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JDN redirects here. For the military IT system, see Joint Data Network. The Joint Data Network (JDN is an interconnected network of Joint Tactical Information Distribution System (JTIDS–based systems which links air and missile defense
For the comic book character Julian Gregory Day see Calendar Man. Calendar Man (real name Julian Gregory Day) is a DC Comics supervillain

The Julian day or Julian day number (JDN) is the integer number of days that have elapsed since the initial epoch defined as noon Universal Time (UT) Monday, January 1, 4713 BC in the proleptic Julian calendar. A day (symbol d is a unit of Time equivalent to 24 Hours and the duration of a single Rotation of planet Earth with respect to the In the fields of Chronology and Periodization, an epoch means an instant in time chosen as the origin of a particular Era. Noon (also midday) is the hour of 1200 in an observer's local time zone or more loosely a time near the middle of the day when workers in many countries take a meal break Monday (pron ˈmʌndeɪ ˈmʌndi is a Day of the week between Sunday and Tuesday. New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC The 5th millennium BC saw the spread of Agriculture from the Near East throughout southern and central Europe The proleptic Julian calendar is produced by extending the Julian calendar to dates preceding AD 4 when its quadrennial leap year stabilized [1] That noon-to-noon day is counted as Julian day 0. Thus the multiples of 7 are Mondays. Negative values can also be used, although those predate all recorded history. Recorded history can be defined as History that has been written down or recorded by the use of Language, whereas History is a more general term referring

Now at 15:03, Tuesday June 10, 2008 (UTC) the JDN is 2454629. Events 1190 - Third Crusade: Frederick I Barbarossa drowns in the Sally River while leading an army to Jerusalem 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common The remainder of this value divided by 7 is 2, an integer expression for the day of the week with 0 representing Monday. The names of the days of the week in various world languages can be classified as either numerical or planetary; however in either case the names of one or more days

The Julian date (JD) is a continuous count of days and fractions elapsed since the same initial epoch. Currently the JD is 2454628. 1277546. The integral part (its floor) gives the Julian day number. In Mathematics and Computer science, the floor and ceiling functions map Real numbers to nearby Integers The The fractional part gives the time of day since noon UT as a decimal fraction of one day or fractional day, with 0. The decimal ( base ten or occasionally denary) Numeral system has ten as its base. Decimal time is the representation of the Time of Day using units which are Decimally related 5 representing midnight UT. Midnight is literally "the middle of the night" In most systems it is when one day ends and the next begins when the date changes Typically, a 64-bit floating point (double precision) variable can represent an epoch expressed as a Julian date to about 1 millisecond precision. '64-bit' CPUs have existed in Supercomputers since the 1960s and in RISC -based workstations and servers since the early 1990s. In Computing, floating point describes a system for numerical representation in which a string of digits (or Bits represents a Real number. In Computing, double precision is a Computer numbering format that occupies two adjacent storage locations in computer memory In Astronomy, an epoch is a moment in time used as a reference for the Orbital elements of a Celestial body. A millisecond (from Milli- and Second; abbreviation ms is one thousandth of a Second.

A Julian date of 2454115. 05486 means that the date and Universal Time is Sunday 14 January 2007 at 13:18:59. Events 1129 - Formal approval of the Order of the Templar at the Council of Troyes. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. 9.

The decimal parts of a Julian date:
0. 1 = 2. 4 hours or 144 minutes or 8640 seconds
0. 01 = 0. 24 hours or 14. 4 minutes or 864 seconds
0. 001 = 0. 024 hours or 1. 44 minutes or 86. 4 seconds
0. 0001 = 0. 0024 hours or 0. 144 minutes or 8. 64 seconds
0. 00001 = 0. 00024 hours or 0. 0144 minutes or 0. 864 seconds.

Almost 2. 5 million Julian days have elapsed since the initial epoch. JDN 2,400,000 was November 16, 1858. Events 534 - A second and final revision of the Codex Justinianus is published Year 1858 ( MDCCCLVIII) was a Common year starting on Friday (link will display the full calendar of the Gregorian Calendar (or a Common JD 2,500,000. 0 will occur on August 31, 2132 at noon UT. Events 1056 - Byzantine Empress Theodora becomes ill dying suddenly a few days later without children to succeed the Throne The 22nd century of the Common Era will span the years 2101&ndash2200 of the Gregorian calendar.

