Using Sun Path Charts

There are a few web sites that calculate sun path charts when given the latitude and longitude of a point on earth. One of the sites is www.sunearthtools.com. It generates a report that you can download as a PDF or view online.

I think the cartesian plots are easier to understand than the polar plot for northern and southern temperate latitudes, so I'll explain that.

The plot shows the apparent paths of the sun across the sky from a point on the earth throughout the solar year. The path traces from left--representing the position of the sunrise--to the right, representing the position of the sunset. Note that for us, the sun's zenith is at about 1:30 PM in the spring and summer months. (we're pretty far west in the Eastern US time zone. In New York City, the zenith is at around 1PM. In Maine it's 12:30PM.) The shape of the paths vary by north-south position. (We're in northern Ohio around 42 degrees north of the equator.)

The uppermost line is the path on the date of the summer solstice (June 21 2017). The lowest arc is the winter solstice (December 21, 2017). The intermediate lines are the paths on two dates, one on the lightening side of the solar year, and one on the darkening side of the solar year. The center line is the spring and fall equinox--note that it runs directly through the east and west cardinal points. The gold line is the day the chart was generated.

The figure 8 is the "analemma". The analemma represents the position of the sun in the sky at the same time of day throughout the year.

Summer Solstice Sunset

The pictures at the left were taken at Observatory Park in Montville, Ohio on the solstices and spring equinox. You can see the compass rose inlaid in the brickwork, and the solstice lines. If you look at the chart, above, the summer solstice sun set is around 300 degrees Azimuth.

0 degrees is North
90 degrees is East
180 degrees is South
270 degrees is West

The summer solstice sets about 30 degrees north of west. You can see that's the case on the photo at the left. The summer solstice sun rises about 30 degrees north of east.

Winter Solstice Sunset

You can see the winter solstice sunset is south of west. On the chart above, it's about 30 degrees south of west, which you can see on the pavement in the photo.

Finally, the equinox sunsets are due west in the spring and fall.

Spring Equinox Sunset

One application of these charts is determining which parts of your yard will be in shadow or sun during the day. How can you do that?

Go out in your yard at some location and face due south. (you can use a compass app on a phone or a real compass for this purpose). Look at the chart at the elevation of the sun on the summer solstice. Point your arm up at that angle. For us, the solstice zenith is about 72 degrees above the horizon. If your phone has an inclinometer app, you can use that to find the angle, or just estimate using your arm. If you're pointing at a tree, you're in the shade. If it's the sky, you're in a sunny spot on that day. You can then repeat the procedure at the zenith -4 hours, and +4 hours positions, and you'll have a really good idea of how sunny that spot is.

If there are some trees in the way, depending on what the species are, the area might still be partially sunny. Locust trees, for example, allow a significant amount of light to reach through their leaves, spindly conifer trees, like scotch pine also allow quite a bit of light through their branches sometimes.