Image source: NREL.gov
Understanding the sun’s motion relative to a site is an important aspect of a good permaculture design, as various elements like plants, animals and solar devices depend on sun for their functionality. A good appreciation of the earth’s rotation about its axis, its revolution around the sun and the consequences of these motions on the sun’s position and availability at a given location on the earth’s surface, is essential for maximizing plant productivity, harnessing maximum energy, minimizing energy usage and maintaining a comfortable indoor environment for humans and animals alike.
For proper functioning and productivity gardens, greenhouses and orchards should be placed at specific locations based on the adequate availability of sunshine for the parts of the year in which the plants are growing or fruiting. Other elements of a permaculture design like solar panels are also needed to be placed thoughtfully to obtain maximum exposure to the sun to harvest as much energy as possible. Houses outside the tropics need to be designed/oriented to make more winter sunlight enter the living spaces, while for those in the tropics, they should be designed/oriented to have more shade and cooler air entering the living spaces. Even animals can derive benefits from thoughtful solar designs, as they too like to have winter warmth and summer shade. Beehives, chickens, fish ponds and livestock, all appreciate sunshine in winter and shade in summer.
For a given site, if we can create a chart that shows the areas of year round shade and year round sunshine, one can plan and design the various elements for optimal functioning. Meaning, we need to chart the sun’s position throughout the year. But how to find the sun motion relative to a site?
Let’s begin by trying to understand the earth’s rotation about its axis, its orbit around the sun and the four seasons.
The four seasons
We all know that summers are hotter and winters are colder. Also, when it is summer in the northern hemisphere, it is winter in the southern hemisphere. But why is that?
The summers are hotter because, the sun’s path is higher in the sky. This makes the days longer and it makes the summer sun more intense. To understand it more clearly, let’s do a simple demonstration using a flash light.
In a darkened room, switch on the flash light on a surface, at a direct 90° degree angle. Make note of the size of the lit area. Now, slowly reduce the angle to make it less direct. You will now observe that the size of the lit area has increased, but the intensity or brightness of it has reduced. What it implies is that, direct 90° degree angle provides more intense light than the inclined one. The same thing happens with the sun. Higher it is in the sky, the more direct and intense the sunlight would be.
Now, this brings us to the next question – why is the sun higher in the sky in summer? The answer lies in the earth’s rotation about its tilted axis and it orbit around the sun. The earth rotates about its own axis, titled at an angle of 23.5° degrees to its orbital plane and at the same time, travels around the sun in a huge circular path through space.
During summer, the North Pole is tilted towards the sun. As a consequence, the sun’s path is higher in the sky, causing the northern hemisphere to receive more light and heat. Around June 21st, the northern hemisphere is tilted the most towards the sun and is called as the Summer Solstice. On this day, which can be referred as the first day of summer, the sun’s path is higher in the sky than it is on any other day in the year. In addition, because the sun is in the sky for more hours, the summer solstice is also the longest day in the year. These extra hours of sunlight gives the sun more time to heat the earth and this is the main reason for summer to be the hottest season.
As the earth continues its orbit around the sun, it reaches a point where its tilt is sideways to the sun. This is called as the Autumnal Equinox, where both the day and the night are of equal length with 12 hours each.
Continuing further in its orbital path around the sun, the earth reaches the other side of the sun, with the northern hemisphere tilted farthest away from the sun. Now, the sun’s path is lower in the sky, causing the northern hemisphere to receive less light and heat. This makes the days shorter and colder. The shortest day in this period is called the Winter Solstice.
As the earth revolves back towards the summer, it passes through another point where the axis is tilted sideways to the sun. Once again, day and night are of the same length. This day is called as the Vernal Equinox.
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.