which astronomical events are observed. User:Arkuat 07:50, 2004 Dec 26 (UTC) :"Scaliger-Herschel Date" is not clearer — if anything, it is decidedly unclear. Article names in Wikipedia should be those by which they are normally known, not names which are esoteric such as your suggestion. Although both Scaliger and Herschel made important contributions to the subject, most who know of Julian days or Julian dates (and want to learn more by consulting an encyclopedia) do not know their names, let alone their contributions. I see no reason for changing the article's name. However, you are correct that the name can be confused with other subjects, such as a date in the Julian calendar, but that is a result of history which cannot be changed at this late date. This is disambiguated by the third paragraph "In other contexts", although this is not normal Wikipedia style. The disambiguation could be improved. — User:Joe Kress 19:25, Dec 26, 2004 (UTC) ::Changing the name of the article before changing the name of the thing referred to would of course be a terrible idea. I was talking about changing the name of the thing referred to. User:Arkuat 21:27, 2004 Dec 26 (UTC) :::Julian days have been in use for more than a century in thousands of publications. Are you going to go back and edit all of those after the fact? I agree that the term ''Julian date'' is very ambiguous and it would have been better had something else been initially chosen, although maybe not what you suggest. (Maybe ''Julian day&time''.) Herschel referred only to the "day of the Julian period". But ''Julian day'' is not a problem, since that term is not used with the Julian calendar, and using it for day-of-year (ordinal date) is considered incorrect, so only that useage needs to be changed. In any event, this is hardly the place to do anything about it. Try the International Astronomical Union, instead. -- User:Nike 04:12, 27 Dec 2004 (UTC) I edited the section on Julian Dates, adding a header and clarifying the language. I simplified the disambiguation part, adding an internal link for ''ordinal dates'' instead of defining them ''in situ''. I also put a comment that this usage (often called ''day-of-year'') is considered incorrect. -- User:Nike 05:04, 27 Dec 2004 (UTC) == Other epochs, etc. == I'm unclear as to the purpose of this section, and exactly how the facts within it are significant to the article. It starts with some dates listed with no reason given and talks about Microsoft Excel without explaining why. Perhaps there could be a section like, Similar Systems with the contents explained, and the day-of-week stuff put somewhere else. -- User:Nike 05:11, 27 Dec 2004 (UTC) OK, I listed the alternatives in this section under Alternatives and moved the day-of-week calculations under Calculations. -- User:Nike 08:03, 29 Dec 2004 (UTC)
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Words begining with Julian_day:
Julian_Day
Julian_day
Julian_day
Julian_day_number
Julian_Day_Numbers
Julian Day
#REDIRECT Julian day
Julian day
The Julian day or Julian day number (JDN) is the number of days that have elapsed since 12 noon Greenwich Mean Time (UT or TT) on Monday, January 1, 5th millennium BC in the proleptic Julian calendar . That day is counted as Julian day zero. The Julian day system was intended to provide astronomers with a single system of dates that could be used when working with different calendars and to unify different historical chronologies. ==Julian Date== The Julian Date (JD) is the Julian day number plus the Decimal time#Fractional Days that has elapsed since noon. Historical Julian Dates were recorded relative to GMT or Ephemeris Time, but the International Astronomical Union now recommends that Julian Dates be specified in Terrestrial Time, and that when necessary to specify Julian Dates using a different time scale, that the time scale used be indicated when required, such as JD(UT1). The fraction of the day is found by converting the number of hours, minutes, and seconds after noon into the equivalent decimal fraction. The term ''Julian date'' is also used to refer to: * Julian calendar dates * ISO 8601#Ordinal dates (day-of-year) The use of ''Julian date'' to refer to the day-of-year (ordinal date) is usually considered to be incorrect. == Alternatives == * The Heliocentric Julian Day (HJD) is the same as the Julian day, but adjusted to the frame of