Celestia Forums

Well, this is not neccecarily a bug in Celestia: it might be a bug in some of the conversion utilities instead - can a developer clarify?

... If i convert a gregorian BCE date to Julian, using the US Naval Observatory??s script found on http://aa.usno.navy.mil/data/docs/JulianDate.html, the date is one year wrong compared to Celestia. That is, if Celestia is set to year 1 BCE, the script shows year 2 BCE.

Similarly, if i use the converter at onlineconversion.com
- http://www.onlineconversion.com/julian_date.htm, the result is the same - one year off.

However the script on this page:
http://www.calendarhome.com/converter/
... are in correspondence with Celestia: Year 1 BCE is year 1.

... It seems that the two first scripts treats 0 - zero - as a year. If so, that is a bad bug. Or is it actually a bug in Celestia?

- rthorvald

You have to be careful as to which calendar system you are using when specifying years BCE.

There is no year 0 in the Christian Gregorian and Julian calendar systems, but there is a year 0 in the Common Era and Astronomical calendar systems. 0 of the Common and Astronomical systems corresponds to year -1 (1 BCE) of the Christian systems. Celestia, of course, uses the Astronomical calendar.

If you try to put the year 0 into the USNO Julian calendar convertor, you'll get an error message. In other words, it's using the Christian calendars, not the Astronomical calendar, so it's off-by-one compared to Celestia.

See http://en.wikipedia.org/wiki/Year_zero and http://www.hermetic.ch/cal_stud/cal_art.html

(I've been trying to find a page on an edu site that's as clear as Peter Meyer's, but haven't yet. But you might browse these calendar pages at NASA: http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/dates.html )


[edited to clarify Christian vs Common]

selden wrote:There is no year 0 in the Christian Gregorian and Julian calendar systems, but there is a year 0 in the Common Era and Astronomical calendar systems. 0 of the Common and Astronomical systems corresponds to year -1 (1 BCE) of the Christian systems. Celestia, of course, uses the Astronomical calendar.

Ok, thank you for the explanation.

- rthorvald