Reasons for the Seasons
by Jon Kahl
(figures from Jack William's The USA Today Weather Book)
Are the seasons caused by the distance between the sun and the
earth? Is winter colder because during that season the earth is
farther away from the sun?
The amount of sunlight the earth receives is called insolation:
incoming solar radiation
Two factors cause insolation to change.
- Length of day
(because more hours of daylight mean more insolation)
- The angle that sunlight strikes the earth's surface
The diagram to the left shows that when
the sun shines directly overhead, the sunlight is concentrated
in a small area. When the sun is lower in the sky, the sunlight
is not as intense because it gets spread out over a larger area.
Why are summer days longer and winter days shorter?
As the earth revolves around the sun, its axis (a line connecting the
North and South poles) is tilted slightly.
On June 21, the summer solstice, the top of the axis
(the North Pole) is pointed directly toward
the sun. Areas north of the equator experience longer days and
On December 21, the winter solstice, the top of earth's axis is pointed
directly away from the sun. Areas north of the equator experience
shorter days and longer nights.
Halfway in between the summer and winter solstices are the equinoxes.
At these times the earth's axis is pointing neither toward nor away
from the sun. On both equinoxes, all locations on earth receive
exactly 12 hours of daylight and 12 hours of night.
What changes the angle at which sunlight strikes the earth?
As the sun rises in the east and sets in the west, it follows a
path that changes with the seasons.
During winter the sun's highest point in the sky is quite low. During
the summer the sun gets much higher and provides more direct, concentrated
- As the earth revolves around the sun, it's actually about 5 million
miles closer in January than it is in July. Despite being closer,
Wisconsin weather is much colder in January than in July. Why?
- Why are noontime shadows longer in winter than in summer?