# 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. Commented Oct 25, 2015 at 15:22
• The docs for JulianDate[] say as much: "All referenced dates are using the proleptic Gregorian calendar." Commented Oct 25, 2015 at 15:33
• Another difference is that astronomical Julian days begin at noon, not midnight, which accounts for the .5 in the USNO date. Commented Oct 25, 2015 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. Commented Oct 25, 2015 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. Commented Oct 25, 2015 at 16:14

## 1 Answer

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.