The Julian day number can be considered a very simple calendar, where its calendar date is just an integer. The word Calendar consist of two words 1 Cal ( in Pashto means Year in Hindi and Persian is Sal- also means Year A date in a calendar is a reference to a particular day represented within a Calendar system This is useful for reference, computations, and conversions. It allows the time between any two dates in history to be computed by simple subtraction. Subtraction is one of the four basic Arithmetic operations it is the inverse of Addition, meaning that if we start with any number and add any number and then subtract

The Julian day system was introduced by astronomers to provide a single system of dates that could be used when working with different calendars and to unify different historical chronologies. Apart from the choice of the zero point and name, this Julian day and Julian date are not directly related to the Julian calendar, although it is possible to convert any date from one calendar to the other. The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 Ab urbe condita

Contents

Julian Date

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). Ephemeris Time ( ET) is a time scale used in Ephemerides of celestial bodies in particular the Sun (as observed from the Earth Moon planets and other members of Terrestrial Time (TT is the modern astronomical standard for the passage of time on the surface of the Earth (for civil purposes Coordinated Universal 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:

The use of Julian date to refer to the day-of-year (ordinal date) is usually considered to be incorrect, however it is widely used that way in the earth sciences and computer programming. The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 Ab urbe condita ISO 8601 is an International standard for date and Time representations issued by the International Organization for Standardization (ISO

Alternatives

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.

Name Current Epoch Calculation Current Value Notes
Julian Date (JD) BC 4713-01-01 12:00, Monday 2454628. 1277546
Julian Day Number (JDN) BC 4713-01-01 12:00, Monday JDN = floor (JD) 2454629 Changes at Noon UT or TT
Chronological Julian Day (CJD) BC 4713-01-01 00:00, Monday JDN = floor (JD + 0. 5) 2454629 (UT) Specific to time zone; UT CJD given
Reduced Julian Day (RJD) 1858-11-16 12:00, Tuesday RJD = JD − 2400000 54628. 1277546 Used by astronomers
Modified Julian Day (MJD) 1858-11-17 00:00, Wednesday MJD = JD − 2,400,000. 5 54627. 6277546 Introduced by SAO in 1957
Truncated Julian Day (TJD) 1968-05-24 00:00, Friday
1995-10-10 00:00, Tuesday		 TJD = JD − 2440000. The Smithsonian Astrophysical Observatory (SAO is a "research institute" of the Smithsonian Institution headquartered in Cambridge Massachusetts, 5
TJD = (JD − 0. 5) mod 10000		 14627. 6277546
4627. 6277545998		 - Definition as introduced by NASA [1]
- NIST definition
Dublin Julian Day (DJD) 1899-12-31 12:00, Sunday DJD = JD − 2415020 39608. 1277546 Introduced by the IAU in 1955
Lilian Day Number 1582-10-16, Saturday (as Day 1) floor (JD - 2299160. 5) 155468 The count of days of the Gregorian calendar
ANSI Date 1601-01-01, Monday (as Day 1) floor (JD - 2305812. The Gregorian calendar is the most widely used Calendar in the world today 5) 148816 The origin of COBOL integer dates
Rata Die 0001-01-01, Saturday (as Day 1) floor (JD - 1721422. COBOL (ˈkoʊbɒl is one of the oldest programming languages still in active use Rata Die ( RD) is a system for assigning numbers to calendar days (optionally with time of day independent of any calendar for the purposes of Calendrical calculations 5) 733206 The count of days of the Common Era
Excel serial date 1990-01-01, Monday
1904-01-02, Saturday		 floor (JD − 2415018. 5)
floor (JD - 2416481. 5)		 39610
38147		 - Definition on PC
- Definition on Mac
Unix Time 1970-01-01, Thursday (JD – 2440587. Unix time, or POSIX time, is a system for describing points in Time, defined as the number of seconds elapsed since Midnight Coordinated Universal 5) × 86400 1213110238 Counts by the second, not the day

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. The Sun (Sol is the Star at the center of the Solar System. 3 minutes, that being the time it takes the Sun's light to reach Earth. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 The Julian day is sometimes referred to as the Geocentric Julian Day (GJD) in order to distinguish it from HJD.

History

The Julian day number is based on the Julian Period proposed by Joseph 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. Joseph Justus Scaliger ( August 5 1540 &ndash January 21 1609) was a French religious leader and scholar known for expanding the The Gregorian calendar is the most widely used Calendar in the world today The word Calendar consist of two words 1 Cal ( in Pashto means Year in Hindi and Persian is Sal- also means Year The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 Ab urbe condita An indiction is any of the years in a 15-year cycle used to date medieval documents throughout Europe, both East and West Metonic cycle or Enneadecaeteris in Astronomy and Calendar studies is a particular approximate common multiple of the Tropical year The solar cycle is a 28-year cycle of the Julian calendar with respect to the Week. In the fields of Chronology and Periodization, an epoch means an instant in time chosen as the origin of a particular Era.

