SOLAR ANGLES:

In clear daylight, the sun can deliver about 310 BTUs per hour to each square foot of surface.  For the surface to absorb or, in the case of a window, to transmit that much energy, conditions would have to be perfect.   It would have to face the sun directly, at right angles to its parallel rays.

For example, for most of the US, the solar energy absorbed by your garden in June is about 25% higher than in April, because the June sun is higher and it rays are closer to being at right angles to the earth's surface.   In April, with a lower sun, each square foot of ground looks narrower  than in June.

In the chart below you can see that, on a July noon in the more northerly states, the sun is 74o  above the horizon, and a simple roof extension will keep it out of the house.   In December at noon, however, the sun is only 27o high, and it will pour in through south-facing windows.

                                   

Date Solar Time Solar Position
am pm altitude azimuth
8 4 22 70
Sept 21 10 2 33 57
12 12 50 0
8 4 6 53
Dec 21 10 2 21 29
12 12 27 0
8 4 22 70
Mar 21 10 2 42 42
12 12 50 0
8 4 37 91
Jun 21 10 2 60 66
12 12 74 0

For a partial understanding of why those solar angles change so much:

The North-South axis around which the earth spins in a day is cocked at an angle to its annual circle around the sun.

In the diagram below, note that the earth's axis of rotation is always pointed toward one point in the sky, the North star, regardless of what side of the sun we are on.  

As a result, the earth is pointed somewhat "toward" the sun for six months, and "away" for the other six. The sun will be much higher in the sky during those months of extra sunlight than it will be when the axis of spin is angled away from the sun.

It is this two-faced aspect of solar angles that permits us to design a house that will let the sun in during the cold days of winter, and keep the sun out during the over-heated days of summer.

    

In June, any point in the Northern hemisphere can “see” the sun more than half of the day.  In December, the Northern hemisphere has its back partly turned to the sun, shortening the days.       

The chapter on Design pointed out the importance of aiming the long wall of the house toward the South.   If possible, it is best to aim that wall at “Solar” South, the point at which the sun will be at the center of its daily arc, and at its highest point.

For a given location, this mid-point will not change with the seasons, or with "Daylight Saving Time".   However, that mid-point can be as much as an hour away from "12:00 Noon" by the town clock.

To find that mid-point (Solar Noon), check the Weather section of the local newspaper for the times of sunrise and sunset.  Figure out what your clock will say midway between sunrise and sunset.  It could be anywhere from 11:00 to 1:00.   At that mid-point time, set a stake in the ground to cast a shadow, or use the shadow of a tree.   That designates the earth’s true North/South line, parallel to the centerline around which the earth rotates.   It won’t change (unless we are hit by an asteroid).  Lay out the line for the building’s long wall at right angles to that shadow.

In the Setting chapter, it was pointed out that trees are great everywhere, except close to the house in the narrow  through which the sun shines in winter.