"An Amazing Grasp of the Obvious"

Chapter 3

Seasons

Ready for another concept? Let's look a bit closer at the Earth's tilt on it's axis. At 23 ½ degrees, the tilt is just right to support the distribution of heat through atmospheric and oceanic mixing.

Close your eyes (yes, and read). Imagine. The Earth, blue with white brush strokes across it. The North Pole is tilted toward the sun relative to the plane of the ecliptic. Oh, sorry. Didn't mean to complicate your imagining! The Plane of the Ecliptic is the somewhat flat imaginary surface on which all of the planets revolve around the sun. All, that is, except Pluto. We don't need to talk about Pluto... so we won't!

When the Earth's North Pole is tilted toward the Sun it is bathed in daylight for 24 hours of every day. The Solar rays speed over the top of the tilted Earth to reach the Arctic Circle beyond it. It's June 22ond, Summer in the northern hemisphere. You see it in it's stark beauty, set against the black night of space.

Now, look again! The lovely blue sphere has traveled for three months. The Earth is not tilted toward the Sun, nor is it tilted away from the Sun. It's tilt is neutral relative to the Sun's warming rays of energy. The North and the South Poles are both at exact sunrise... or sunset... for the entire 24 hours of that day. It's September 22ond!

How can this be? 3 more months have elapsed. Now the Earth's tilt is away from the Sun at the North Pole . The day is completely dark there... but the Antarctic icecap is reflecting the brilliance of it's magnificent heat-benefactor through the glory of a 24 hour day! It is Winter in the northern hemisphere, and Summer in the southern hemisphere.

Another moment and it's March 22ond. Spring is in the air in Minnesota and the Earth's tilt is again neutral to the Sun's heat energy. Ready for a diagram?

source: http://csep10.phys.utk.edu/astr161/lect/time/seasons.html

 

This diagram lists the seasons as they occur in the northern hemisphere. The blue lines through each of the Earth locations in the above diagram represent the Earth's tilt. Notice that the tilt doesn't change as the Earth travels through it's seasonal migration around the Sun. The blue line is tilted toward the left in all four of the Earth's seasons. Because the tilt doesn't change, the seasons do change!

We have a wonderful balance on the blue planet provided by the seasons, which result from the perfect tilt of the Earth relative to it's orbit around the Sun. My wife and I live in San Diego where the seasons are all pretty mild. Nonetheless I love seasonal extremes and have traveled to some cold places in winter and some hot places in Summer. My wife and I even spent a night, in the winter of 2002, at the Ice Hotel near Quebec City, Canada! That was FUN!! And, as I write, plans are developing to travel with a Russian group to the North Pole! Holey Moley! That'll be the trip! They go in April, when the sun is up 24 hours each day there, but the temperatures are still cold enough to insure solid ice. The Russians have a floating ice airport near 89o North Latitude... about 69 miles from the North Pole! And I've been to Death Valley, California, and the Dead Sea in the Middle East, and even to the Sahara Desert in summer! Why not?

The reason that seasons exist at all is a result, primarily, of solar angle of incidence and length of daylight which both vary throughout the Earth by season because of the Earth's tilt.

Angle of incidence is the primary agent of seasonal change. It refers to how high the sun is in the sky. A 90o angle of incidence only occurs in the tropical regions and means that the sun is at the zenith... shining straight down onto the Earth. On June 22ond (give or take a day) each year, for example, the Sun is shining directly overhead at the Tropic of Cancer. That day is called the June Solstice. Havana, Cuba and Cabo San Lucas (Baja California, Mexico) are located very near the Tropic of Cancer. If you travel there on June 22ond you can look straight up (at noon) and you'll see the full blazing energy of the Sun! The angle of incidence will be 90o. That's because the Tropic of Cancer is located at 23 ½o North Latitude and the Earth's 23 1/2o tilt is causing that Tropic (which is in the northern hemisphere) to most directly face the sun on the June Solstice.