reference of the Sun, and thus can differ from the Julian day by as much as 8.3 minutes, that being the time it takes the Sun's light to reach Earth. The Julian day is sometimes referred to as the Geocentric Julian Day (GJD) in order to distinguish it from HJD. * Another version of the Julian day, introduced by Peter Meyer, is the chronological Julian Day (CJD), in which the starting point is set at midnight at the beginning of January 1, 5th millennium BC (proleptic Julian calendar) local time rather than noon UT. Chronographers found the ''Julian day'' concept useful, but they didn't like noon as the starting time. So CJD = JD + 0.5 (in the Greenwich time zone, anyway). Note that JD may use Universal Time (UT) or Terrestrial Time (TT), and so it is the same for all time zones and is independent of Summer Time or Daylight-Saving Time (DST). On the other hand, CJD is not, so it changes with different time zones and takes into account the different local DSTs. Users of CJD sometimes refer to the Julian day as astronomical Julian Day (AJD) to distinguish it from CJD. Because the starting point is so long ago, numbers in the Julian day can be quite large and cumbersome. A more recent starting point is sometimes used, for instance by dropping the leading digits, in order to fit into limited computer memory with an adequate amount of precision. * The Modified Julian Day (MJD), introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik 1, is defined in terms of the Julian day as follows: ::MJD = JD - 2400000.5 :The offset of 0.5 means that MJD started at midnight at the beginning of November 17, 1858, and that every Modified Julian Day begins and ends at midnight UT or TT. * The Reduced Julian Day (RJD) is also used by astronomers and counts days from the same day as MJD, but from noon UT or TT, and thus is defined as: : RJD = JD - 2400000 * The Truncated Julian Day (TJD) was introduced by NASA for the space program. TJD began at May 24, 1968. Since TJD exceeded four digits on October 10, 1995, some now count TJD from this date in order to maintain a four-digit number. It can be defined as: ::TJD = JD - 2440000.5 :or ::TJD = (JD - 0.5) mod 10000 * The Dublin Julian Day (DJD) is a count of days from midnight at the beginning of January 1, 1900. The source of the name is unknown. It is used in computer programs, such as Lotus 1-2-3 and Microsoft Excel. In these particular programs, this date is counted as day 1, instead of day 0, because the year 1900 was [http://support.microsoft.com/default.aspx?scid=kb;en-us;214326 erroneously] treated as a leap year. * The Lilian day number defines day 1 as October 15, 1582, which was the first day of the Gregorian Calendar. It was named for Aloysius Lilius, the principal author of the Gregorian Calendar. * The ANSI Date defines January 1, 1601 as day 1, and is used as the origin of COBOL integer dates. This epoch is the beginning of the previous 400-year cycle of leap years in the Gregorian Calendar, which ended with the year 2000. * Rata Die is the epoch used in ''Calendrical Calculations'' by Edward M. Reingold and Nachum Dershowitz, where day 1 is January 1, 1, that is, the first day of the Christian Era or Common Era in the proleptic Gregorian Calendar. ==History== The ''Julian day number'' is based on the ''Julian Period'' proposed by Joseph Justus Scaliger in 1583, at the time of the Gregorian calendar reform, but it is the multiple of three calendar cycles used with the Julian calendar: : 15 (Indiction cycle) × 19 (Metonic cycle) × 28 (Solar cycle) = 7980 years Its epoch falls at the last time when all three cycles were in their first year together — Scaliger chose this because it pre-dated all historical dates. ''Note:'' although many references say that the "Julian" in "Julian day" refers to Scaliger's father, Julius Caesar Scaliger, in the introduction to Book V of his ''Opus de Emendatione Tempore'' (''Work on the Emendation of Time'') he states: "''Iulianum vocauimus: quia ad annum Iulianum dumtaxat accomodata est''" which translates more or less as "''We call this Julian merely because it is accommodated to the Julian year''". This "Julian" refers to Julius Caesar, who introduced the Julian calendar in 46 BC. In his book ''Outlines of Astronomy'', first published in 1849, the astronomer John Herschel wrote:
