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JULIAN DAY NUMBERING The method of counting years from the supposed year of birth of Jesus of Nazareth is biased toward Christianity and implies an acceptance of Christianity. There is a need for a genuinely secular way to indicate dates and times. Today many different methods of marking time and dates are in use. Most of these methods are inconsistent with one another, illogical, and based on particular ethnic traditions. As I write this, it is Friday in the United States, but Saturday in New Zealand. It is 3:00 p.m. in the Eastern United States, but 12:00 p.m. further west in the same country. Times everywhere differ by time zones, savings time, and even notation (3:00 p.m. can also be 15:00). Different cultures have different months and days of the month, and years may be counted in many different ways. Years are usually counted from a date of importance for a particular religion or ethnic group. In much of the world today, the official calendar is an awkward combination of astrological weeks and day names, Roman months and day numbers, intercalary leap days according to the Gregorian reform, and years counted from the supposed birth-year of Jesus of Nazareth, founder of Christianity. There is a better way to reckon days and times. This more rational and culturally objective system is already used by scientists, and it is called Julian day numbers. We can use Julian day (JD) numbers to precisely indicate the equivalent of year, month, day, hour, minute, second, and fraction of a second. The Julian day number is the same everywhere on earth at once, and does not advance by leap days, irregularly long months, or mixed base 7, 12, 24, and 60 units of time. What is a Julian day number? A Julian day number is the number assigned to a day in a continuous, serial count since an arbitrary, prehistoric time which may be described as noon, Universal Time, January 1, 4713 B.C. in the proleptic (extrapolated backward) Julian calendar and Christian year. The Julian day number may be expressed as a whole number or the day number plus the fraction of the day (in decimal extension) corresponding to any instant in time. An example of a Julian day number is 2452970.45384 This number indicates the equivalent of an era, a year, a month, a day, an hour, a minute, a second, and fraction of a second in the Gregorian calendar and civil time -- in this case, namely, November 26, A.D. 2003, 5:53 p.m. and 32 seconds EST (Eastern Standard Time, or the R Time Zone). The Julian day number for present and past Gregorian dates and civil times may be obtained from the U.S. Naval Observatory Web site: http://aa.usno.navy.mil/data/docs/JulianDate.html or you may use the above calculator.
Where Did Julian Day Numbering Come From? In 1593 an Italian-French scholar named Joseph Julius Scaliger reconciled three calendars important in his day to derive a long period with an arbitrary, prehistoric epochal date. This date, in Christian Era reckoning, was January 1, 4713 B.C. (in the Julian calendar, extrapolated backwards). The astronomer John Herschel in 1849 used Scaliger's epochal date for a serial day
count, starting with noon of the epochal day as day 0. This serial day
count became widely used by astronomers and was officially adopted by
the International Astronomical Union. The Julian day number appears or
lies in the background computations of many applications today, such as
those used by NASA, computer software programs, and GPS navigation
systems. Too Many Digits? The Julian day number may seem like a long number, but it actually uses fewer digits than the equivalent Christian year, Gregorian date, and civil time. Example: the Gregorian date 2003-11-26 is 8 digits long, compared to the Julian day number, 2452970, which is 7 digits long. The civil time 17:53:32 is 6 digits long, compared to the Julian date's decimal fraction .45384, which is 5 digits long. The Julian date may look longer because it is expressed as a single number, instead of being broken up into units. It may help to break the number down into smaller units. The first digit, 2 in the example, doesn't change more often than about once every 2,739 years. It is a million days, a megameron (mega, one million, + -ameron, from Greek hemera, day, as in decameron, ten days). The first three digits do not change more than once every 27.397 years. This myriameron, or 10,000 days, is a convenient unit to set off in one's mind. You might write the number 245-2970 to make this more evident. It is the
last four digits of the Julian day number which have to be noted most
frequently,
since they change more often. The last four digits of the Julian
day number, 2970 in the example, indicate the equivalent of the Gregorian calendar's year, month, and day of the month. These four
digits might be used
as a shorthand date whenever the precise myriameron can be assumed.
Day Fractions The decimal fraction of a Julian day number functions like a time even though it may be described as part of the date. The first digit after the decimal point is a day-tenth. A day-tenth lasts roughly 2 and one-half hours, if you need a civil time equivalent to compare it to and remember it by. The first two digits after the decimal point are the number of hundredths of a day, or day-hundredths. A Julian day-hundredth turns over roughly once every fifteen minutes, which is a convenient unit of civil time to compare it to. The Julian day number should be calculated with standard (SI) seconds in the Terrestrial Time (TT) reference frame, a uniform timescale. However, it may also be computed with other reckonings such as UTC, which incorporates leap-seconds (not recommended) or atomic time, TAI. All of these will be fairly close, so the distinction is mainly important for occasions requiring precision, in which case the precise time measurement system should be stated. Eventually, more
precise time scales will be devised, and when successors of humanity
begin to inhabit interplanetary space, they will be able to use the Sun
rather than Earth as a frame of reference (as in the so-called Heliocentric
Julian Day, or HJD, for which, see http://www.physics.sfasu.edu/astro/javascript/hjd.html). Synchronizing the Earth The Julian day number and its fraction of a day are the same everywhere in the world at the same time. The day begins, or has its zero hour, at midnight on the International Date Line. One consequence of this is that the Julian day spans different local times in different time zones. While this may seem inconvenient, it actually brings the world together by getting past local diurnal cycles to a time that is synchronized throughout the Earth. Besides, throughout history, the timing of the beginning of the day has been arbitrary. In the a.m.-p.m. system, the hours roll over at noon as well as midnight. In many cultures, day begins at sunrise. For the ancient Celts and Hebrews, it began at sundown. For the Romans, as in current civil time, it began at midnight. With Julian day numbers, the day begins locally at whatever local time corresponds to midnight UT on the International Date Line. For example, in Eastern Standard Time in the United States (EST, the R Time Zone), the Julian day begins locally at 7:00 a.m. |