Sunlight Duration Planner
Track your sunlight duration with our free sports calculator. Get personalized stats, rankings, and performance comparisons.
Reviewed by Sher, Sports Science & Nutrition Specialist
Formula
Hour Angle = arccos(-tan(lat) x tan(dec)); Daylight = 2 x HA / 15
The sunrise equation calculates daylight duration using latitude and solar declination. Solar declination is 23.45 x sin(2pi/365 x (day - 81)). The hour angle determines when the sun crosses the horizon, and daylight hours equal twice the hour angle divided by 15 degrees per hour.
Worked Examples
Example 1: Summer Hiking Trip in Colorado
Problem:Plan daylight for a hike at 39.7N latitude on day 172 (June 21), starting at 6 AM, 8-hour duration, Mountain Time (UTC-7).
Solution:Solar declination: 23.45 degrees\nHour angle: cos^-1(-tan(39.7) x tan(23.45)) = 109.5 degrees\nDaylight hours: (2 x 109.5) / 15 = 14.6 hours\nSunrise: ~5:32 AM\nSunset: ~8:10 PM\nActivity window: 6:00 AM to 2:00 PM\nDaylight in activity: 8.0 hours (100%)\nSolar altitude at noon: 73.7 degrees
Result:14.6 hrs daylight | Sunrise: 5:32 AM | 100% daylight activity | UV Index: 11.1
Example 2: Winter Photography Session in New York
Problem:Plan golden hour photography at 40.7N on day 355 (Dec 21), starting at 3 PM, 2-hour session, Eastern Time (UTC-5).
Solution:Solar declination: -23.45 degrees\nDaylight hours: 9.2 hours\nSunrise: ~7:16 AM\nSunset: ~4:29 PM\nActivity: 3:00 PM to 5:00 PM\nDaylight in activity: 1.5 hours (75%)\nGolden hour: starts ~3:29 PM\nMost of session in golden light
Result:9.2 hrs daylight | Sunset: 4:29 PM | 75% daylight | Perfect golden hour timing
Frequently Asked Questions
How is daylight duration calculated for any location and date?
Daylight duration is calculated using the solar declination angle and the geographic latitude of the observer. The solar declination varies from positive 23.45 degrees at the June solstice to negative 23.45 degrees at the December solstice, following a sinusoidal pattern throughout the year. The hour angle formula uses the inverse cosine of negative tangent latitude times tangent declination to determine when the sun crosses the horizon. At the equator, daylight remains close to 12 hours year-round, while at higher latitudes the variation becomes extreme, ranging from near-continuous daylight in summer to very short days in winter. Sunlight Duration Planner accounts for all these astronomical factors to provide accurate sunrise and sunset times.
What timezone offset should I use for accurate sunlight calculations?
The timezone offset represents the difference between your local clock time and Coordinated Universal Time (UTC) and is critical for accurate sunrise and sunset predictions. Eastern Standard Time is UTC minus 5, Central is UTC minus 6, Mountain is UTC minus 7, and Pacific is UTC minus 8. During Daylight Saving Time, add one hour to each. European timezones range from UTC plus 0 for the United Kingdom to UTC plus 2 for Eastern Europe. Using the wrong timezone offset will shift all calculated times by the error amount. Also note that locations at the eastern or western edges of a timezone can see sunrise and sunset differ by up to 30 minutes from the timezone center. Longitude within your timezone affects the precise timing of solar events.
References
Reviewed by Sher, Sports Science & Nutrition Specialist · Editorial policy