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If I ask Mathematica for the Julian Date for the battle of Agincourt with

WolframAlpha["battle of agincourt date"]

it reports 2238175. But this does not agree with the value of 2238183.50 reported by the US Naval Observatory.

Why are these reported dates different?

orome
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    Alpha is assuming a Gregorian date, when it's supposed to be a Julian one. To check, you can use my jd[] function from here: jd[{1415, 10, 25}, "Calendar" -> "Julian"] matches USNO's output, and jd[{1415, 10, 25}, "Calendar" -> "Gregorian"] is close to Alpha's. – J. M.'s missing motivation Oct 25 '15 at 15:22
  • So the USNO is using a "calendar of date" system, so to speak, while Alpha is using a proleptic Gregorian calendar? – orome Oct 25 '15 at 15:30
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    The docs for JulianDate[] say as much: "All referenced dates are using the proleptic Gregorian calendar." – J. M.'s missing motivation Oct 25 '15 at 15:33
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    Another difference is that astronomical Julian days begin at noon, not midnight, which accounts for the .5 in the USNO date. – m_goldberg Oct 25 '15 at 15:54
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    I'm a bit confused. Then the Wolfram|Alpha result is incorrect, right? As "25 October 1415" is valid in the Julian calendar for this event. – Szabolcs Oct 25 '15 at 15:58
  • @Szabolcs: It would seem so. And that would be a surprise? – orome Oct 25 '15 at 16:04
  • For something like Wolfram|Alpha, that wants to know everything, yes, it's quite surprising ... It's exactly the kind of tool I'd expect to be able to navigate complicated calendar conversions. – Szabolcs Oct 25 '15 at 16:07
  • @Szabolcs, they covered their bases with a caveat, as I mentioned. – J. M.'s missing motivation Oct 25 '15 at 16:09
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    @J.M. But that does't apply here because the question posted to W|A wasn't "What is the Julian day number of Oct 25, 1415". It was, "When was the Battle of Agincourt?". Then it gives a plainly wrong answer to that. – Szabolcs Oct 25 '15 at 16:14
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    @rax, you could answer your own question, also. – J. M.'s missing motivation Oct 25 '15 at 16:18
  • @Szabolcs, Alpha did return the right date; unfortunately, not knowing about the "switch", it computed JD the wrong way. – J. M.'s missing motivation Oct 25 '15 at 16:25
  • @J.M.: No, it returned the wrong date: 25 October 1415 Gregorian (16 October Julian). That's not the date on which the battle took place. – orome Oct 25 '15 at 16:33
  • I think we're cross-talking here. What I was trying to say was that it knew it was "October 25, 1415"; the problem was that it then interpreted that as a Gregorian date instead of a Julian one. It knows nothing about the switch. – J. M.'s missing motivation Oct 25 '15 at 16:37
  • @J.M.: Not really. Where it says "Result" on the Alpha panel, it states "October 25, 1415" without specifying a calendar. Later it resolves this ambiguity by giving a corresponding Julian date (of 16 October 1415). Taken together we can conclude that the date it returned as an answer is Gregorian (a reasonable default). And that date is wrong. – orome Oct 25 '15 at 16:47
  • This bug remains unaddressed as of 2017-09-14. – orome Sep 14 '17 at 12:40

1 Answers1

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The date is different because Wolfram Alpha is wrong about when the Battle of Agincourt took place.

Alpha is returning 25 October 1415 in the Gregorian calendar as the date of the battle, but the battle took place on that date in the Julian calendar (the calendar in use at the time).

That date corresponds 3 November 1415 in the proleptic Gregorian calendar used by JulianDate, which produces the discrepancy in Julian Dates reported by Alpha and the USNO.

orome
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