Note: although many references say that the Julian in "Julian day" refers to Scaliger's father, Julius Scaliger, in the introduction to Book V of his Opus de Emendatione Temporum ("Work on the Emendation of Time") he states, "Iulianum vocavimus: quia ad annum Iulianum dumtaxat accomodata est", which translates more or less as "We have called it Julian merely because it is accommodated to the Julian year. Julius Caesar Scaliger or Giulio Cesare della Scala ( April 23, 1484 &ndash October 21, 1558) was an Italian scholar and physician " 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. Sir John Frederick William Herschel 1st Baronet KH, FRS ( March 7, 1792 &ndash May 11, 1871)was an 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 nineteenth century, but used the meridian of Greenwich instead of Alexandria, after the former was made the Prime Meridian by international conference in 1884. Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena The Prime Meridian is the meridian (line of Longitude) at which longitude is defined to be 0° This has now become the standard system of Julian days. Julian days are typically used by astronomers to date astronomical observations, thus eliminating the complications resulting from using standard calendar periods like eras, years, or months. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study The word Calendar consist of two words 1 Cal ( in Pashto means Year in Hindi and Persian is Sal- also means Year They were first introduced into variable star work by Edward Charles Pickering, of the Harvard College Observatory, in 1890. For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson Edward Charles Pickering ( July 19 1846 – February 3 1919) was an American Astronomer and Physicist, brother The Harvard College Observatory (or HCO is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Department of Astronomy [3]

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. Claudius Ptolemaeus ( Greek: Klaúdios Ptolemaîos; after 83 &ndash ca 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. A water clock or clepsydra ( Greek kleptein to steal; hydro water) is any timekeeper operated by means of a regulated flow of liquid into (inflow Nevertheless, he double-dated most nighttime observations with both Egyptian days beginning at sunrise and Babylonian days beginning at sunset. Ancient Egypt was an Ancient Civilization in eastern North Africa, concentrated along the lower reaches of the Nile River in what is now Babylonia was an Amorite state in lower Mesopotamia (modern southern Iraq) with Babylon as its capital This would seem to imply that his choice of noon was not, as is sometimes stated, made in order to allow all observations from a given night to be recorded with the same date.

Calculation

The Julian day number can be calculated using the following formulas:

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. Astronomical year numbering is based on AD ( Anno Domini)/CE ( Common Era) year numbering but follows normal Decimal Integer numbering more strictly In all divisions (except for JD) the floor function is applied to the quotient (for dates since 1 March −4800 all quotients are non-negative, so we can also apply truncation). In Mathematics and Computer science, the floor and ceiling functions map Real numbers to nearby Integers The Events 86 BC - Lucius Cornelius Sulla, at the head of a Roman Republic army enters in Athens, removing the Tyrant In Mathematics, truncation is the term for limiting the number of digits right of the Decimal point, by discarding the least significant ones

\begin{matrix}a & = & \left\lfloor\frac{14 - month}{12}\right\rfloor \\ \\y & = & year + 4800 - a \\ \\m & = & month + 12a - 3 \\\end{matrix}

For a date in the Gregorian calendar (at noon):

\begin{matrix}JDN & = & day + \left\lfloor\frac{153m + 2}{5}\right\rfloor + 365y + \left\lfloor\frac{y}{4}\right\rfloor - \left\lfloor\frac{y}{100}\right\rfloor + \left\lfloor\frac{y}{400}\right\rfloor - 32045\end{matrix}

For a date in the Julian calendar (at noon):

\begin{matrix}JDN & = & day + \left\lfloor\frac{153m + 2}{5}\right\rfloor + 365y + \left\lfloor\frac{y}{4}\right\rfloor - 32083\end{matrix}

