Day Length Calculator

The driver is the sun's declination, which follows a sine wave through the year — and a sine changes fastest at its zero crossings (the equinoxes) and pauses at its peaks (the solstices — literally 'sun stands still'). At London's latitude that translates to nearly 4 minutes of daylight gained per day in late March but mere seconds per day around June 21. Human perception agrees: spring feels like switching on, midsummer like a plateau — the calculus is doing the feeling.

Two thumb-on-the-scale effects, both favoring daylight: refraction lifts the sun's image ~34 arc-minutes (you see it before it geometrically rises), and sunrise/sunset are defined by the sun's upper LIMB, not its centre — another 16′. Together they stretch the equinox day to about 12h08m at the equator and slightly more at high latitudes. The date when day truly equals night (the 'equilux') falls a few days before the spring equinox and after the autumn one.

Directly but not linearly: PV yield follows daylight hours times sun elevation (winter's short days are also LOW-sun days — a double penalty that makes December output at 50° latitude a quarter of June's, not half). Agriculture keys on photoperiod thresholds: many plants flower by measuring night length (soybeans, rice are short-day; spinach, barley long-day), and poultry lay by artificial photoperiod — every commercial greenhouse and hen house runs on the curve this tool computes.

No — the equation of time skews it: solar noon drifts against clock noon by up to ±16 minutes through the year, so daylight gets added (or taken) unequally between sunrise and sunset. The notorious case: in early January, evenings already brighten while mornings keep darkening for another week or two — both true simultaneously because solar noon is sliding later. Our sunrise-sunset calculator shows the asymmetry; this tool's total is immune to it (length depends only on the hour angle).