Saturday, July 27, 2013

Seasonal Change

Mississippi River near Lock & Dam #1
Early Autumn / Early Spring
Mississippi River, north
of Lock and Dam No. 1
Seasons are distinguished by changes in weather, ecology, and hours of daylight. What accounts for seasonal changes?
“One must maintain a little bit of summer, even in the middle of winter.” Henry David Thoreau
A common misconception is that seasons occur because of differences in the distance between the Earth and the Sun throughout the year.

Seasons are not caused by variations in distance between the Earth and the Sun. The variation is distance between the Earth and the Sun is relatively small.

The perihelion, when the Earth is closest to the sun (94,445,000 miles), occurs around January 4th (position 1, left).

The aphelion, when the Earth is furthest from the sun (94,555,000 miles), occurs around July 3rd (position 2, left).

A number of years ago, the documentary “A Private Universe,” about how we misperceive the world around us, was filmed at Harvard’s commencement. Twenty-three faculty members, alumni and graduating seniors were asked, “Why is it warm in the summer and cold in the winter?” All but two answered incorrectly, saying the Earth was closer to the sun in the summer than in the winter (it’s actually closer in January). John Edward Huth
Four Seasons

The four seasons of the year are caused by variations is sunlight due to the 23.5° tilt of Earth's axis.

When summer occurs in a hemisphere, it is receiving more direct sunlight than the opposite hemisphere where it is winter.

In winter, sunlight hits the earth at oblique angles so it is less concentrated.

Seasonal changes become more pronounced as our absolute latitude increases (i.e., as we move farther from the equator).

In the northern hemisphere, summer is warmer than winter because sunlight hits the Earth at a more direct angle. Summer days in the northern hemisphere are much longer than the nights.

Much of our literature has been written with a northern hemisphere bias:
“What good is the warmth of summer, without the cold of winter to give it sweetness.”
John Steinbeck, Travels with Charley: In Search of America
The carousel-like change experienced traveling on what Buckminster Fuller popularized as Spaceship Earth, is captured in Joni Mitchell's song The Circle Game:
And the seasons they go 'round and 'round
And the painted ponies go up and down
We're captive on the carousel of time
We can't return we can only look behind
From where we came
And go round and round and round
In the circle game