Explaining Earth’s Four Seasons

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How our seasons originated is more complicated than most may realize.
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The “Mother Earth News Almanac” by the Mother Earth News staff provides readers with information about self-sufficient lifestyles of the past and present.

Mother Earth News Almanac: A Guide Through the Seasons (Voyageur Press, 2016), by the MOTHER EARTH NEWS staff is a collection of helpful information and advice to living a self-sufficient lifestyle. The book provides fun and practical ideas on topics such as raising animals, canning, making compost, and more! The following excerpt is from Chapter 1, “The Planet Earth.”

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The “black void of space” — as we sometimes say — is neither as black nor as empty as popular folklore would indicate. It now seems fairly certain that the detectable universe contains at least one trillion galaxies, each of which is made up of billions of stars. Around each star, in turn, may revolve several planets, many of which have satellites of their own.

It is estimated that a single galaxy, the Milky Way, contains a minimum of 100 billion stars. On the edge of that cluster, nearly 27,000 light years from its center, is our own sun…circled by nine known planets.

One of those planets (we call it Earth) revolves — at a distance ranging from about 91,500,000 to 94,500,000 miles — once around the sun every 365 days, 6 hours, and 53 seconds. At the same time, the earth rotates on its own axis (which is tipped at an angle of almost 23-1/2 degrees to the plane of orbit) once each 23 hours, 56 minutes, and 4.09 seconds.

We call every one of the earth’s complete circuits of the sun a year — and, if the globe on which we live “sat up straight” (instead of being canted over about 23-1/2 degrees the way it is), every day of every year would be almost exactly the same at any given spot on the planet’s surface. As it is, however, that seemingly insignificant little tilt is the major reason that the earth’s Northern and Southern Hemispheres enjoy opposing summer-winter and spring-fall seasons.

Thanks to that tilt, you see, the planet’s North Pole points the whole 23-1/2 degrees toward the sun only once — on June 21 — every year. In the same fashion, the earth’s South Pole also points the whole 23-1/2 degrees toward the sun just once — on December 22 — each year. And for brief moments twice every orbit — on March 21 and September 23 — the globe’s slant is directly “crossways” to the sun.

These four specific positions along each circuit that the earth makes around the sun have been precisely plotted. They have also been named. The two instances when one of the planet’s poles momentarily points as directly as possible at the sun — in June and December — are both known as a solstice (Latin for “sun stands still”). The positions halfway between — in March and September — when the earth’s tilt is exactly sideways to the sun, and day and night are of equal length at all points on the globe’s surface, are both called an equinox.

It should be apparent that the Northern Hemisphere receives the maximum possible sunlight on June 21 and the minimum possible solar energy on December 22, whereas the southern half of the planet does just the opposite. It seems logical, then, to expect

June 21 to be the warmest day north of the equator and December 22 to be the coldest — and vice versa in the Southern Hemisphere.

Just as the hottest part of the day is never at high noon but a couple of hours later, the earth’s atmosphere moderates the heating and cooling of the planet and causes its temperature fluctuations to “lag” — by several weeks. Thus, the four neat dates pinpointed on its orbit around the sun. For this reason, the two equinoxes and two solstices mark the beginnings — rather than the midterms — of each hemisphere’s four yearly astronomical periods, called seasons.

It should also be noted that — due to influences exerted by some of the other heavenly bodies mentioned above, due to the slightly flattened shape of the earth itself, and due to some other complicating factors — our planet’s orbit is not exactly circular nor is the globe’s speed entirely uniform at all points along its path. For these reasons the March 21, June 21, September 23, and December 22 beginning dates of the four seasons for each hemisphere are only averages. Even more interesting is the fact that the length of the seasons vary by as much as four and a half days, as shown in the accompanying table.

All in all, the origins of the seasons are a lot more complex than most of us realize. The forces of the universe seem to be in good hands, however, so we might just as well settle back and enjoy our yearly swings around the sun…which is exactly what this almanac is designed to help you do.

Approximate Length of the Seasons

Northern Hemisphere Southern Hemisphere Length of Season: Days Length of Season: Hours
Spring Autumn 92 20
Summer Winter 93 14
Autumn Spring 89 19
Winter Summer 89 1

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More from Mother Earth News Almanac:
Making Eco-Friendly Choices
Purple Martin Birdhouse

Excerpted from Mother Earth News Almanac (Voyageur Press, 2016) by the MOTHER EARTH NEWS staff, used with permission from Voyageur Press, 2016. Original text and illustrations © Ogden Publications.