HOWARD'S "SOLAR" GREENHOUSE

In 1999, the "Dirt Doctor," Howard Garrett, a registered landscape architect, constructed in the yard behind his house a tall, woodframe "solar" greenhouse.  It's closed on the north and west and open (glassed in) on the south and east, with a solid, shingled, shed roof of a very special design, as he explained in some detail in the April, 2000, issue of The Dirt Doctor's Dirt.

"The key is to pay attention to the way the sun tracks across the sky," he wrote.  "In the winter, the sun goes from east to west on the southern horizon.  In the summer it takes the same east-west route, but in the northern sector."

Then he explained, "My new solar greenhouse has a solid roof that slopes at a 22º angle from the south down to the north.  This is the exact angle of the sun at the winter solstice, the lowest path it travels in the southern sky on December 22.  During this time the sun shines in the greenhouse and hits the top of the back wall shining on all the plants in the greenhouse...During the summer, with the sun on its northern path, no direct sun enters the house except early in the morning."

We'll take his word that the roof slopes at 22º, and we'll give him the benefit of the doubt and assume he meant to say that this is "the highest point it [the sun] reaches at noon on December 22 and the lowest path it takes during the year."  In any case, his obvious intent was to maximize the amount of sunlight in the greenhouse during the winter months, while minimizing it during the summer.  That being the case, it's a shame he, himself, didn't pay more attention to "the way the sun tracks across the sky" because his design is based on two completely false premises.

First, nowhere in the continental United States does the sun's path ever enter the northern sector of the sky.  To see the sun north of the zenith (the point directly overhead), you'd have to travel south of the Tropic of Cancer, which is about the latitude of Havana, Cuba.  (The northernmost point in the sun's travel through the sky is what determines the latitude of the Tropic of Cancer.)

Second, the noontime winter sun is never at an angle less than about 34º above the horizon in Dallas (see explanation below).  To find it as low as 22º, you'd have to travel to the latitude of Green Bay, Wisconsin, about 825 miles north of Dallas.  Thus, the interior of the "Dirt Doctor's" greenhouse will never be in full, midday sun, as he undoubtedly discovered in December of 1999.  But he continues to carry on this charade and has yet to publicly admit that he made an embarrassing (and costly) mistake.

His overall reasoning becomes even sillier when you realize that, even if he'd used the correct roof angle of 34º, the interior of his greenhouse would be fully lit by the sun only on the day of the winter solstice.  Wouldn't it have been more sensible to have used an angle of about 42º?  Then the interior would have received full sun from around Thanksgiving to near the end of January.

In any case, a competent landscape architect should know how to determine the varying angle of the sun (and the lengths of shadows) throughout the year.  Even the Druids and the Anasazi managed to figure that one out.  But maybe it shouldn't be expected from someone who wrote in the February, 1998, issue of The Dirt Doctor's Dirt, "My calendar declares that April 21 is the beginning of Spring."

Use your Back key to return from whence you came -- OR

If you're curious, here's how to figure the sun's highest point in the sky at the beginning of each of the four seasons (followed by a practical application for your own garden).

Because of the tilt of the earth, the noonday sun is directly overhead along the Tropic of Capricorn (south of the equator) on December 22, which is the winter solstice and the first day of winter.  It's directly overhead at the equator on September 22 and March 21, the first days of autumn and spring (the autumnal and vernal equinoxes), respectively.  And it's directly overhead along the Tropic of Cancer (north of the equator) on June 21, which is the summer solstice and the first day of summer.

According to data in a Rand McNally atlas, Dallas is at a latitude of 32º 47' or 32.78º north of the equator.  The Tropic of Cancer is 23.45º north of the equator.  And the Tropic of Capricorn is 23.45º south of the equator.  (Of course the equator is at 0º.)

Therefore, Dallas is 32.78 - 23.45 = 9.33º north of the Tropic of Cancer.  It's 32.78º north of the equator, and it's 32.78 + 23.45 = 56.23º north of the Tropic of Capricorn.

As a consequence, the sun in Dallas is 9.33º south of the zenith (the point directly overhead) at noon on the first day of summer.  And it's 90 - 9.33 = 80.67º above the southern horizon.  It's 32.78º south of the zenith and 90 - 32.78 = 57.22º above the horizon at noon on the first day of spring and again on the first day of fall.

And the sun reaches a maximum elevation of 56.23º south of the zenith, or 90 - 56.23 = 33.77º above the horizon, at noon on the first day of winter, the so-called winter solstice.

Practical Application: Competent landscape architects use this kind of knowledge in their work.  But even the average home gardener can use it to help solve the common problem of where to locate plants around fences and walls so that they receive the necessary amount of sun or shade.

For example, suppose you want to put a sun-loving plant, such as a rose, on the north side of a 6-foot, solid wooden fence.  How far away from the fence does it need to be in order to receive full, noon-day sunlight down to the soil line?

The formula, without any long-winded, boring explanation, is, as follows:

Distance = Barriar Height x Tangent of Lowest Zenith Angle

where, the Lowest Zenith Angle is how far south of the point directly overhead the noonday sun ever appears during the part of the year in question.  (Conveniently, the zenith angle at the two equinoxes is equal to your latitude.)

So, if the rose was to be in full sun at noon from March 21 through September 22,

Distance = 6 ft. x Tangent of 32.78º = (6)(0.64396) = 3.86 ft.

Therefore, in Dallas (or anywhere else at a latitude of about 33º) locate your roses at least 4 feet from the north side of a 6-foot wall or fence, and they should receive adequate sunlight.

On the other hand, to receive full noon-day sun throughout the year, a plant would have to be located

Distance = 6 ft. x Tangent of 56.23º = (6)(1.49548) = 8.97 ft.

or 9 feet away from the fence.

But how do I get the tangent of the zenith angle?  Easy.  There are all sorts of inexpensive hand-held calculators with trigonometric functions.  Borrow one from a student or ask for a demonstration of one at Radio Shack.  (You don't need to buy a calculator.  After all, you need only that one number.)  Or go to any library or book store and find the tangent in a book of math tables.  Or, if all else fails, you can get it with the math functions in your word processing program or from the Works spreadsheet.

Okay, but how do I find my latitude?  Also easy. Get it from an atlas, or call your local TV weatherman.  Or you can get it from a website such as this: http://www.realestate3d.com/gps/uslatlongdegmin.htm.

Now you can use your Back key to return from whence you came.