Instructions for Use
To find today's Pre-Christian style date is fairly simple, although it does require that you know roughly where you are.
- Enter your Latitude and Longitude in the appropriate textboxes using decimal notation
- Select either East or West of the Greenwich meridian
- Select either North or South of the equator
- Select whether you're currently on Daylight Savings Time (or local equivalent) or not
- Click on the 'Riman!' button
As an example the settings default to my home location and will work without alteration, although if you are greatly removed from the English Midlands (eg in the USA) then you should see a warning that something is not quite right. Just follow the instructions shown and try again.
A couple of assumptions have been made in my calculations:
- Your computer has the system time and date set fairly correctly.
- Your computer has the time zone set fairly correctly.
I've had to do this in order to keep the user interface as simple as possible, as a lot of time and location information is required for the calculations of local sunrise and sunset times.
Limitations:
- All of the calculations used are based on the sunrise and sunset times and, most most importantly, on the first possible date for observing the new crescent moon. Whereas the apparent motion of the sun can be found quite easily for any geograhical location, the same is unfortunately not true for the moon.
For the purposes of this JavaScript programme all of the lunar observations are based on an observing location about 12 miles NE of Birmingham, England. This should not present too much of a problem as similar observations made elsewhere are most unlikely to be more than a day or so different. Given that a clear observation of the new crescent depends upon the vagaries of local weather I don't think that the discrepancy is worth worrying about.
- It has thus far proved beyond the capabilities of both JavaScript and myself to build an eternal pre-christian calendar which could reside comfortably in one web page. The present calendar is only good for the year 2005 (1556AE) and will return anomalous results if your computer clock is set for a different year.
Acknowledgements
My thanks to Robert Stone for the inspiration behind this set of pages and a helpful correspondence, and also to Christopher Gronbeck at CREST for the sunrise / sunset algorithms.