Photoperiod

What does Photoperiod mean?

Day length is one of the few things about the sky you can know in advance, and photoperiod is its name: the stretch of a day between sunrise and sunset at a given place and date. It comes out of geometry, so a date and a set of coordinates are enough to compute it and no weather record is involved. It grows and shrinks through the year as Earth's tilt swings each hemisphere toward and away from the sun, and latitude sets how wide that swing gets. Near the equator the day barely changes length across the year. Far north or south, the gap between the shortest and longest day is measured in hours. Cloud changes how bright a day feels without changing the calculated moments the sun crosses the horizon, which is what makes day length a steady way to read the season when today's sky is unusual. It is worth keeping separate from two things it gets blurred with: how high the sun climbs, and how much light or ultraviolet actually reaches the ground.

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