The opposite Earth location, the Tropic of Capricorn, will experience a 90o angle of solar incidence on the December Solstice (typically occurring on December 22). Tropic of Capricorn? Think Rio De Janeiro! The "Girl From Ipanema!" My hips sway from side to side in rhythmic motion (impressing no one but me) at every thought!

The significance of angle of incidence is that the more direct the Sun's rays are to the surface of the Earth, the more energy they convey. A lesser angle of incidence stretches the rays out over the spherical (curved) surface of the Earth. One ray of Solar energy is no stronger than another, whether it reaches the Earth at the Equator or at the North Pole. But the curve of the Earth in either direction away from the Tropic of Cancer (on June Solstice) stretches Solar rays so that they warm a larger surface of the Earth. Thus, as a space heater can more efficiently heat 100 square feet of area than 300 square feet, Solar energy rays that reach the Earth at a direct 90o angle warm the Earth far more effectively than rays that reach the Earth at a 30o angle of incidence.

OK. Can't put this off any longer. The question is, why are the Tropics called Cancer and Capricorn? I alone, amongst all Earth dwellers, know the answer. You know that both of those names are found in Astrology. So, one must assume that if you live on Cancer, you're fate will be far different than if you live on Capricorn. It's all about one's love life, actually. And, if your love life is like mine, you'll probably be better off at the OTHER tropic!

No. Scratch that. I was drifting from science into non-science (nonsense?). If you gaze into the night sky, craning your neck, you will find the southern constellation "Capricorn" at heaven's zenith at Rio on the December Solstice and the northern constellation "Cancer" at Cabo on the June Solstice. Capricorn means "goat." and Cancer means "crab."

I've looked for those shapes in the heavenly star-mural and, although I've found them by using star-charts, I suspect that the people who originally chose the names for the constellations failed their ink-blot exams!

The reason that I alone know this stuff is that I make it up as I go. It facilitates originality... That's good, right?

Now a 0o angle of incidence is found where the Sun just touches the horizon at noon on a given day and then dips below the horizon for most of the day. On June Solstice, when the Sun's rays are concentrated in the northern hemisphere, we must travel far to the south, to the Antarctic Circle, to find the 0o angle of incidence location. The Antarctic Circle is at 66 1/2o South Latitude. Note that 66 ½ is to 90 what the Earth's tilt is! Subtract the first from the second numbers and you have 23 ½! What that means is that, on June Solstice, no Solar Rays ever reach the South Pole, but they do just touch the Antarctic Circle.

We can reverse this for December Solstice and find that the North Pole is dark and Solar Rays just touch the Arctic Circle for a moment that day with a 0o angle of incidence. You've heard about where the sun doesn't shine, I'm sure. That's the place!

Sounds a lot like Winter, doesn't it!

Northern hemisphere Winter exists because the Earth's tilt is away from the sun causing Solar ray angle of incidence to reduce. Thus the Sun's energy rays are stretched out and less energy per square inch reaches the Earth. And it gets colder!

The other effect of the Earth's tilt is that day length changes with the seasons. The only exception to this fact is found at the Equator, where every day has 12 hours of light and 12 hours of darkness. The reason, both for the exception at the Equator and for the norm in the rest of the Earth, has to do with the Circle of Illumination.

The Circle of Illumination is the line around the Earth which separates daylight from darkness. On the drawing below, the line reveals much more daylight in the northern hemisphere on the left (Summer Solstice) than on the right. This relates to the number of hours of daylight and darkness for each 24 hour period that occur at varying latitudes on the Earth's surface.

source: http://www.geog.ouc.bc.ca/physgeog/contents/6h.html

Because about most of the northern hemisphere's 45th parallel (45o North Latitude) is in daylight on Summer Solstice, daylight lasts for about 15 hours of the day and darkness lasts for a relatively shorter nighttime period (about 9 hours). Now Minneapolis, Minnesota is near the 45th parallel and so is Milan, Italy. Does it sound right that in the summer the sun is up from 6am to 9pm? In winter the sun goes down around 4pm. It works just as well, by the way, in Dunedin, New Zealand. Dunedin is located near the southern hemisphere's 45th parallel. Summer there, however, begins on December 22!

