Calculator Inputs
Enter latitude, longitude, time zone, and dates to estimate daylight duration, sunrise, sunset, seasonal differences, and solar geometry values.
Sunlight Trend Graph
The chart visualizes estimated daylight duration for the selected latitude. Monthly mode shows midpoint values. Weekly mode highlights annual seasonal variation.
Example Data Table
These example values show approximate daylight duration patterns across key dates for a subtropical location near 25° north latitude.
| Date | Latitude (°) | Declination (°) | Estimated Sunlight Hours | Season Note |
|---|---|---|---|---|
| 2026-03-21 | 24.8607 | 0.00 | 12.00 | Equinox balance |
| 2026-06-21 | 24.8607 | 23.45 | 13.56 | Near longest daylight |
| 2026-09-22 | 24.8607 | 0.00 | 12.00 | Equinox balance |
| 2026-12-21 | 24.8607 | -23.45 | 10.44 | Near shortest daylight |
Formula Used
1) Solar declination
δ = 23.45 × sin[(360 / 365) × (284 + n)]
2) Sunrise hour angle
cos(H0) = −tan(φ) × tan(δ)
3) Daylight duration
Daylight Hours = (2 × H0) / 15
4) Equation of time
EoT = 9.87 × sin(2B) − 7.53 × cos(B) − 1.5 × sin(B)
Variables
n = day of year, φ = latitude, δ = declination, H0 = sunrise hour angle in degrees, B = seasonal correction angle.
This model estimates geometric daylight duration from Earth-Sun position. It is suitable for planning, comparison, and educational solar analysis.
How to Use This Calculator
- Enter the site latitude in decimal degrees.
- Enter longitude and the local time zone offset.
- Select a primary date for sunlight estimation.
- Choose a comparison date for seasonal analysis.
- Pick monthly or weekly graph mode.
- Press Calculate Sunlight to display results above the form.
- Review daylight hours, sunrise, sunset, solar noon, declination, and seasonal change.
- Use the CSV or PDF buttons to export the current result summary.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates daily sunlight duration from latitude and calendar date. It also shows sunrise, sunset, solar noon, declination, and a seasonal comparison.
2. Is this the same as peak sun hours?
No. Daily sunlight hours measure daylight length. Peak sun hours represent solar energy intensity converted into equivalent full-sun hours for power estimation.
3. Why do daylight hours change through the year?
Earth’s axial tilt changes the Sun’s apparent path. That shifts sunrise and sunset times, producing longer or shorter days through seasons.
4. Why are longitude and time zone included?
They help estimate local solar noon, sunrise, and sunset clock times. Daylight duration itself mainly depends on latitude and date.
5. Can this calculator show polar day or polar night?
Yes. At extreme latitudes, the hour-angle term can indicate continuous daylight or no sunrise. The result will label these cases clearly.
6. How accurate is the result?
It is a strong geometric estimate for planning and education. Local terrain, refraction, and atmospheric conditions can slightly shift real sunrise and sunset.
7. How does this help solar planning?
It helps compare seasons, anticipate daylight availability, and support site review. Pair it with irradiance and shading analysis for full solar design.
8. What chart mode should I choose?
Use monthly mode for a simple annual pattern. Use weekly mode when you want finer seasonal detail across the year.