English: Approximate length of daylight, in hours, as a function of latitude and time of year. Here, the length of daylight is defined as the period from the beginning of civil twilight in the morning to the end of civil twilight in the evening. Morning civil twilight begins before sunrise when the sun is geometrically 6 degrees below the horizon and ends when the sun rises; evening civil twilight lasts from sunset until the sun is geometrically 6 degrees below the horizon. Civil twilight is roughly the period before sunrise and after sunset during which, under clear skies, outdoor activities can be carried out without artificial light. Times were calculated using the FORTRAN program sunae.f found here. Contours are not necessarily smooth because of the 0.5-degree latitude spacing used in the calculations. Calculations were performed for the 365 days in 2007 on the Greenwich Meridian. For each minute during 2007 the sun's elevation (the angle between a flat horizon and the apparent location of the center of the sun, accounting for refraction) was calculated at -90, -89.5, -89, ..., 89.5, and 90° latitude and 0° longitude. The start of the morning twilight was found by linearly interpolating to the time when the solar elevation was -6° and increasing; the end of the evening twilight was found the same way but for the elevation decreasing.