generalize sundial API

[HankB]

Hi Zack,
Thanks for providing these calculations.
I'm not certain if you accept pull requests. If not, just ignore (delete?) If yes, read on.

Changes:

• separate main() from calculations to facilitate using calculations in another program.
• generalize public API to include time zone offset and time for which next sunrise/sunset is calculated
• update README to reflect code changes.
• add 'beer clause' to license.

best,
hank

(I will offer to buy you a beer should you ever be near Chicago or I near your location.)

[schollz]

@HankB Ha, thanks! Submitting this PR is gratitude enough :)

I never thought anyone would use this...I wrote this for Arduino to control the gate on my chicken coop without having to phone home to figure out when sunset/sunrise was. Turns out Arduino doesn't have enough precision in its double to allow this calculation (I think it was off by hours). Glad you found it useful!

[HankB]

One of the reasons for pulling the 'time from' out of the calculation was so I could easily test results. Based on your comments I probably should do that. I'm surprised the calculations didn't work on the Arduino but I don't think I've ever tried any floating point on that architecture.

FWIW my use case is to turn outdoor lights on and off. I should probably spring for a light sensor but I like S/W solutions. ;)