Day length by latitude

On the June solstice London gets about 16 hours 38 minutes of daylight and a town on the equator about 12 hours 7 minutes.

The circle of illumination

At any moment half of Earth is lit and half is dark. The line between them, drawn in orange in the model, is a great circle called the circle of illumination, or terminator. It always faces the Sun squarely. Because the axis is tilted, this circle usually cuts the circles of latitude unevenly. As Earth turns once a day, a place on a given latitude travels around its circle and spends part of the trip in light and part in darkness. The lit fraction of that circle is the fraction of 24 hours that is daytime.

The equator is special. The circle of illumination always cuts it exactly in half, whatever the season, so day and night at the equator are both about 12 hours all year. Move away from the equator towards the hemisphere that leans to the Sun and more of your latitude circle is lit; move towards the other pole and less is lit. That is why the swing in day length grows with latitude.

The sunrise equation

The day length can be calculated. Two numbers are needed: your latitude φ, and the Sun's declination δ, which is the latitude where the Sun is directly overhead that day. The Sun rises and sets when the hour angle H satisfies cos H = −tan φ × tan δ. The Earth turns 15° per hour, so the day lasts 2H ÷ 15° hours. NOAA's solar calculator uses a slightly fuller version, cos H = (cos 90.833° − sin φ sin δ) ÷ (cos φ cos δ), because the air bends sunlight over the horizon and because sunrise is counted from the top edge of the Sun's disc, not its centre. That correction adds a few minutes, which is why the model shows 12 h 7 min at the equator instead of exactly 12 hours.

The table was computed with the NOAA formula for 2026, the same one the model uses. Notice the pattern. At the equator nothing changes. At 23.44° N the longest and shortest days differ by about 3 hours, at Seoul's latitude by about 5 hours, at London's by almost 9 hours, and at 60° N by 13 hours. Also notice that even on the equinox, day is a little longer than night everywhere, again because of the refraction correction. Australia's Bureau of Meteorology gives the same figure for most of Australia: about 12 hours 8 minutes of daylight on the equinox.

Reading the chart

The chart under the model plots day length for every day of the year at the latitude you choose. At the equator it is a flat line. At mid-latitudes it is a smooth wave with its peak on the June solstice and its low point on the December solstice in the north (the other way round in the south). The dashed lines mark the equinoxes and solstices. The wave is steepest around the equinoxes, which is why daylight seems to change fastest in March and September and hardly at all in late June.