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Daylight Calculator

Determine exactly how many hours of daylight you'll experience on any given day at your location on Earth. By inputting your geographic coordinates and the date, this calculator computes sunrise and sunset times accounting for latitude, longitude, time zone, and atmospheric refraction effects. Essential for astronomers, photographers, farmers, and anyone curious about seasonal light variation.

Last updated:

Creators Davide Borchia, PhD
7,531 people find this calculator helpful

What Defines Daylight?

Daylight is strictly the period between sunrise and sunset—the time when the Sun is above the horizon at your location. This extends beyond mere visibility; even if clouds obscure the Sun, daylight still occurs as long as the Sun's disk is above the horizon line.

A critical subtlety exists: atmospheric refraction bends incoming sunlight, making the Sun visible even when geometrically below the horizon. This optical effect adds approximately 2–3 minutes to sunrise and subtracts the same from sunset, extending effective daylight by roughly 5–6 minutes compared to pure geometric calculation. This is why the equinox delivers slightly more than 12 hours of sunlight everywhere on Earth.

Daylight length is fundamentally determined by one factor: latitude. Longitude and time zone affect clock times of sunrise and sunset but not their duration.

Calculating Daylight Duration

The length of daylight is computed by determining local sunrise and sunset times, then subtracting:

Day length = tsunset − tsunrise

Latitude (φ) = ±latitude value (positive North, negative South)

Longitude (λ) = ±longitude value (positive East, negative West)

  • t<sub>sunrise</sub> — Time of sunrise in local time, derived from latitude, longitude, date, and time zone
  • t<sub>sunset</sub> — Time of sunset in local time, derived from latitude, longitude, date, and time zone
  • φ (Latitude) — Angular distance from the equator in degrees; determines the seasonal variation in daylight
  • λ (Longitude) — Angular distance from the prime meridian; affects clock time but not daylight duration

Latitude's Dominant Role in Daylight Variation

Latitude is the sole geographic factor controlling daylight length. At the equator, the Sun's position in the sky oscillates by only ±23.44° throughout the year, producing fairly consistent 12-hour days year-round.

At higher latitudes (such as Rome, Chicago, or Tokyo around 40°N), this swing becomes more pronounced. During summer, the Sun travels a longer arc across the sky, extending daylight to 14–16 hours. Winter reverses this, shrinking daylight to 8–10 hours.

Beyond the polar circles (66.56° latitude), extreme behavior emerges:

  • Midnight Sun: During local summer, the Sun never fully sets, creating continuous daylight for weeks or months.
  • Polar Night: During local winter, the Sun never rises above the horizon, producing continuous darkness.

The mathematical relationship between latitude and day length follows the Earth's 23.44° axial tilt, which determines the solar declination for each date of the year.

Twilight Phases and Extended Light Periods

Sunrise and sunset are not instantaneous; they occur across distinct twilight phases defined by the Sun's angular distance below the horizon:

  • Civil Twilight: Sun between 0° and 6° below the horizon. Enough ambient light for outdoor activities without artificial illumination.
  • Nautical Twilight: Sun between 6° and 12° below the horizon. Dim; stars and planets become visible, but the horizon remains discernible.
  • Astronomical Twilight: Sun between 12° and 18° below the horizon. Darkness is near-complete; faint stars are fully visible.

Duration of these phases depends on latitude and date. At high latitudes, twilight can extend for hours during summer, delaying true darkness. At the equator, twilight is compressed into roughly 20–30 minutes year-round due to the Sun's steep angle relative to the horizon.

Key Considerations When Calculating Daylight

Several practical factors affect the accuracy and interpretation of daylight calculations.