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 B.C., 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 Julian Days in the late 19th century, but using the meridian of Greenwich instead of Alexandria, after the former was made the Prime Meridian by international conference in 1884. This has now become the standard system of Julian days. Julian days are typically used by astronomers to date astronomy observations, thus eliminating the complications resulting from using standard calendar periods like eras, years, months, or weeks. Julian days begin at noon because when Herschel recommended them, the astronomical day began at noon (it did so until 1925). The astronomical day had begun at noon ever since Ptolemy chose to begin the days in his astronomical periods 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 clocks. Nevertheless, he double dated most nighttime observations with both Egyptian days beginning at sunrise and Babylonian days beginning at sunset. Thus the astronomical day did ''not'' begin at noon to allow all observations of a single night to be in a single day. ==Calculation== The Julian day number can be calculated using the following formulas: ''All divisions (except for JD) are integer divisions, meaning the remainder in the division is discarded. The months January to December are 1 to 12. Astronomical year numbering is used, thus 1 BC is 0, 2 BC is −1, and 4713 BC is −4712.'' For a date in the Gregorian calendar (at noon): For a date in the Julian calendar (at noon): For the full Julian Date (divisions are real numbers): The day of the week can be determined from the Julian day number by calculating it modular arithmetic 7, where 0 means Monday. {|border=1 cellspacing=0 cellpadding=3 |- | align=center | JD mod 7 || align=center | 0 || align=center | 1 || align=center | 2 || align=center | 3 || align=center | 4 || align=center | 5 || align=center | 6 |- | Day of the week || Mon || Tue || Wed || Thu || Fri || Sat || Sun |} ==See also== * epoch * epoch (astronomy) * era * time * time scales * Decimal time ==Footnotes == * This equals November 24, 4714 BC in the proleptic Gregorian calendar. ==References== * Gordon Moyer, "The Origin of the Julian Day System," ''Sky and Telescope'' 61 (April 1981) 311-313. * ''Explanatory Supplement to the Astronomical Almanac'', edited by P. Kenneth Seidelmann. University Science Books, 1992. ISBN 0935702687 ==External links== * [http://hermetic.nofadz.com/cal_stud/jdn.htm Article 'Julian Day Numbers'] by Peter Meyer * [http://aa.usno.navy.mil/data/docs/JulianDate.html U.S. Naval Observatory Julian Date Converter] * [http://www.nr.com/julian.html Julian Day and Civil Date calculator] * [http://tycho.usno.navy.mil/mjd.html U.S. Naval Observatory Time Service article] * [http://visualiseur.bnf.fr/Visualiseur?Destination=Gallica&O=NUMM-94926 Outlines of Astronomy by John Herschel] * [http://www.iers.org/iers/earth/resolutions/UAI_b1.html International Astronomical Union Resolution 1B: On the Use of Julian Dates] * [http://emr.cs.iit.edu/home/reingold/calendar-book/Calendrica.html Calendrica] * [http://isotropic.org/uw/date/ Another Julian Day calculator with conversions to many other calendars] valid from 1 January 100 proleptic Gregorian calendar * [http://sourceforge.net/projects/solarclock/ Open-source date conversion software] Calendars Astronomy vi:Ngày Julian
Julian day
==Solar Cycle== The 28-year solar cycle mentioned in the "history" section is clearly not the 11-year solar cycle linked to in that section. Where should the link be pointing? --User:Carnildo 23:27, 27 Oct 2004 (UTC) :The appropriate article does not seem to exist yet in Wikipedia. It should describe the 28-year cycle of the days of the week on which any specific date in the Julian calendar recurs—the product of a quadrennium (a four-year period with one leap day) and a week (of seven days) (4 × 7 = 28). Another possibility would be an article on the Julian Period (also not yet in Wikipedia) within which its three cycles would be described. — User:Joe Kress 06:51, Oct 28, 2004 (UTC) == any Greenwich time == "any Greenwich time" does not parse. Is there more than one Greenwich time? I think they would be Greenwich Mean Time, Greenwich Apparent Time, Greenwich Sidereal Time, etc. Universal Time and Terrestrial Time are time scales derived from GMT, but I don't think that they are called "Greenwich times". The [http://www.iers.org/iers/earth/resolutions/UAI_b1.html IAU] states: :it is recommended