About Sun Angles...
Welcome to the Sun Angles. Explore solar energy the easy way with our star product, a clever tool called a heliodon (HEE-leo-don).
Demonstrating the portable heliodon.
- The earth is always “full”, like the full moon.
- Half the ball of the earth is always illuminated.
- The solar radiation the earth intercepts can be viewed as parallel rays of energy.
- The earth perpetually intercepts a column of parallel solar rays.
To see how a ball deals with parallel rays, push a small ball like a golf ball half way into a column of dry spaghetti. The angle any individual spaghetti ray makes to a site on the ball depends on the site’s location within the column—at the edge, at the center, or in between. The ray at the center of the ball strikes its surface at 90 degrees. The center ray is perpendicular to the ball. The rays at the edges are parallel to the surface of the ball. They barely graze its surface. The angle between the ray and the ball determines the intensity of the direct sun on the surface--or would if there were no atmosphere.
Latitude, time of day, and date determine your site’s location on the earth ball within the current column of parallel solar rays. “Latitude” locates the site on the north-south curve of the earth between the Equator and one of the Poles. “Time of day” (or solar hour angle) locates the site as it turns east around the axis from sunrise to sunset. “Date” (or solar declination) locates the whole earth on its annual orbit as an astronomical angle.
“Date” (solar declination) also names the latitude at the center of the solar beam. The center of the solar beam moves almost 50 degrees of latitude over the surface of the earth during the year as the date changes. The center beam glides from the Tropic of Capricorn to the Tropic of Cancer and back again, visiting each latitude between the Tropics twice in the year. The rest of the beam glides with it. This is the movement that changes the seasons in the temperate zones.
Earthrise over the realm of the moon.
Photo: NASA 1968
The change takes place because the earth’s axis is both tilted and "fixed", pointing always to the North and South Celestial Poles. A good globe shows the earth’s tilt. You can use a globe to show yourself the effect of the "fixed" axis. Draw a line across the center of a round table from edge to edge. Put the globe at one end of the line with the globe's curved frame parallel to the line, South Pole toward the center of the table. If you squint along the line on the table from its other end, centering your eye on the Tropic of Capricorn, you will see what the sun sees at the December solstice. To see what a day looks like at that solstice, have someone carefully turn the earth eastward (New York chases London) a full turn. To see the June solstice, push the globe straight down the line to the other side of the table without turning the globe's frame. Squint from the far end of your line at the Tropic of Cancer. Have someone turn the earth eastward again. All possible positions of the earth on its orbit can be represented around the edge of the circle. In every position the globe’s axis should be parallel to the line you have drawn. The frame should always be parallel to the original line with the South Pole pointing in the original direction.
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