  1. Atmospheric Refraction Adds Hidden Minutes — The standard refraction correction adds about 34 arc-minutes to the apparent solar disk diameter, extending calculated daylight by roughly 5–6 minutes. Without accounting for this, sunrise and sunset predictions may appear 3–6 minutes earlier or later than observed reality.
  2. Local Horizon Obstruction Isn't Factored In — Calculations assume a sea-level, unobstructed horizon. Mountains, buildings, or hills near you can shift sunrise times later and sunset times earlier, reducing actual daylight. A calculator provides the theoretical daylight for an ideal horizon at your coordinates.
  3. Time Zone Shifts the Clock but Not Duration — Longitude and time zone offset affect what time sunrise and sunset occur locally, but they do not change day length. A location at 0° longitude experiences the same daylight duration as one at 180°; only the clock time differs.
  4. Polar Regions Show NaN Results for Extreme Dates — Above the polar circles on extreme summer or winter dates, the Sun may not rise or set at all. Calculators return NaN (not a number) for these cases, correctly indicating that the concept of sunrise or sunset becomes meaningless.

Frequently Asked Questions

How does latitude determine the length of daylight throughout the year?

The Earth's 23.44° axial tilt causes the Sun's highest point in the sky to shift northward in Northern Hemisphere summer and southward in Southern Hemisphere summer. At the equator, this tilt has minimal effect, so the Sun's arc remains relatively constant, yielding near-equal day and night year-round. At higher latitudes, the Sun's arc length varies dramatically: a 50°N location experiences sunlight arcs of roughly 16 hours in June but only 8 hours in December. The mathematical relationship connects solar declination (which changes with day of year) to latitude, determining the duration the Sun spends above the horizon.

Why is daylight on the equinox nearly equal everywhere despite latitude differences?

On the equinoxes (March 20 and September 22), the Sun's declination is exactly 0°, meaning the terminator—the day-night boundary—passes through both poles. Everywhere on Earth, the Sun rises due east and sets due west, spending 12 hours above the horizon. Atmospheric refraction adds roughly 15 minutes more, so you observe about 12 hours 15 minutes of daylight. This near-universal 12-hour day is a unique property of equinox dates and breaks down on any other day of the year.

What causes the Midnight Sun and Polar Night phenomena?

Above the Arctic and Antarctic circles (66.56° latitude), the Sun's seasonal declination range combined with the location's latitude causes the Sun to remain above the horizon continuously (Midnight Sun) or below it continuously (Polar Night) for extended periods. At the exact poles, this lasts six months in each direction. At the polar circles themselves, the effect occurs only on the respective solstices. This happens because the horizon plane, tilted relative to the ecliptic, allows the Sun to circle the sky without crossing the horizon plane during extreme seasons.

How does atmospheric refraction extend perceived daylight?

Light from the Sun bends when passing through layers of increasingly dense atmosphere near Earth's horizon. This refraction bends the light path downward, making the Sun appear higher in the sky than its true geometric position. Consequently, we see the Sun rise a few minutes earlier and set a few minutes later than calculations accounting only for geometry would predict. The standard refraction correction adds about 34 arc-minutes to the Sun's apparent disk, extending daylight by roughly 5–6 minutes on average. This effect is consistent worldwide because it depends on atmospheric density, not latitude.

Why does twilight last longer at higher latitudes?

At the equator, the Sun's path crosses the horizon at a steep angle (nearly perpendicular), so it descends below the horizon rapidly, compressing twilight into 15–30 minutes. At higher latitudes, the Sun's path becomes increasingly oblique to the horizon plane. Near the poles, the Sun grazes the horizon almost tangentially, taking hours to dip below the geometric horizon. This shallow angle means the Sun spends extended time in each twilight zone (civil, nautical, and astronomical), creating extended periods of partial darkness lasting several hours during summer months at high latitudes.

Can longitude or time zone change the total length of daylight on a given date?

No. Daylight duration depends exclusively on latitude, date, and the Earth's axial tilt. Longitude and time zone shift when sunrise and sunset occur on the clock (e.g., a location 180° away has sunrise and sunset 12 hours shifted), but they do not alter the time span between the two events. A city at 20°E and another at 20°W, both at 50°N latitude, experience identical daylight lengths on the same date despite their clocks showing different times for sunrise and sunset.

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