Daylight Calculator.
Daylight Calculator
Natural daylight hours, sunrise, sunset, and photoperiod by latitude, longitude, and date.
Why this matters
Plants don't experience clock time — they experience day length. For greenhouse operators and outdoor growers, knowing exactly how much natural daylight is available on any given date at any given latitude is foundational to:
- Photoperiod-sensitive cultivars (cannabis, chrysanthemums, strawberries, soybeans) — natural daylight determines when flowering triggers
- Supplemental lighting planning — the gap between target DLI and natural DLI tells you how much supplementation you need
- Outdoor planting timing — frost dates aside, daylight thresholds determine when crops will germinate, set fruit, or bolt
- Greenhouse blackout scheduling — to force flowering at high latitudes in summer, blackout cloth must extend the dark period to mimic shorter days
- Light-deprivation cannabis (cannabis "light dep") — outdoor cannabis growers in long-day regions force flowering earlier with structured light deprivation
Photoperiod thresholds for common cultivars
| Cultivar | Type | Critical photoperiod | Notes |
|---|---|---|---|
| Cannabis (photoperiod-sensitive) | Short-day | ~12 hours dark | Most photoperiod-sensitive; outdoor flowers when nights exceed ~12h |
| Cannabis (autoflower) | Day-neutral | N/A | Flowers based on age, not photoperiod |
| Chrysanthemum | Short-day | ~13 hours dark | Greenhouse flowering forced by blackout |
| Strawberry (June-bearing) | Short-day | ~13 hours dark | Sets fruit in spring/fall |
| Strawberry (everbearing) | Day-neutral | N/A | Fruits regardless of photoperiod |
| Soybean | Short-day | ~13 hours dark | Variety-specific |
| Spinach | Long-day | ~13 hours light | Bolts in long days |
| Lettuce | Long-day (mild) | ~13 hours light | Bolts at long-day + heat combination |
| Tomato | Day-neutral | N/A | Heat / DLI matter more |
| Hops | Short-day cone production | ~16 hours light cessation | Cone formation triggered by shortening days |
| Poinsettia | Short-day | ~12 hours dark, sustained | Famous fall-flowering response |
The flagging widget above checks your date / latitude against these thresholds and indicates whether the natural photoperiod would or would not trigger flowering for short-day cultivars.
Outdoor cannabis flowering (light deprivation)
Outdoor cannabis growers at temperate latitudes face a problem: by the time natural day length drops below ~13 hours of light (around mid-September in the northern hemisphere), the season is already cooling, and frost may arrive before harvest.
Light deprivation ("light dep") solves this by manually shortening the day with blackout cloth. Pulling tarps over the canopy at, say, 7pm during August forces a 12/12 photoperiod and triggers flowering 6-8 weeks earlier than nature would. By harvest, weather is still warm and plants finish before frost risk.
The Daylight Calculator helps plan light dep schedules: at your latitude, you know exactly when natural day length crosses critical thresholds and can plan blackout timing accordingly.
Greenhouse photoperiod control at high latitudes
At 50°N+ in summer, day length exceeds 16 hours. For short-day flowering crops (chrysanthemums, poinsettias, strawberries), this is a problem — blackout cloth must extend the dark period to trigger flowering.
For long-day flowering or vegetative-promotion crops, supplemental lighting can extend day length artificially in winter. At 60°N in December, day length is barely 6 hours; supplementing to 14-16 hours requires substantial lighting.
The annual daylight curve above shows your latitude's full season at a glance, useful for planning year-round greenhouse operations.
The math
The calculator uses standard solar geometry:
Solar declination δ = 23.45° × sin((360° / 365) × (284 + N)) where N is day of year Sunrise hour angle ω = arccos(−tan(latitude) × tan(δ)) Day length = 2 × ω / 15 (hours) Solar elevation at noon = 90° − |latitude − δ|
This is a simplified model that ignores atmospheric refraction (about 4 minutes earlier sunrise and later sunset than this calculation), elevation, and obstructions. For most agricultural planning purposes, the simplified math is accurate within a few minutes — well within the resolution of useful decisions.
References
Solar geometry is standard astronomy and well-documented. References include the NOAA Solar Calculator, the Astronomical Almanac, and Spencer's 1971 Fourier-series approximation. Photoperiod thresholds for cultivars synthesized from peer-reviewed horticulture literature.
Free under CC BY 4.0. Cite as "OAT Daylight Calculator (openagriculturetechnology.com)".