that JD be specified as SI seconds in Terrestrial Time (TT) where the length of day is 86,400 SI seconds. :In some cases it may be necessary to specify Julian Date using a different time scale...The time scale used should be indicated when required such as JD(UT1). I also do not see it mentioned, but Herschel's proposal for Julian Days (or "days of the Julian Period") had the day starting at noon in Alexandria, Egypt, since Greenwich was not yet accepted by all countries as the Prime Meridian, although he used GMT elsewhere: :The last 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 B.C., 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 he 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. I will make the edit when I have time.User:Nike 07:59, 18 Dec 2004 (UTC) ==Alternative name== I know that this is no place for original research, which is why I'm putting this on the talk page. I think the subject of this article is extremely important, and am a bit distressed at how confusing the current name is. Wouldn't it be a lot clearer for us to call this something like "Scaliger-Herschel Date" or so? Scaliger set the epoch, but Herschel established the pattern of its modern use, in which it serves to record the times at which astronomical events are observed. User:Arkuat 07:50, 2004 Dec 26 (UTC) :"Scaliger-Herschel Date" is not clearer — if anything, it is decidedly unclear. Article names in Wikipedia should be those by which they are normally known, not names which are esoteric such as your suggestion. Although both Scaliger and Herschel made important contributions to the subject, most who know of Julian days or Julian dates (and want to learn more by consulting an encyclopedia) do not know their names, let alone their contributions. I see no reason for changing the article's name. However, you are correct that the name can be confused with other subjects, such as a date in the Julian calendar, but that is a result of history which cannot be changed at this late date. This is disambiguated by the third paragraph "In other contexts", although this is not normal Wikipedia style. The disambiguation could be improved. — User:Joe Kress 19:25, Dec 26, 2004 (UTC) ::Changing the name of the article before changing the name of the thing referred to would of course be a terrible idea. I was talking about changing the name of the thing referred to. User:Arkuat 21:27, 2004 Dec 26 (UTC) :::Julian days have been in use for more than a century in thousands of publications. Are you going to go back and edit all of those after the fact? I agree that the term ''Julian date'' is very ambiguous and it would have been better had something else been initially chosen, although maybe not what you suggest. (Maybe ''Julian day&time''.) Herschel referred only to the "day of the Julian period". But ''Julian day'' is not a problem, since that term is not used with the Julian calendar, and using it for day-of-year (ordinal date) is considered incorrect, so only that useage needs to be changed. In any event, this is hardly the place to do anything about it. Try the International Astronomical Union, instead. -- User:Nike 04:12, 27 Dec 2004 (UTC) I edited the section on Julian Dates, adding a header and clarifying the language. I simplified the disambiguation part, adding an internal link for ''ordinal dates'' instead of defining them ''in situ''. I also put a comment that this usage (often called ''day-of-year'') is considered incorrect. -- User:Nike 05:04, 27 Dec 2004 (UTC) == Other epochs, etc. == I'm unclear as to the purpose of this section, and exactly how the facts within it are significant to the article. It starts with some dates listed with no reason given and talks about Microsoft Excel without explaining why. Perhaps there could be a section like, Similar Systems with the contents explained, and the day-of-week stuff put somewhere else. -- User:Nike 05:11, 27 Dec 2004 (UTC) OK, I listed the alternatives in this section under Alternatives and moved the day-of-week calculations under Calculations. -- User:Nike 08:03, 29 Dec 2004 (UTC)
See other meanings of words starting from letter:
J
JA | JB | JC | JD | JE | JF | JG | JH | JI | JK | JL | JM | JN | JO | JP | JR | JS | JT | JU | JW | JX | JY | JZ |Words begining with Julian_day:
Julian_Day
Julian_day
Julian_day
Julian_day_number
Julian_Day_Numbers
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