To properly evaluate the solar potential for your greenhouse site, you need to know where the sun is (and isn't) in the winter. To find where the sun will be you'll need: a compass, a protractor, 8-12" of heavy thread, and a little washer or other kind of small weight.

Before you begin to evaluate your site you need to know that south on a compass does NOT point to true south. Solar principles operate based on true south. In Cochise County true south is about 12 degrees EAST of south on your compass. So, when you find south, turn to the east 12 degrees and you'll be facing true south. A good solar green house should face as close to true south as possible, probably no more than 30 degrees off either direction.

If the south face of your future greenhouse isn't close to true south, it's not impossible to design one that will work, but the design will be far different from what will be discussed in this column.

The latitude of Cochise County is approximately 32 degrees north. Our latitude affects where the sun rises and sets throughout the year as well as the sun's altitude in the sky. We know days are longer and that the sun climbs higher into the sky in the summer. In winter, days are shorter and the sun is much lower in the sky. This is because of the angle of our earth on its axis.

This axis angle also causes the sun to rise and set south of true east and west in the winter but far north of east and west in the summer. At winter solstice, the sun rises 60 degrees east of true south and sets 60 degrees to the west. The sun rises about 7:00 and sets about 5:00, climbing only to an altitude of 32 degrees at noontime. This "window" of sun shine needs to be as clear as possible so that few shadows will fall on the south side of the greenhouse between the hours of 9 and 3:00.

Standing at what will be the south side of your greenhouse, use your compass to mark off angles to represent the position of the sun throughout the day. The angles are: 60 degrees east (7:00am), 50 degrees (8:00), 40 degrees (10:00), 15 degrees (11:00), and true south at noon. At 1:00 the sun is about 15 degrees west, about 30 degrees at 2:00, 40 degrees at 3:00, 50 degrees at 4:00 and setting around 5:00 at 60 degrees west. When you're done making marks, you'll have a little fan type shape in the dirt. Label each mark with the time of day it represents. (See Figure 1.)

You will use the protractor, thread and weight to check whether the sun will clear nearby houses and trees at different hours throughout the day. Tie your weight to one end of the thread. Put the other end through the little hole in the rounded side of the protractor and tie it. Holding the pro tractor with the flat side up and the weight hanging under the curved side, you have a tool to measure altitude angles.

Squat down in front of the fan you made in the dirt, low to the ground, near where the bottom of your greenhouse windows will be. Looking along the line going south, tilt the protractor until the weighted string hangs along the 32? mark on the protractor as in Figure 2. If nothing rises above this sight you'll get sun on your greenhouse at noon.

Use the same method to check the sun's altitude at the other times of day. The altitudes to check for are: 10 degrees at 8:00 and 4:00; 20 degrees at 9:00 and 3:00; 26 degrees at 10:00 and 2:00; and 30 degrees at 11:00 and 1:00. If nothing, or at least very little, blocks the sun's access at these times, you have plenty of sun for your greenhouse. Congratulations! If you find problems with shade, there are ways to compensate. Call the Cochise County Extension Office to find either Cathd Fish or Emilie Vardaman in Bisbee, and we can answer some of your questions. Next month: sizes and shapes of green houses, plus plants that love to live in them.