# Astronomy Math Question **URL:** \ **Category:** Factual Questions\ **Created:** [October 27, 2017, 11:23am UTC](https://boards.straightdope.com/t/astronomy-math-question/799978 "2017-10-27T11:23:08Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [October 27, 2017, 11:23am UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/1 "2017-10-27T11:23:08Z")
Many astronomy calculations (ie: eclipses, rise and set times, etc) take the current date and convert it to a Julian date. But, why should that matter? Does anyone know why the Julian date still has relevance? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 27, 2017, 11:27am UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/2 "2017-10-27T11:27:34Z")
It’s a lot easier to work with than years, months, days, etc. That’s about it, really. How many days is it from January 17, 2004 to June 12, 2007? That’s not so easy to figure out. But with two Julian days, all you have to do is subtract. Yes, yes, computers. Computers can figure the number of days between two dates. But how do you think they do that? They do it by converting both dates to some continuous-count number or other and then subtracting. Also note that the word you’re looking for is “Julian day”, not “Julian date”, by the way. That might help you get better search results. ---
**Author:** ![bibliophage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bibliophage/32/7615_2.png) [@bibliophage](https://boards.straightdope.com/u/bibliophage)\ **Post date:** [October 27, 2017, 1:22pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/3 "2017-10-27T13:22:53Z")
Jinx, it’s not quite clear to me where your confusion lies, so I apologize if I’m telling you something you already know. [Julian day](https://en.wikipedia.org/wiki/Julian_day) is something distinct from a date in the [Julian calendar](https://en.wikipedia.org/wiki/Julian_calendar). ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [October 27, 2017, 5:53pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/4 "2017-10-27T17:53:54Z")
> [@Chronos](#): > > It’s a lot easier to work with than years, months, days, etc. That’s about it, really. > > How many days is it from January 17, 2004 to June 12, 2007? That’s not so easy to figure out. But with two Julian days, all you have to do is subtract. > > Yes, yes, computers. Computers can figure the number of days between two dates. But how do you think they do that? They do it by converting both dates to some continuous-count number or other and then subtracting. > > Also note that the word you’re looking for is “Julian day”, not “Julian date”, by the way. That might help you get better search results. When I worked on payroll programs, a lot of payroll functions used the Julian day - just for that reason - i.e. what percentage of the year’s entitlements (thus, say, what percent of your holidays, sick days per year, etc.) have you earned? They had special calendars (commercially available) that had the Julian Day number in small print on each day. That you could buy calendars with that number on them suggests it was reasonably widely used in accounting-type functions. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 27, 2017, 6:05pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/5 "2017-10-27T18:05:40Z")
> [@md2000](#): > > When I worked on payroll programs, a lot of payroll functions used the Julian day - just for that reason - i.e. what percentage of the year’s entitlements (thus, say, what percent of your holidays, sick days per year, etc.) have you earned? They had special calendars (commercially available) that had the Julian Day number in small print on each day. That you could buy calendars with that number on them suggests it was reasonably widely used in accounting-type functions. Aren’t those “Julian days” just a count of the days in the current year? The astronomical Julian Day counts from 1-Jan-4713 BCE, which is well before any possible written astronomical observation. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 27, 2017, 6:07pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/6 "2017-10-27T18:07:25Z")
> [@Chronos](#): > > Also note that the word you’re looking for is “Julian day”, not “Julian date”, by the way. That might help you get better search results. Actually Julian Date is correct. And it’s Julian Day Number you’re talking about integer dates, without a fraction to indicate the time. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [October 27, 2017, 6:08pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/7 "2017-10-27T18:08:01Z")
> [@Chronos](#): > > It’s a lot easier to work with than years, months, days, etc. That’s about it, really. > > How many days is it from January 17, 2004 to June 12, 2007? That’s not so easy to figure out. But with two Julian days, all you have to do is subtract. > > Yes, yes, computers. Computers can figure the number of days between two dates. But how do you think they do that? They do it by converting both dates to some continuous-count number or other and then subtracting. > > Also note that the word you’re looking for is “Julian day”, not “Julian date”, by the way. That might help you get better search results. Computers aren’t a big fan of [calendars and times](https://www.youtube.com/watch?v=-5wpm-gesOY). ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [October 27, 2017, 8:07pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/8 "2017-10-27T20:07:51Z")
> [@dtilque](#): > > Aren’t those “Julian days” just a count of the days in the current year? The astronomical Julian Day counts from 1-Jan-4713 BCE, which is well before any possible written astronomical observation. Yes, that’s correct, and it’s confusing. The original astronomical Julian day/date for today (10/27/2017 PM) is 2,458,054, a simple integer count of days since 1/1/4713 BC on the proleptic Julian calendar. However, many computer programs and calendars display days as YYYYDDD, so that today would be 2017300. This is a sort of halfway house in which the days and months are combined into an integer, but not the year. People also refer to these as Julian day/dates, which I don’t like, but it’s the weirdest thing, nobody cares whether I like it or not. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 27, 2017, 9:09pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/9 "2017-10-27T21:09:20Z")
