Daylight & Darkness

Daylight is the most predictable variable in this almanac and the most frequently misjudged. It is deterministic, it is not symmetrical around the calendar, and it ends later than sunset.

Instrument

Daylight window & turnaround

Sunrise, solar noon, sunset, civil dusk and the latest safe departure time for your outing.

Scope

What this section covers

Sunrise, sunset, civil twilight, solar noon and circadian light. When usable light is available, and what it does to you.

Field notes

Reference

DaylightMorning Light Is the Signal Your Body Clock Is Waiting ForHow intrinsically photosensitive retinal ganglion cells set circadian timing, why indoor light barely registers, and what the phase response curve means for when you should go outside.
DaylightSunset Is Not When the Light EndsCivil, nautical and astronomical twilight defined, how much usable light each provides, and why twilight duration varies enormously with latitude and season.
DaylightWhat Shorter Days Do to Mood, and What the Evidence SupportsSeasonal affective disorder, the phase-shift and photoperiod hypotheses, light therapy evidence, and the role of outdoor daylight exposure.
DaylightGolden Hour, Blue Hour, and the Physics Behind BothRayleigh scattering, atmospheric path length, and how to predict the quality of light at any hour rather than guessing.
DaylightWhy Day Length Changes Fast in March and Barely Moves in JuneAxial tilt, the sinusoidal rate of change in day length, and why the earliest sunset does not fall on the shortest day.