The Earth takes 24 hours to rotate once. Minneapolis, therefore, makes a single rotation in a day. Because the North Pole is tilted toward the Sun on the June Solstice (as is seen in the diagram above, left side), the Circle of Illumination is casting shadow on a smaller slice of the northern hemisphere than it is on the southern hemisphere. Minneapolis will therefore spend less of it's rotation in darkness than it will in daylight.

The Earth spins on it's axis, which is the "axle" of the Earth (so to speak). This "axle" connects the Poles through the center of the Earth. Thus Minneapolis does not rotate facing the Sun, but at an ever changing angle to the Sun. In summer the Sun rises northward in the east and sets northward in the west, and hovers south of the zenith at noon.

An excellent example of this took my breath away one day in late June, 1981, after a long day of fishing with my dad and my son in northern Saskatchewan, Canada. I had walked out on the pier of a 30-mile-long north-south lake at nearly midnight and watched the sun plunge into the lake at a point near to due north! Although I knew the principal of this, the blue planet struck me almost dizzy when the Sun did not set in the west! The farther north you travel, in summer, the farther north the sun will rise and set until you pass the Arctic Circle where (on June Solstice) the Sun won't set at all! Above the Arctic Circle the Sun lowers in the sky until it bottoms at due north, then begins it's daily lifting exercise toward noon.

For the sake of my southern hemisphere mates, let me add a disclaimer. All of this information is reversed in the southern hemisphere. The sun sets increasingly south of west as one travels southward in the southern hemisphere summer. Must go to Antarctica, however, to gain the fullest perspective!

Thus it is that in summer, with more direct sunlight lasting many more hours, the days get progressively warmer. And in winter, with less direct sunlight and fewer daylight hours, the days get progressively cooler.

Note that the Circle of Illumination, as it constantly changes through the seasons, must always pass the mid-point of the north-south axis of the Earth at it's center. The center of the north-south axis will always be the Equator. Thus the Circle of Illumination must always illumine the Equator for 12 hours and leave it in darkness for an equal time period each day. (The words "equator" and "equal" may not be coincidentally similar!)

Besides the seasons, this constant change of Solar Radiation on the Earth has another very significant effect. Air moves because the Earth's surface is not heated evenly. Air, you see, is not heated by the Sun. Not primarily, at any rate. The oceans and continents are warmed by the Sun. Air, however, transmits the short-wave radiation from the Sun (lets it pass through) but air captures (absorbs) the long wave radiation which rises from the Earth. Thus air is mostly warmed by warm ocean and continental surfaces. Cold air, which is found above cold land and water surfaces, has a higher pressure than warm air and thus moves toward warm air, causing atmospheric circulation. For this reason the atmosphere... the Earth's air envelope... is constantly being mixed.

The mixing permits the differential heating of the Earth to be moderated, causing hot places to cool and cold places to warm. It also causes the air to be continually oxygenated as it endlessly sweeps over the plankton of the oceans and the great forests of the continents. Because warm air rises, pollutants are continually being shed by gravity in a natural filtration system resulting from the ever shifting warm surfaces of the Earth.

All of this is natural, obviously. What isn't obvious is why these things are so. Natural science can describe gravity and air pressure and the relationship of temperature to both, but they cannot answer the simple question: why is it so? We may be told that the answer is "physics." But that is not any real answer! Science has determined that the inflexible laws of physics, as we learned them, do not apply at the sub-atomic level. Different physical laws apply there! Why?

Might it all be so because it works so wonderfully to provide the vast variety and longevity of an Earth environment fit so perfectly for life? Might it all be so by design?

To say that it cannot be so is to vary from science. To say that it definitely is so also varies from science. Yet, all alone in my easy chair in the wee hours, I like to consider the odds that such a magnificent design might result from random chance. Sometimes I like to ponder why the laws of nature so powerfully hold their subjects in obedience. Why do they not just randomly change on occasion?

And, I do love the seasons.

Chapter 4