> [@Freddy\_the\_Pig](#): > > However, many computer programs and calendars display days as YYYYDDD, so that today would be 2017300. This is a sort of halfway house in which the days and months are combined into an integer, but not the year. Sure I have seen calendars with each day numbered, 1–365, but what you describe sounds quite unusual. Even in the context of a computer program, those hybrid integers seem useless as you cannot perform arithmetic on them like Julian days, nor does the representation save space or memory compared to Julian days. Where have you encountered them, and was it really as single integers and not as two columns, one for the year and one for the day, jammed together? I presume it was not in an astronomical context or in an almanac. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [October 28, 2017, 12:27am UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/10 "2017-10-28T00:27:49Z")
> [@DPRK](#): > > Where have you encountered them, and was it really as single integers and not as two columns, one for the year and one for the day, jammed together? I presume it was not in an astronomical context or in an almanac. Astronomically, definitely not. I’m thinking back to Twentieth Century COBOL, when dates were often stored in YYDDD format. The features of COBOL made it easy to prise apart the first two digits and the last three, and do a double subtraction to determine the difference between two dates. This was invariably described in the documentation as Julian dates. On a calendar, I agree, you usually just see the three-digit day integer, and I have seen this described as a Julian date or Julian date number as well. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [October 28, 2017, 5:43am UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/11 "2017-10-28T05:43:33Z")
Doesn’t local Julian Day start at noon … easier for astronomers who typically sleep during the day? … ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 28, 2017, 6:11am UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/12 "2017-10-28T06:11:56Z")
Basically yes, except there is no “local” Julian day since it refers to Universal Time or Terrestrial Time; I have no idea about the anecdote involving astronomers sleeping during the day, but one story explaining why astronomical and nautical days historically began at noon is that it was pretty easy to observe the sun anywhere to get the instant of local noon, while marking the instant of midnight required more subtlety. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 28, 2017, 6:22am UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/13 "2017-10-28T06:22:57Z")
Wikipedia says > [@](#): > > Julian days begin at noon because when Herschel recommended them, the astronomical day began at noon. 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. 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. When this practice ended in 1925, it was decided to keep Julian days continuous with previous practice. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 28, 2017, 2:32pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/14 "2017-10-28T14:32:44Z")
I had always heard (from other astronomers) that astronomical days changed at noon because most observations happened at night and it was convenient for the day to not change during an observation. But that could, of course, have been a folk etymology, and the point about noon being easy to determine precisely sounds at least as compelling. And in any event, nowadays with astronomers at every longitude, whatever the reason was, it’s irrelevant now. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [October 28, 2017, 3:38pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/15 "2017-10-28T15:38:08Z")
It makes sense that local noon was easier to determine than local midnight … especially with 4713 BC technologies … Astronomers work at night, sleep during the day … maybe these fancy “professional” grade telescopes are just set them and forget them, most of my experience is with amateur backyard equipment … clock drives are close but someone still has to man the eyepiece and correct; up, down, forward, backwards; for a six hour exposure … nothing like sitting stock still on some mountain-top in the middle of winter all night long … burrrrrrrrr … ---
**Author:** ![SCAdian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scadian/32/3292_2.png) [@SCAdian](https://boards.straightdope.com/u/SCAdian)\ **Post date:** [October 28, 2017, 6:57pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/16 "2017-10-28T18:57:09Z")
> [@DPRK](#): > > Sure I have seen calendars with each day numbered, 1–365, but what you describe sounds quite unusual. Even in the context of a computer program, those hybrid integers seem useless as you cannot perform arithmetic on them like Julian days, nor does the representation save space or memory compared to Julian days. > > Where have you encountered them, and was it really as single integers and not as two columns, one for the year and one for the day, jammed together? I presume it was not in an astronomical context or in an almanac. We use them a lot in the Navy, in YDDD format. Don’t ask me why, though, or even when. On Supply Dept forms, I think… ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [October 28, 2017, 7:36pm UTC](https://boards.straightdope.com/t/astronomy-math-question/799978/17 "2017-10-28T19:36:05Z")
> [@Freddy\_the\_Pig](#): > > Astronomically, definitely not. I’m thinking back to Twentieth Century COBOL, when dates were often stored in YYDDD format. The features of COBOL made it easy to prise apart the first two digits and the last three, and do a double subtraction to determine the difference between two dates. This was invariably described in the documentation as Julian dates. On a calendar, I agree, you usually just see the three-digit day integer, and I have seen this described as a Julian date or Julian date number as well. Yes. I often see “Day of Year” referred to as “Julian Day” or “Julian Date” (if you Google “Julian Date calendar” many of the responses will be Day of Year calendars) Year and Day of Year information isn’t as easy to do calculations with as proper astronomical Julian Date is, but it’s a lot easier than using the month/day representation is.