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I had noticed for several months ago that the light and shadow on an object can change very suddenly and dramatically. This phenomenon was especially noticeable around 1:30 in the afternoon. I figured that the sun must shift from the east side of the sky to the west side of the sky around 1:30. My solution was simply to plan for a lunch break around 1:30. Everything worked fine until yesterday, when I was working in the morning and experienced the same sudden shift in lighting. I decided it was time to learn more about exactly how the sun tracks across the sky.

early morning
photo

late morning
photo

Here is an example of what I encountered. I took the first photo at the beginning of the painting session, around 9:00 a.m. I took the second photo about two hours later. The switch from the lighting conditions of the first photo to the lighting of the second photo happened very suddenly a little after 10 a.m.

When I got home, I found a utility at the U. S. Naval Observatory that will calculate the altitude and azimuth of the sun at specified intervals for any location on any day. This is a very handy tool for learning how to predict shifts in outdoor lighting. Later in the day, I returned to the cemetery with compass and an azimuth table in hand and realized that the tables confirmed what I had observed while painting.

Working outdoors in the summer, there are 4 ways an object can be lighted. Assuming that a building is on a north/south axis*, the lighting changes when the azimuth of sun reaches 90 degrees, 180 degrees, and 270 degrees. In the winter and early spring, the sun crosses the 90 and 270 degree thresholds before sunrise or after sunset. Here are a few illustrations of these phases of light:

Sunrise to 90 degrees

The north and east sides of a building are in sunlight. In early July in Atlanta, this stage lasts from sunrise to 10:20 a.m.

90 degrees to 180 degrees

The east and south sides of a building are in sunlight. This stage lasts from 10:20 a.m. to 1:40 p.m.

180 degrees to 270 degrees

The south and west sides are in sunlight. This stage lasts from 1:40 p.m. to 5:10 p.m.

270 degrees until sunset

The west and north sides are in sunlight. This stage lasts from 5:10 p.m. until sunset.

* – Note: If the building is not on a north/south axis, it is still easy to calculate when the light will change. Simply figure out how many degrees the building is oriented from north, and then add or subtract that interval to the light shift intervals. For example, the north side of my house is actually about 26 degrees west of north. To figure out when there will be a major light shift, I subtract 26 from 90, 180, and 270 respectively, and then read the azimuth table. (return to text)