Why Are There Seasons?

The seasons in middle and high latitudes depend on the variation in sunlight received on the Earth’s surface throughout the year. The amount of energy from the Sun (called “insolation”) changes because of the variation in the inclination of Earth's axis from the vertical to the orbital plane. At the summer solstice in the northern hemisphere, the north pole of the axis points toward the Sun, such that the Sun reaches its highest noontime position in the northern sky, being directly overhead at 23.5°N, at the tropic of Cancer. The day of northern summer solstice has the longest daylight and the shortest night in the year in the northern hemisphere. At the winter solstice, the north pole of the axis points away from the Sun, and the Sun reaches its lowest noontime position in the northern sky, being directly overhead at 23.5°S, at the tropic of Capricorn. The day of northern winter solstice has the shortest daylight and the longest night in the year on the northern hemisphere.

At an equinox, the Sun is directly overhead at noontime on the equator. The equinox following winter is called spring equinox, and the one following summer is the fall equinox. During an equinox, day and night are exactly of equal length everywhere on Earth. The seasons on northern and southern hemispheres are exactly opposite, and the Tropic of Cancer and the Tropic of Capricorn get their names from a time, over 2000 years ago, when summer and winter solstice were reckoned by following the path of the Sun through the constellations of the zodiac, in the sky (rather than looking in a printed calendar, as we do now). During this time the Sun was in the constellations Cancer and Capricorn during northern summer and winter solstice, respectively. Precession of the Earth (i.e. periodic circular motion of the Earth’s axis) has resulted in a shift in the Sun’s position since then such that it is now in the constellation Gemini during the summer solstice.

Sketch illustrating the different amounts of sunlight (i.e. insolation) received by the Earth on the northern summer solstice.

Questions to ask yourself: How high above the horizon is the Sun at noon in Boulder, Colorado, (located at 40°N) on the day of northern summer solstice? What about Oslo, Norway (located at 60 °N)? From where can the sun be seen on that day at midnight? How high above the horizon is the Sun at noon, in Boulder, on the day of northern winter solstice? In Oslo, Norway? From where can the sun be seen on the winter solstice at midnight? See the figure below to help you picture some of the scenarios described above.