Daylight Duration Calculator

About Daylight Duration Calculator

The Daylight Duration Calculator answers a deceptively complex question: how many hours of usable sunlight will you get on a given day at a given place? We compute sunrise, sunset, civil/nautical/astronomical twilight, and total daylight using the same NOAA Solar Calculator algorithms that power weather services worldwide. Photographers use it to plan golden hour shoots, farmers to schedule planting around frost-free days with sufficient sun, solar installers to estimate panel output, hikers to budget trail time before nightfall, and Arctic/Antarctic travelers to verify the existence of polar night or midnight sun on their visit dates. Coordinates input lets you check anywhere on Earth, not just preset cities.

What's the difference between sunrise, civil twilight, nautical twilight and astronomical twilight?

These mark the sun's center crossing specific angles below the horizon. Sunrise/sunset = sun's upper edge at the horizon (sun center at -0.833° accounting for refraction and the sun's apparent radius). Civil twilight ends when the sun is 6° below the horizon — you can still read outdoors, streetlights come on, the first bright planets appear. Nautical twilight (sun at 12° below) is when the sea horizon is no longer distinguishable, traditionally when sailors could no longer use a sextant. Astronomical twilight (18° below) is the end of any detectable sky glow from the sun; only then is the sky truly dark for deep-sky astronomy. The calculator shows all four boundaries so you can pick the threshold relevant to your activity.

How do I read each field — golden hour, civil/nautical/astronomical dawn and dusk, and solar noon?

Map each on-page field to a job. Golden Hour (Morning End) is when the sun reaches 6° elevation after sunrise; Golden Hour (Evening Start) is when it drops back to 6° before sunset — the soft, warm light window photographers and filmmakers chase, sitting between sunrise/sunset and full daytime. Civil Dawn/Dusk (sun 6° below) bracket the workable outdoor-light window and define legal lighting-up time for many jurisdictions. Nautical Dawn/Dusk (12° below) mark when the sea horizon becomes visible/invisible against the sky — the window mariners use a sextant for star sights. Astronomical Dawn/Dusk (18° below) bound true darkness: between Astronomical Dusk and the next Astronomical Dawn the sky is fully dark for deep-sky astrophotography. Solar Noon is the moment the sun is highest and exactly due south (north in the Southern Hemisphere) — use it to aim fixed solar panels and to set a true-south sundial, since it is the real midpoint of daylight regardless of clock time.

Why do daylight hours not change linearly through the year?

The rate of daylight change is sinusoidal because Earth's axial tilt (23.44°) interacts with our orbit as a smooth cosine function. Near the equinoxes (March 20 and September 22 ish), the sun's declination is changing fastest — day length changes by about 2-4 minutes per day at mid-latitudes. Near the solstices (June 21 and December 21), declination is at its extreme and barely moves; day length changes by only seconds per day. That's why people notice 'the days are getting longer' more in February-March and August-September than in December or June. At the equator the variation is tiny (12h ± 5 min year-round); at 60° latitude (Oslo, Anchorage) it swings from 6h to 18h.

How does daylight duration work above the Arctic or Antarctic Circle?

Above 66.5° latitude (the Arctic Circle), there are days where the sun never rises (polar night) or never sets (midnight sun) — daylight duration becomes 0 hours or 24 hours respectively. The closer to the pole, the longer these periods last. At the North Pole, the sun rises around the March equinox and doesn't set until the September equinox — six months of continuous daylight followed by six months of darkness. But civil twilight extends usable light beyond 'sunset': Tromsø (Norway, 69.6°N) has the sun below horizon late November to mid-January, but enjoys 'polar day-blue' twilight for hours. Our calculator handles these edge cases correctly, returning 'sun never rises/sets' messages where appropriate.

Why might the sunrise/sunset times differ from my phone's weather app?

Three common reasons: (1) coordinates precision — phone apps often use city centroid while you might be 20 km away; sunrise can differ by 1-3 minutes per 10 km east-west. (2) Elevation — our standard calculation assumes sea level; mountains higher than the horizon delay sunrise and advance sunset by minutes (about 1 minute per 1.5 km of elevation observed). (3) Refraction model — we use the standard 0.567° atmospheric refraction at the horizon, but actual refraction varies 0.3-0.9° with temperature inversions, especially in deserts and polar regions where the famous 'Novaya Zemlya effect' can advance sunrise by hours. For most uses our answer is accurate within 1-2 minutes.

Does Daylight Saving Time affect the calculation?

Yes — we display times in the local civil time of the location, honoring DST rules from the IANA timezone database. So a sunrise at 06:30 UTC in Paris would show as 07:30 in winter (CET) and 08:30 in summer (CEST). DST itself doesn't change when the sun rises in real terms — that's set by astronomy — but it shifts what number you see on the clock. Some countries (Iceland, most of Africa, Russia post-2014, Japan, China) don't observe DST, so the times stay simpler year-round. If you're planning across time zones, switch the calculator's display to UTC for unambiguous comparison.

How can I use daylight data for solar panel or garden planning?

Solar: peak sun hours (PSH) — the equivalent hours of 1000 W/m² sunshine — is roughly 60-80% of geometric daylight hours, depending on cloud cover and panel tilt. A site getting 14 hours of daylight in June produces about 5-6 PSH on a clear day. Garden: most edible plants need 6+ hours of direct sun (full sun); 4-6 hours = partial sun (suits lettuces, herbs); under 4 hours = shade plants only. Use our annual chart to identify months where your target site exceeds these thresholds. Combine with frost dates from local agricultural extensions for accurate growing-window predictions. Tomatoes, peppers, and squash need 8+ hours and warm soil simultaneously.

What algorithm do you use, and how accurate is it?

We implement the NOAA Solar Position Algorithm (SPA) as documented by Reda and Andreas (2003) — the same one used by US government weather and energy agencies. It accounts for Earth's elliptical orbit (equation of time), precession, nutation, and aberration, accurate to about ±0.0003° in solar position over the years 2000-2050. Sunrise/sunset times are accurate to within seconds for sea-level observers in standard atmospheric conditions, and within 1-2 minutes accounting for typical atmospheric variability. For extreme accuracy needs (eclipses, scientific observation), use NASA HORIZONS, which incorporates real-time atmospheric data.