Seasons of the Earth

One of the great joys, or pains (depending on your perspective) of living in Minnesota is the changing of the seasons. We begin with Spring flower blooming and the emergence of wood ticks with Lyme's Disease, giving way to Summer picnics and mosquito plagues at sunset, followed by Autumn with the change in leaf colors that are typically blown away by rain and wind before we can appreciate it, and lastly the Winter ... which seems to last as long as all three other seasons put together. While many complain about the seasons, enough still live in Minnesota and somehow endure these changes. What causes such tremendous fluctuations in temperature? Why is it so cold in winter and hot in summer? Why is it dark at 5:00 PM in December and light after 9:00 PM in July?

The simple answer lies in the fact that Earth is tilted on its axis relative to the plane of its orbit. Earth orbits the Sun in an elliptical path, sometimes coming within 147,100,000 km (a place known as PERIHELION, the closest approach) and sometimes going as far away as 152,100,000 km (a place known as APHELION, the most distant approach). Twice each year, the Earth is at its average distance of 149,600,000 km (points known at EQUINOX). Most students would conclude that when Earth is at perihelion, it would be summer, because we are 5,000,000 km closer than aphelion. While these changing distances in the orbit do have an effect on temperatures, this change in distance is not sufficient to cause the marked seasonal differences. Indeed, it is the tilt of the planet that is the most significant cause of seasons.

Referring to the diagram to your right, you will see the tilt of the planet relative to the path of orbit. At all times, no matter where the Earth is in its yearly journey around the Sun, the north-south axis always points to the tail star of Ursa Minor ... Polaris, also known as The North Star. In this diagram, when the Earth is in the farthest right position, Earth is closest to the Sun, at Perihelion, and this point is near a day called Winter Solstice. When the Earth is most distant from the Sun, at Aphelion, as in the left side position in the diagram, this point is near a day called Summer Solstice. When the Earth is at the top and bottom positions, it is at its average distance, and at positions called Autumnal and Vernal Equinoxes. Now it is time to learn what all of this means, and how these positions lead to the seasons we experience up here in the Great White North.

The image below is pretty big, but I liked it when I found it, and chose to keep it at its present size for your viewing pleasure. The top image depicts the way the Sun strikes the Earth during Summer Solstice. Remember from the image above, Earth is at its farthest point of its orbit, but the Sun is shining directly down on the Earth at a special latitude line called the Tropic of Cancer. This latitude line is 23.44o north of the Equator. This same degree value is equal to the Earth's axial tilt. During summer, more solar radiation per square meter strikes the Northern Hemisphere. At summer solstice, the Sun shines in the sky for its longest amount of time per day, giving more time for heating to occur. The nighttime is shortest, resulting in less time for cooling. All of these factors result in Summer. The Sun is up for a long time, rises to its highest point above the horizon, and rises and sets from its most northerly positions. Minnesotans get out on their boats, sit on their decks, play golf until 9:00 PM, get nice tans, and dream of the hockey season soon to begin. While we revel in the summer, Australians are receiving their equivalent of winter since they are tilted away from the Sun and receiving less solar energy. Sorry, mates! We have no worries in June. G'day.

Once again, referring to the first diagram, notice the Earth may be closest to the Sun at perihelion and near the Winter Solstice, but the image directly above shows how North America is tilted AWAY from the sunlight. The sun shines down directly on the Tropic of Capricorn, which is in the southern hemisphere, and obliquely on the northern hemisphere. This southern latitude lies 23.44o below the Equator. This means that the daytime is going to be shorter, and nighttime longer. With less time for daytime heating and more time for nighttime cooling, it never really warms up very much. Even more significant is what this tilt does to the location of the Sun in the sky. In the dead of North American winter, the Sun never gets up very high in the sky, seemingly hugging the southern horizon. This causes the Sun to shine its light less directly on the ground, and spread out over more surface area. The reduced amount of sunlight per square meter of ground results in a reduction of surface warming. The result is Winter, when Minnesotans lace up their hockey skates, rev up the snowmobiles, cross-country ski, and pretend to ski on little imitation hills. Meanwhile, Argentinians are experiencing their summer for the same reason we experience summer when we are tilted toward the Sun. Buenos Aires really has "better air" in December than we have up north in our winter. The long night of winter can result in a psychological condition known as Seasonal Affective Disorder (SAD) ... I'm not making this up. The lack of sunshine and long dreary nights weighs heavily on the spirit. However, Minnesota astronomers revel in the longer nights. It gets dark at 5:00 PM, and this means more time for observing the night sky ... and without those pesky mosquitoes that can ruin a summer observing run. While you can only do some much to endure the summer heat and humidity, in winter, you can put on as many clothes as you want. And ... the sky is much more stable in winter due to less heat convection from the ground at night. Just one more reason to live up north!

This final image to your right shows a few other interesting results of the Earth's tilt. For all regions between the Tropic of Cancer and the Tropic of Capricorn, the Sun will shine directly down from twelve o'clock high at least one afternoon per year. The weather is always warmer here.

There is a latitude 66.56o north of the Equator called the Arctic Circle. As you can see from this diagram to your right, the Sun will shine all day and night during Summer Solstice at Earth's aphelion. Meanwhile, at the same latitude south, the Sun will never rise above the horizon during the same solstice. It is weird to think of it, but places in Alaska have entire weeks to even months of no sunlight in their winters, and weeks to months of continual daylight in their summers. I remember climbing a mountain in the extreme north Northwest Territories at 11:30 PM in September, and it was still plenty light enough for me to see clearly.

Twice each year, the Sun shines directly down on the Equator, at the Autumnal and Vernal Equinoxes. During these two days, the sun is up for exactly 12 hours and down for the same time ... everywhere on Earth. The Sun will rise from a point directly to the East, and set at a point directly to the West. The dates for any of these occasions vary slightly, but in general:

Winter Solstice = December 21/22

Spring Equinox = March 20/21

Summer Solstice = June 21/22

Autumn Equinox = September 22/23

To know the exact Times and Dates of Seasonal Events, click on the website.

Move now to Precession and then to other Earth Rhythms before you start the Moon Unit, or you can return to the Earth Introduction Page.


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