# Astronomy Math Question

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?

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.

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 is something distinct from a date in the Julian calendar.

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.

Actually Julian Date is correct. And it’s Julian Day Number you’re talking about integer dates, without a fraction to indicate the time.

Computers aren’t a big fan of calendars and times.

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.

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.

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.

Doesn’t local Julian Day start at noon … easier for astronomers who typically sleep during the day? …

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.

Wikipedia says

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.

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 …

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…

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.