Is there an Objective-C (or C) library (that is compatible with core location) that can tell me the time of sunrise and sunset for any given calendar day?
EDSunriseSet is an open source and free Objective-C wrapper for the C languages routines created by Paul Schlyter.
Calculation is done entirely by the C-code routines. EDSunrisetSet bridges those calculations to common Cocoa classes (NSDate, NSTimeZone, ...)