How to use it
- Choose a place and a year.
- Read the chart of daylight for every day. Move over it to read a day.
- Read the longest and shortest days and the table by month.
5 practical uses
- Plan a garden, a greenhouse or a solar installation.
- Compare places to live or to travel to by their winter days.
- Plan outdoor work through the seasons.
- Explain the seasons to a class with a chart.
- Plan a trip to see the midnight sun or the polar night.
The maths behind it
For each day the sun's declination δ is computed from the Julian day, and the half-day arc from cos H₀ = (sin(−0.833°) − sin φ sin δ) / (cos φ cos δ). The day length is 2H₀ in hours. When the right-hand side is beyond ±1 the sun does not rise or does not set, and the day is 0 or 24 hours.
Why the curve looks like that
The Earth's axis is tilted 23.4°, so as it goes round the sun each hemisphere leans towards it for half the year and away for the other half. At the equator the day is always about 12 hours; at 50° it runs from about 8 hours to 16; at the Arctic Circle from 0 to 24. The curve is a sine-like wave with its peaks at the solstices (around 21 June and 21 December) and its middle at the equinoxes. Day length includes the extra minutes from refraction and the sun's width, which is why the equinox day is a few minutes longer than the night.
Times are computed in the browser with the SunCalc library, which follows the astronomical almanac formulas to within a minute or so, and are shown in the local time of the place, including summer time, from its time zone. Sunrise and sunset are the moments the top edge of the sun meets a flat horizon, with the usual allowance for the bending of light in the air; a hill to the east delays your sunrise.