The Relationship Between Sun Altitude and Seasons
An easy explanation of why noon altitude and day length change from season to season.
What noon altitude is
Noon altitude is the sun's altitude when it is at its highest point of the day (when it is due south). The larger this value, the more vertically the sunlight comes in — making it stronger and the day longer.
Why it changes with the seasons
Because Earth's rotation axis is tilted, the sun's declination changes with the season even at the same latitude. In summer the noon altitude is high and the day is long; in winter it is low and short.
- Summer solstice: maximum noon altitude and longest day in the Northern Hemisphere
- Winter solstice: minimum noon altitude and shortest day in the Northern Hemisphere
- Spring and autumn equinoxes: day and night are almost equal in length
The effect of latitude
At the equator the difference in day length between seasons is small, but the higher the latitude, the larger the difference becomes. You can compare this difference directly in the seasonal table on a city's detail page.
Noon altitude is also used for setting the angle of solar panels and designing building daylighting.