2.1.7: Day Length and Seasons

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Page ID: 15547

Michael E. Ritter
University of Wisconsin-Stevens Point via The Physical Environment

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Day length is determined by the length of time the Sun is above the horizon. Day length changes through the year as the orientation of the Earth to the Sun changes. The circle of illumination is the imaginary circle that separate day from night.

Figure \(\PageIndex{1}\): Variations in day length.

Figure \(\PageIndex{1}\) shows two extreme cases, the December and June solstices. Note during December that more of a given latitude in the Southern hemisphere is exposed to the Sun. This is the longest day of the year for those living poleward of the Equator. In June the opposite occurs with longer day length in the Northern hemisphere. Note that in both cases, the circle of illumination bisects the Equator (cuts it in half). The Equator is the only place on Earth that experiences equal day length every day of the year. The Earth's axis is neither inclined toward or away from the Sun on the equinoxes and so the circle of illumination cuts all latitudes in half resulting in equal day length for the entire Earth.

To summarize the changing seasonal conditions:

Table \(\PageIndex{1}\): Earth- Sun Relations and Season
	SPRING EQUINOX	WINTER SOLSTICE	AUTUMNAL EQUINOX	SUMMER SOLSTICE
Date	March 21	December 22	Sept. 23	June 21
Subsolar Point	0^o	23 1/2^oS	0^o	23 1/2^o N
Tangent Rays	North and South Poles	Arctic and Antarctic Circles	North and South Poles	Arctic and Antarctic Circles
Day length	12 hour day length everywhere	24 hours of darkness at North Pole; 24 hours day light at South Pole; 12 hours day light at Equator	12 hour day length everywhere	24 hours of darkness at South Pole; 24 hours day light at North Pole; 12 hours day light at Equator