# Why does Mathematica's date for the battle Agincourt differ from the US Navy's?

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?

• 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. will be back soon Oct 25 '15 at 15:22
• The docs for JulianDate[] say as much: "All referenced dates are using the proleptic Gregorian calendar." – J. M. will be back soon Oct 25 '15 at 15:33
• 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
• 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
• @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

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.