Compute the position of the sun, and local solar time using Meeus' formulae. Compute day and/or night length using different twilight definitions or arbitrary sun elevation angles. This package is part of the 'r4photobiology' suite, Aphalo, P. J. (2015) <doi:10.19232/uv4pb.2015.1.14>. Algorithms from Meeus (1998, ISBN:0943396611).
copied from cf-post-staging / r-suncalcmeeus