Seasons and sky
Photoperiod
Direct answer
Photoperiod is the duration of light within a daily cycle, commonly measured as day length for biological response.
Photoperiod is the daily duration of light experienced by an organism or place.
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At a glance
- Found in
- Seasons & Earth Almanac
- Related
- Midnight Sun, Solar Insolation, Astronomical Season, Equinox
Photoperiod is the daily duration of light experienced by an organism or place.
A Sun-Earth orientation for seasonal, atmospheric, tidal, and terrestrial terms.
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The story behind Photoperiod
Experimental biology showed that many seasonal responses depend on light-duration signals rather than temperature alone.
Why Photoperiod matters
Plants and animals use photoperiod as a seasonal cue for flowering, migration, dormancy, and reproduction.
Photoperiod in practice: an example
A long-day plant may flower only after daily light exceeds a species-specific critical duration.
Where readings of Photoperiod go wrong
It is a mistake to assume astronomical sunrise-to-sunset length equals the effective photoperiod under shade or artificial light.
Sources and editorial method
Research references used for this entry include:
- NOAA NCEI: U.S. Climate Normals - Official methodology and data for climate normals, degree days, growing seasons, and freeze dates.
- NASA Science: Seeing Equinoxes and Solstices from Space - NASA explanation of axial tilt, the terminator, seasonal illumination, equinoxes, and solstices.
- Source
- The Great Almanac Dictionary
- Category
- Seasons & Earth Almanac
- Last updated
- 2026-09-26
Compare light duration and life cycles
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Common questions
What is photoperiod?
Photoperiod is the duration of light within a daily cycle, commonly measured as day length for biological response.
What do people get wrong about photoperiod?
It is a mistake to assume astronomical sunrise-to-sunset length equals the effective photoperiod under shade or artificial light.
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