The constants used at the end of the Gregorian and Julian formulas are required to return the same JDN for the same date in both calendars between March 1, 200 and February 28, 300. Events 86 BC - Lucius Cornelius Sulla, at the head of a Roman Republic army enters in Athens, removing the Tyrant Events By Place World Human population reaches about 257 million Events 202 BC - coronation ceremony of Liu Bang as Emperor Gaozu of Han takes place initiating four centuries of the Han Dynasty 's rule Events By place Roman Empire The Franks penetrate into what is now northern Belgium (approximate date The constants are the JDNs of February 29, −4800 in each calendar. Leap years Although the modern calendar counts a year as 365 days a complete revolution around the sun takes approximately 365 days and 6 hours In the proleptic Gregorian calendar the Julian day zero is November 24, 4714 BC which is 32045 days apart from the start of the Gregorian quadricentennial cycle (i. The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582. Events 380 - Theodosius I makes his adventus, or formal The 5th millennium BC saw the spread of Agriculture from the Near East throughout southern and central Europe e. 400-year cycle starting and ending in a year divisible by 400) containing the Julian day zero, which begins on March 1 4801 BC in the proleptic Gregorian calendar. Events 86 BC - Lucius Cornelius Sulla, at the head of a Roman Republic army enters in Athens, removing the Tyrant The 5th millennium BC saw the spread of Agriculture from the Near East throughout southern and central Europe The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582.

For the full Julian date, not counting leap seconds (divisions are real numbers):

\begin{matrix}JD & = & JDN + \frac{hour - 12}{24} + \frac{minute}{1440} + \frac{second}{86400}\end{matrix}

So, for example, 1 January 2000 at midday corresponds to JD = 2451545. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. 0

The day of the week can be determined from the Julian day number by calculating it modulo 7, where 0 means Monday. A week (also called sennight or sevennight) is a unit of Time longer than a Day and shorter than a Month. In Mathematics, modular arithmetic (sometimes called modulo arithmetic, or clock arithmetic) is a system of Arithmetic for Integers

JDN mod 7 0 1 2 3 4 5 6
Day of the week Mon Tue Wed Thu Fri Sat Sun

Gregorian calendar from Julian day number

We can then develop these formulas into a single inlined formula per component, computed as above. All this computing requires only integers and so is not sensitive to rounding errors caused by floating point approximations (most decimal fractions have an inexact representation within the binary format used by floating point arithmetic used by most computer software, so using them would produce false results on some dates because of roundoff errors).

The formulae below (which use Euclidian division — integer division (div) and modulo (mod) — without any negative numbers) are valid for the whole range of dates since −4800. For dates before 1582, the resulting date components are valid only in the Gregorian proleptic calendar. This is based on the Gregorian calendar but extended to cover dates before its introduction, including the pre-Christian era. For dates in that era (before year 1 CE), astronomical year numbering is used. Astronomical year numbering is based on AD ( Anno Domini)/CE ( Common Era) year numbering but follows normal Decimal Integer numbering more strictly This includes a year zero, which immediately precedes 1 CE. Astronomical year zero is 1 BCE in the proleptic Gregorian calendar and, in general, year n BCE = astronomical year 1 − n, and for astronomical year A (A < 1), the BCE year is 1 + abs(A).

J = Julian day number
j = J + 32044
g = j div 146097
dg = j mod 146097
c = (dg div 36524 + 1) × 3 div 4
dc = dg − c × 36524
b = dc div 1461
db = dc mod 1461
a = (db div 365 + 1) × 3 div 4
da = db − a × 365
y = g × 400 + c × 100 + b × 4 + a
m = (da × 5 + 308) div 153 − 2
d = da − (m + 4) × 153 div 5 + 122
Y = y − 4800 + (m + 2) div 12
M = (m + 2) mod 12 + 1
D = d + 1. 5

See also

Footnotes

  1. ^ This equals November 24, 4714 BC in the proleptic Gregorian calendar. In Astronomy, a Julian year (symbol a) is a unit of measurement of Time defined The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 Ab urbe condita Decimal time is the representation of the Time of Day using units which are Decimally related In the fields of Chronology and Periodization, an epoch means an instant in time chosen as the origin of a particular Era. In Astronomy, an epoch is a moment in time used as a reference for the Orbital elements of a Celestial body. An era is a commonly used word for long period of time When used in science for example geology eras denote clearly defined periods of time of arbitrary but well defined For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of An ordinal date is a Calendar date consisting of a year and a day of year ranging between 1 and 366 (starting on January 1) In historical materials dates will often be indicated with what appears to be duplicate or excessive digits sometimes separated by a hyphen or a slash The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582.
  2. ^ Why Is Wednesday November 17, 1858 The Base Time For VAX/VMS?. Retrieved on 2007-12-13. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1294 - Saint Celestine V abdicates the papacy after only five months Celestine hoped to return to his previous life
  3. ^ An Introduction to the Study of Variable Stars by Caroline Ellen Furness page 206.

References

External links

New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC
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