HOW TO CHART THE SUN’S MOTION IN RELATION TO YOUR HOMESTEAD
THE SUN’S PATH ACROSS THE SKY
Let’s try to perceive this planetary motion from a location on the earth’s surface. Imagine the sky as a huge hemispherical dome above our head and the sun moving on the inner surface of this dome. Let’s draw the sun’s path on summer solstice and winter solstice. As can be clearly seen the two paths are different, with it been longer and higher in the sky in summer and shorter and lower in the sky during winter. The longer the path, longer is the duration of the sun’s stay in the sky. Throughout the year, the sun’s path keeps shifting up and down between these two extremes.
CHARTING THE SUN’S LOCATION AT ANY TIME IN A DAY
With the help of these two arc lines, we can chart the sun’s position at any time in a day, during the year, at a given location on the earth’s surface. On December 21st, 9.00 a.m., the sun has risen in the east and is low in the southern sky. On June 21st, at the same time of the day, it is higher in the sky. If we draw a line on the sky dome between these two 9 a.m. positions, we can exactly pin point the sun’s location at 9 a.m. during the rest of the year. As the sun’s path moves up and down through the seasons, the sun’s position at 9 a.m. will always be somewhere on this line.
In a similar way, a line can be drawn for the sun’s position at 3 p.m. throughout the year. We now have a rectangle on the sky dome, showing the sun’s position between 9.00 in the morning and 3.00 in the afternoon throughout the year. Part of the summer solstice arc and winter solstice arc along with the 9.00 a.m. and 3.00 p.m. arc’s make the four sides of the rectangle. The area inside this rectangle is called the solar window for a given location.
This solar window is very important because, it is between 9.00 a.m. and 3.00 p.m. that we receive the maximum energy from the sun. To harness the maximum possible energy at a given geographic location, the solar window should be clear and not be shaded by trees or any other obstacle between 9.00 a.m. and 3.00 p.m., during most part of the year. It is for this reason that, before installing any solar device, one should analyze the installation site’s solar window and make sure that, there aren’t any shading obstacles.
MEASURING THE SUN’S POSITION
But to actually chart the boundaries of a solar window for a real geographic location, one needs to know, how to measure the sun’s position in the sky. To pin-point the sun’s position on the sky dome, we need to make two measurements.
The first measurement is the sun’s direction on a compass. A straight line drawn from the sun to the horizon intersects a specific degree on a compass. This angle is a measure of the sun’s azimuth or elevation. The second measurement is the sun’s altitude or vertical angle. The combination of azimuth and altitude describes a specific spot on the sky dome.
CHARTING THE SUN’S PATH
Using these two measurements of solar azimuth and solar elevation, a chart can be drawn which will show how the sun’s position changes throughout the year. This chart is called the sun path chart and it shows how the sun’s path looks like from the earth’s surface.
The images show the sun path chart for San Francisco, California. It shows what we would see, if we look towards the South Pole from this location. The y-axis represents the altitude or solar elevation, measured from 0° degrees on the horizon to 90° degrees directly overhead. The x-axis represents the sun’s direction on the compass, the solar azimuth. The center of chart is 180° or due south. The intersection of the sun’s altitude and azimuth on the graph shows the sun’s position in the sky.
The chart can be overlaid with the locations of shading obstacles like trees or buildings. Further, one can overlay the sun’s paths of the shortest and longest day of the year. On top of that, the position lines of the hours of the day can also be graphed. This gives the solar window – the area of the sky when the sun would be between 9.00 a.m. and 3.00 p.m., throughout the entire year.
Lastly, the five monthly sun paths for the rest of the year can also be included. There are only five paths as the sun’s path is duplicated as it moves up and down through the seasons. For example, its path on March 21st is the same as its path on September 21st.
Now we can see where the sun would be shaded during the year. As an example of how to make use of this chart, notice the lowest sun path, the path for Dec 21st. Between 9.00 and 10.00 a.m. on this day, the sun would be shaded by a tree which stands between 135° and 150° degrees on the compass.