The time elapsed in hours between the specified sun angle from 90 degree in am and pm. +ve above the horizon, -ve below the horizon.
dayLength(doy, lat, angle = -6)
day of year number
latitude of site (deg)
angle to measure time between, such as twilight (deg). angular distance between 90 deg and end of twilight - altitude of sun. +ve up, -ve down.
day length in hours