Daylight Hours Calculator
Calculate total daylight hours, sunrise/sunset times, and solar geometry for any date and latitude. Includes seasonal comparison at equinoxes and solstices.
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Result
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Daylight Hours Calculator: Compute Sunrise, Sunset, and Total Daylight for Any Date and Latitude
The Daylight Hours Calculator computes the total hours of daylight, approximate sunrise and sunset times (solar time), and solar geometry parameters (declination, hour angle) for any date at any latitude. In "Solar Energy Planning," "Agriculture," and "Architectural Design," daylight duration directly affects system sizing, crop scheduling, and building orientation. According to NOAA's Solar Calculator, daylight hours vary from 0 (polar night) to 24 (midnight sun) depending on latitude and date. At the equator, daylight varies by only 10 minutes throughout the year; at 60 degrees latitude, it ranges from 5.5 to 18.8 hours.
How is daylight duration calculated?
Daylight duration is computed from the solar hour angle, which depends on the solar declination and the observer's latitude. The solar declination (the angle between the sun and the equatorial plane) varies sinusoidally throughout the year: -23.45 degrees at the winter solstice to +23.45 degrees at the summer solstice. The hour angle omega is computed as: omega = arccos(-tan(latitude) x tan(declination)). Daylight hours = 2 x omega / 15. This formula is derived from spherical astronomy and follows the equations published in Jean Meeus' Astronomical Algorithms (1991).
Daylight Hours by Latitude
| Location | Latitude | Jun 21 (Summer) | Dec 21 (Winter) | Annual Range |
|---|---|---|---|---|
| Equator | 0 deg | 12.1 hrs | 12.1 hrs | 0.2 hrs |
| Miami | 25.8 deg N | 13.8 hrs | 10.5 hrs | 3.3 hrs |
| London | 51.5 deg N | 16.5 hrs | 8.0 hrs | 8.5 hrs |
| Stockholm | 59.3 deg N | 18.5 hrs | 6.1 hrs | 12.4 hrs |
| Tromso | 69.6 deg N | 24.0 hrs | 0.0 hrs | 24.0 hrs |
6 Professional Use Cases
- Solar Panel Sizing: Solar energy engineers use peak sunlight hours (a fraction of total daylight) to size photovoltaic systems. A location with 14 hours of daylight typically has 5-6 peak sun hours due to atmospheric attenuation and angle.
- Agricultural Scheduling: Farmers use daylight hours to determine planting and harvesting windows. Long-day crops (wheat, spinach) require 14+ hours; short-day crops (rice, poinsettias) require less than 12 hours.
- Construction Planning: Construction managers plan outdoor work schedules based on available daylight. Winter construction in high-latitude regions may have only 6-7 usable daylight hours.
- Architectural Design: Architects orient buildings to maximize natural light penetration. Daylight calculations determine window placement and size per LEED v4 Daylighting Credit.
- Wildlife Research: Ecologists correlate animal behavior patterns with photoperiod (daylight length). Bird migration, hibernation onset, and breeding cycles are triggered by daylight duration changes.
- Event Planning: Outdoor event organizers determine sunset time to plan lighting requirements and schedule activities appropriately.
How to Use the Daylight Hours Calculator
- Enter Date: Select any date using the date picker.
- Enter Latitude: Input the latitude in decimal degrees (-90 to 90). Positive values are North, negative are South. Default is London (51.5074).
- Execute: Click "Calculate." The tool computes daylight duration, sunrise/sunset times, and solar geometry.
- Compare Seasons: The seasonal comparison table shows daylight hours at the equinoxes and solstices for the entered latitude.
Polar Day and Polar Night
At latitudes above approximately 66.5 degrees (the Arctic/Antarctic circles), the sun does not set during summer solstice (polar day / midnight sun) and does not rise during winter solstice (polar night). The calculator handles these edge cases: when the hour angle calculation produces NaN (the sun never sets or never rises), it returns 24 hours (continuous daylight) or 0 hours (continuous darkness) based on the sign of tan(latitude) x tan(declination). These phenomena affect approximately 4 million people living within the Arctic and Antarctic circles.
Frequently Asked Questions
Are the sunrise/sunset times in local time?
The times are in approximate solar time, not local clock time. Solar noon occurs when the sun is at its highest point, which may differ from 12:00 local time due to time zone boundaries and the Equation of Time (up to +/- 16 minutes).
Does the calculator account for atmospheric refraction?
The calculator uses the geometric sunrise/sunset definition (sun center at horizon). Atmospheric refraction extends visible daylight by approximately 3-4 minutes at both sunrise and sunset, adding about 6-8 minutes to the total.
What happens at the exact equator?
At latitude 0 degrees, daylight duration is approximately 12.1 hours throughout the year. The slight variation from exactly 12 hours is due to the sun's non-zero angular diameter and atmospheric refraction.
Can I enter southern hemisphere latitudes?
Use negative latitude values for the southern hemisphere. At -33.9 degrees (Sydney), summer solstice (December 21) produces the longest day, and winter solstice (June 21) produces the shortest.