Seasons
Rhythmically varying angles of the sun in the sky are a basic feature of physical
geography because they represent systematic changes in the amounts ofsolar
energyreceived in various locations. It is the appearance of the sun in the sky that
separates day from night and winter from summer. From time immemorial humans
recognized the latter, more subtle, effect and attached great societal importance to
it. For instance, a millennium before the Egyptian pyramids were built, Celtic
tombs overlooking Ireland’s River Boyne were outfitted with portals over the
entrances. During a few sunrises in the third week of December the sun shines
through the portal and illuminates the tombs’ central chamber. This event marked
the time of year when the sun was lowest in the sky. Clearly, the builders under-
stood there was a relationship between the path of the sun in the sky and expec-
tations oftemperatureandprecipitationpatterns. Worldwide, societies have
incorporated such notions into religious and social rites and planned planting and
harvesting. Today, the reasons for seasonality are well understood and seasonal
effects are still vital to life on the planet.
Earthorbits the sun approximately once per year and it is the position of Earth in
its orbit that causes changes in seasons. Earth’s orbit is elliptical with about 5 million
km of difference between Earth’s closest approach to the sun (perigee) and furthest
swing from the sun (apogee). This orbital shape affects seasons in only very minor
ways. It isnotchanges in Earth/sun distance that causes seasons. Two observations
prove this: (1) summer in the Northern Hemisphere occurs during the apogee of the
orbit and (2) there are opposite seasons in the Northern and Southern hemispheres;
that is, when one is experiencing winter the other is in summer.
The reason for seasons is rooted in the fact that Earth’s axis is tilted at an angle
of 23.5°away from perpendicular to the Plane of the Ecliptic. The Plane of the
Ecliptic is the imaginary surface that bisects the sun and Earth and is the orbital
plane of Earth. As Earth orbits the sun the angle of tilt is maintained at 23.5°.
The north end of Earth’s axis always points in the same direction. In our part of
Earth history the north end axis always points to Polaris, the North Star. (In fact,
there is a 26,000-year axial wobble that can be ignored on a year-to-year basis.)
However, the angular relationship between the axis and the position of the sun sys-
tematically changes during the course of a single orbit. In June the north end of the
axis points toward the sun, in December the north end is pointed away from the sun,
and in March and September it is in an intermediate position. This configuration
causes warmer temperatures in whichever hemisphere is pointed toward the sun.
The progression of seasons is controlled by themigrationof the solar path in the
sky. On any date, there is a single latitude at which the sun can be viewed straight298 Seasons