Calculate daylight hours by latitude and month.
The daylight calculator estimates how many hours of daylight a garden at any latitude receives in each month of the year — the figure that drives day-length-sensitive decisions like when onions bulb, when spinach bolts, and whether autumn sowings will mature before light runs out. Enter a latitude from −90° to 90° (positive north, negative south) and a month, and the tool returns the approximate day length at mid-month plus a season label: 14 hours or more counts as peak light, 10–14 as moderate, and under 10 as low.
The tool uses the standard astronomical day-length formula. Solar declination at mid-month is 23.45° × sin(360/365 × (day-of-year − 81)); the hour angle H of sunrise satisfies cos H = −tan(latitude) × tan(declination); day length = 2H ÷ 15 hours, since the sun moves 15° per hour. When the cosine clamps past ±1 — inside the polar circles near the solstices — the result pins to 24 hours (midnight sun) or 0 hours (polar night). Mid-month day-of-year values (15 Jan, 166 Jun, 349 Dec…) keep each month's estimate representative.
At latitude 40° in June the formula gives about 14.8 hours of daylight — labelled peak. Long-day onions, which bulb at roughly 14–16 hour day lengths, are well served at this latitude.
December at 52°N yields only about 7.5 hours — deep in the low band. Even in an unheated greenhouse, most crops sit nearly dormant below 10 hours of light, so December sowings wait for February.
Southern Scandinavia at 60°N swings from about 5.6 hours in December to 18.4 in June. The same latitude that starves winter crops of light gives summer vegetables extremely long photoperiods.
At 0° latitude every month returns essentially 12.0 hours — cos H stays at 0 regardless of declination. Tropical growers schedule by wet and dry seasons, not by changing day length.
It uses the standard declination approximation evaluated at mid-month, ignoring atmospheric refraction and elevation, so results are typically within roughly 10–20 minutes of published almanac day lengths — ample precision for garden planning.
They are the tool's thresholds: 14+ hours is peak, 10–14 is moderate, and below 10 is low. Many gardeners treat the 10-hour line as the practical floor for active winter growth.
Yes — enter latitude as a negative number. At −35° (Buenos Aires, Cape Town, Sydney), June returns the short winter days and December the long summer ones, mirroring the northern pattern.
Inside the Arctic and Antarctic circles, the sunrise equation has no solution near the solstices: cos H falls outside ±1. The calculator clamps these to 24 hours (midnight sun) or 0 hours (polar night), matching reality.