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'''Julian day''' is a count of days since the beginning of the Julian period, used  to calculating days between any two events.
 
'''Julian day''' is a count of days since the beginning of the Julian period, used  to calculating days between any two events.
  
The calendar, with twelve months of the year, each with varying number of days, and February with 28 or twenty nine days in leap years, makes it difficult and complicated to calculate the time between any two events. Joseph Scaliger proposed that a Julian Period be used to time events, beginning at a time sufficiently far in the past as to precede dates in recorded history. The Julian period begins Monday, 1 January 4713 B.C. and is 86400 seconds of time. The Julian Year is 365.25 days and the Julian century is 36525 days.
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The calendar, with twelve months of the year, each with varying number of days, and February with 28 or twenty nine days in leap years, makes it difficult and complicated to calculate the time between any two events. Joseph Scaliger proposed that a Julian Period be used to time events, beginning at a time sufficiently far in the past as to precede dates in recorded history. The Julian period begins Monday, 1 January 4713 B.C.  
  
 
'''Julian Day''' counts the number of days from the beginning of the Julian period allowing the calculating of event times. '''Julian Day Number''' is an integer number assigned to any one day, the JD beginning at noon. The '''Julian Date''' is an instant given as a decimal part of the day since the previous noon.
 
'''Julian Day''' counts the number of days from the beginning of the Julian period allowing the calculating of event times. '''Julian Day Number''' is an integer number assigned to any one day, the JD beginning at noon. The '''Julian Date''' is an instant given as a decimal part of the day since the previous noon.
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'''To convert a Gregorian calendar date to Julian Day Number, used for dates on or after 15 October 1582 for Catholic countries, other countries changed on other dates):'''
 
'''To convert a Gregorian calendar date to Julian Day Number, used for dates on or after 15 October 1582 for Catholic countries, other countries changed on other dates):'''
  
JDN = (1461 × (Y + 4800 + (M − 14)/12))/4 +(367 × (M − 2 − 12 × ((M − 14)/12)))/12 − (3 × ((Y + 4900 + (M - 14)/12)/100))/4 + D − 32075<ref name="WikiJD">[[w:Julian Day|Julian Day at Wikipedia]]</ref>
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JDN = (1461 × (Y + 4800 + (M − 14)/12))/4 +(367 × (M − 2 − 12 × ((M − 14)/12)))/12 − (3 × ((Y + 4900 + (M - 14)/12)/100))/4 + D − 32075<ref>[[w:Julian Day|Julian Day at Wikipedia]]</ref>
  
 
where  
 
where  
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'''To convert a Julian calendar date to Julian Day Number, used for dates on or before 4 October 1582 for Catholic countries, other countries changed on other dates):'''
 
'''To convert a Julian calendar date to Julian Day Number, used for dates on or before 4 October 1582 for Catholic countries, other countries changed on other dates):'''
  
JDN = 367 × Y − (7 × (Y + 5001 + (M − 9)/7))/4 + (275 × M)/9 + D + 1729777<ref name="WikiJD"/>
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JDN = 367 × Y − (7 × (Y + 5001 + (M − 9)/7))/4 + (275 × M)/9 + D + 1729777
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where  
 
where  
 
*Y = year  
 
*Y = year  
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'''To convert Julian Day Number to Julian Date, given time of day:'''
 
'''To convert Julian Day Number to Julian Date, given time of day:'''
JD = JDN + (h - 12)/24 + m/1440 + s/86400<ref name="WikiJD"/>
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JD = JDN + (h - 12)/24 + m/1440 + s/86400
  
 
where
 
where
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'''To convert Julian Date to Modified Julian Date as used by Orbiter:'''
 
'''To convert Julian Date to Modified Julian Date as used by Orbiter:'''
MJD is used since the beginning of the space program, eliminates the initial '24' from the number and begins the Date at midnight.
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MJD is used since the beginning of the space program.
  
MJD = JD - 2400000.5<ref name="WikiJD"/>
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MJD = JD - 2400000.5
  
  
 
== references ==
 
== references ==
 
<references/>
 
<references/>
 
[[Category: Glossary]]
 

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