A MODEL COMPARISON FOR DAYLENGTH AS A FUNCTION OF LATITUDE AND DAY OF YEAR

A model that calculates the length of the day for a flat surface for a given latitude and day of the year is described. Calculated daylengths are within 1 minute of values published in Smithsonian Meteorological Tables and the Astronomical Almanac for latitudes between 40-degrees North and South with a maximum error of 7 minutes occurring at 60-degrees latitude. The model allows the use of different definitions of sunrise/sunset and the incorporation of twilight. Comparisons with other daylength models indicate that this model is more accurate and that variation in accumulated hours of daylight of up to one week over the course of the growing season can be accounted for by how sunrise/sunset are defined.