Daylight Hours Calculator

Daylight hours (also called day length, photoperiod, or daylength) is the number of hours of sunlight between sunrise and sunset on a given date at a given location. It varies continuously throughout the year, reaching its maximum at the summer solstice and its minimum at the winter solstice, with equal day and night (approximately 12 hours each) at the equinoxes. The amount of daylight directly affects human mood, plant growth, animal behavior, solar energy generation, and agricultural planning.

At the equator, day length varies very little throughout the year — staying close to 12 hours in all seasons. The variation increases with latitude: at 40° N (the latitude of New York, Madrid, and Beijing) day length ranges from about 9.3 hours in December to about 15 hours in June. At 60° N (the latitude of Helsinki and Anchorage) the range is roughly 5.5 hours in December to nearly 19 hours in June. Above the Arctic Circle (66.5° N), there are periods of midnight sun (no darkness) in summer and polar night (no daylight) in winter.

This calculator computes daylight hours for any date and latitude, along with the precise sunrise and sunset times, the civil, nautical, and astronomical twilight durations, and a graph of how daylight changes through the year at that latitude.

Daylight hours are derived from the solar hour angle at sunrise and sunset, which depends on the observer's latitude and the Sun's declination on the given date.

The cause of seasonal daylight variation is Earth's axial tilt of 23.44 degrees relative to its orbital plane. As Earth orbits the Sun, the Northern Hemisphere tilts toward the Sun from March to September (more daylight in summer) and away from it from September to March (less daylight in winter). The Southern Hemisphere experiences the opposite pattern — its summer (more daylight) coincides with the Northern Hemisphere's winter.

At the summer solstice (around June 21 in the Northern Hemisphere), the Sun traces its highest and longest arc across the sky, producing the longest day of the year. At the winter solstice (around December 21), the Sun traces the shortest, lowest arc, producing the shortest day. At the spring and autumn equinoxes (around March 20 and September 22), the Sun rises due east and sets due west, and day and night are approximately — but not exactly — equal in length.

The equinox approximation is imperfect because of the Sun's finite angular size (the full disk must clear the horizon, not just the center) and atmospheric refraction (which bends sunlight to make the Sun appear above the horizon when it is geometrically below it). Both effects add about 8-12 minutes of "extra" daylight around the equinoxes, so the day with exactly 12:00 hours of sunlight is actually a few days before the spring equinox and a few days after the autumn equinox.

Beyond the daylight hours between sunrise and sunset, twilight extends the usable light period. Civil twilight occurs when the Sun is up to 6 degrees below the horizon — there is enough natural light for most outdoor activities without artificial lighting. Nautical twilight (6-12 degrees below horizon) provides enough light for navigators to see the horizon while still seeing stars — historically crucial for celestial navigation. Astronomical twilight (12-18 degrees below horizon) is the threshold beyond which the sky is dark enough for astronomical observation.

Total usable light per day is often better measured by "civil daylight" (sunrise-sunset plus the two civil twilight periods) than by strict daylight hours alone. In summer at high latitudes, civil twilight can extend well past midnight, creating nearly continuous usable light. This is the basis of the "midnight sun" experience at latitudes above 66.5° and the famous long summer evenings in Scandinavia, Scotland, and Alaska.

Solar energy system design requires accurate daylight hour data to size photovoltaic systems. The number of peak sun hours (a measure of usable solar irradiance) directly determines how many solar panels are needed to meet a given energy demand. Systems in Minnesota must be oversized to compensate for winter's 8-9 hours of daylight, while systems in Arizona benefit from nearly 14 hours in summer.

Circadian health and seasonal affective disorder (SAD) treatment protocols are tied to daylight hours. Light therapy guidelines recommend specific daily light exposure durations that must be supplemented when natural daylight falls below 10-12 hours per day. Knowing the exact daylight duration for your latitude helps calibrate supplemental light therapy schedules. Agricultural photoperiod management (controlling flowering in greenhouse crops) also depends on precise daylight data for specific dates and latitudes.