Summer World_ A Season of Bounty - Bernd Heinrich [1]
Both the duration and the incidence of illumination at any one place depend on the Earth’s tilt, or inclination, toward the sun, and the seasons are a consequence of this tilt. At all points of the Earth’s approximately 365-day (actually 365.2422-day) orbit around the sun, which we define as a year, the Earth’s axis of rotation (an imaginary line connecting the north and south poles) is 23.5 degrees with respect to the plane of its orbit around the sun. This angle does not affect the total energy that the entire Earth receives over the year; rather, it shifts the distribution of energy between the northern and southern hemispheres. When one hemisphere gets a lot of energy, the other gets little, and thus when it is summer in one it is winter in the other. At the equator the energy input is equal year-round, the sun is directly overhead at noon, and days and nights are always equal.
When the Earth is at the point in its orbit where the north pole is inclined at its maximum, 23.5 degrees, toward the sun, that is defined as the summer solstice in the north. At this time the far north is in continuous light and the far south is in continuous darkness. As the Earth continues its journey around the sun (while still maintaining its own same axis of rotation) the tilt that was toward the sun decreases gradually until solar radiation falls equally slanted onto both poles. At this point, the autumnal equinox, day and night are of equal length everywhere.
The solstices, the asteorological relationships during the Earth’s annual journey around the sun, proximally cause the seasons and the overall weather patterns to which life adjusts. However, ultimately the seasons are due to an ancient catastrophe. Astronomers believe that about 4 billion years ago a body the size and mass of Mars slammed into the Earth at 18,000 miles per hour, possibly tipping the Earth’s axis of rotation. Additionally, the matter that was ejected by this colossal collision produced the moon. Life arose near that time, about half a billion (500 million) years later, and it has adjusted to summer versus winter ever since. Different species each have their own schedules of preparation for summer, although for most summer is the season of reproduction, feeding, growing, and trying to avoid being eaten. It’s the season of courting, mating, and birthing; of living and dying.
Fig. 1. The Earth’s annual journey around the sun, showing the seasons in relation to the planet’s tilt. At the solstices, the durations of day and night are different in the north versus the south polar regions. At the equinoxes these durations are equal at every point on Earth.
THE MEMBERS OF MANY SPECIES, MYSELF INCLUDED, become more alive again at the first “scent” of summer. Skunks come out of their dens, and we get the first whiffs of their presence. Chipmunks emerge from underground and leave their first tracks on the softening snow. The yearling beavers leave their dens as their parents get ready to have new pups. Flower buds of the willows, alder, beaked hazel, poplars, and elms are poised to respond to the first warmth, to open and reveal their beautiful colors and varied forms. Some of the birds that overwintered begin to sing, and the migrant birds are plying the skies by the millions on their way north from the tropics. The first are starting to arrive. Nature is about to burst at the seams. As George Harrison’s song, performed by the Beatles in 1969, goes: “Here comes the sun—da da da da.