The Julian Day Number (JDN) is the number of a day in the Julian Day Calendar. For example, the JDN of January 1, 1960 is 2,436,934. (The definition and example came from Webster's Third New International Dictionary).



The Prime Meridian of the world is the meridian of longitude that passes through the Greenwich Observatory in London, United Kingdom and was adopted as the reference point from which both longitude and time should be measured. The Universal Time (UT) is the time at this reference point. Greenwich Mean Time (GMT) is the civilian equivalent of Zulu Time (Z) which is used by the military and aviation.



The Julian Number (JN) is the combination of JDN, and UT expressed in decimal of a day. This means that UT's hours, minutes and seconds have to be converted to fraction of a day in decimal. If UT, for example is 19:30:47 or 7:30:47 pm, the equivalent of this is 0.8130438 day. This is derived by adding the quotients after converting the hours, minutes and seconds to fraction of a day, as follows: 7pm or 19H/24H per day = 0.7916666 day; 30 minutes/60/24 = 0.0208333 day; and 47 seconds/60/60/24 = 0.0005439 day.  The sum of 0.7916666 + 0.0208333 + 0.0005439 is 0.8130438.


If the JDN of January 1, 1960 is 2,436,934 and UT is 19:30:47, JN therefore is 2,436,934.8130438.



The Aristean Decimal time (ADT) is the time of the day expressed in decimal. There are 100 aristos in one day. The day begins at 6:00 pm like in the Jewish calendar, and not at midnight. The first 50 aristos is for nighttime and the next 50 aristos is for daytime.


An aristo is about a quarter of an hour. Each aristo is divided into 10 deciaristos. A deciaristo is about 1.5 minutes. A deciaristo is divided into 100 milliaristos. A milliaristo is 0.864 second. A day therefore has 100,000 milliaristos, which is about the number of times the human heart beats in a day.


To express time in ADT, we use o'day, meaning "of a day". The unit of time aristo is fraction of a day and is abbreviated as "R" or "Ar". The following are the equivalent of the present times in both 24- and 12-hour times in ADT:

24-hour clock

12-hour clock


Fraction of day


9:00 pm

12.5 o'day

0.12500 R


12:00 mn

25.0 o'day

0.25000 R


3:00 am

37.5 o'day

0.37500 R


6:00 am

50.0 o'day

0.50000 R


9:00 am

62.5 o'day

0.62500 R


12:00 nn

75.0 o'day

0.75000 R


3:00 pm

87.5 o'day

0.87500 R


6:00 pm

100.0 o'day

1.00000 R



The date and time of ADT at the International Date Line (IDL) is used as the reference time in the military, in aviation, and in astronomy instead of those at Greenwich. IDL is in Zone 1 of the Aristean Time Zone. The phonetic letter of Zone 1 is Alpha. Therefore, the time of this zone will be called the Alpha Time.  If Alpha Time is 6.304 o’day, this can be written as 6.304A.


Instead of using the longitude passing through Greenwich as the reference meridian, we will be using the IDL, or zero meridian, or zero aristitude (Aristean Longitude). Instead of Zulu Time, we will use Alpha Time. Instead of Universal Time, we will use World Time or WT. Instead of starting the day at IDL at midnight, we will start the day at sunset, the equivalent of 6:00pm.



The Julian-Aristean Time (JAT) is the new and simpler way of expressing JN, i.e. combining JDN and ADT. The time used is that at IDL or Zone 1. The JAT, for example, of January 1, 1960 at 6.304 o'day at the International Date Line is 2,436,934.06304.


If, for example, new moon occurred at 62.5 o'day on January 1, 1960, the JAT is 2,436,934.62500. If the moon orbits the earth every 29.53059 days, the next occurrence of a new moon would be at 2,436,964.15559. This is equivalent to January 31, 1960 at 15.559 o'day.


Doesn't this look easier than expressing the time in sexagesimal with colons separating the hours from the minutes and the minutes from the hour and converting the time to decimal? Now that the time is expressed as decimal, we can do arithmetic calculation with time.


Wouldn't this time system in decimal format be an improvement of the present sexagesimal format?


File jat.htm - Last updated: November 3, 2004


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