Environmental Factors 2

Daily and yearly rotations of the sun are important in planning landscape designs.

Not only do these variations contribute notably to climate and which plants grow in a region, but also both cycles, in combination with inanimate objects and large plants, shape microclimates.

Most importantly, they determine seasons and amounts of sun and shade in any given location. Watching and becoming familiar with the sun’s seasonal and daily positions in our locales will make better designers and plants people of us all.

It is easiest for me to visualize the sun’s yearly positions using an arc from the south horizon toward the north. The optimum time for observing the sun and becoming familiar with its patterns is solar noon, when a compass reading shows that the sun is directly south.

Here in the northern hemisphere, the sun’s position along this arc increases 1¬∞ in the sky every four days from December 21 to June 21; it then decreases 1¬∞ every four days from June 21 to December 21. The sun is lowest on this arc, and days are shortest, at the winter solstice (about December 21) and highest, with days the longest, at the summer solstice (about June 21). The equator is the one place on the globe where the sun is ever directly overhead and at the highest point on this arc. We see the sun at the equinoxes (about March 21 and September 21) as midway along this arc. The equator is also the one place where hours of dark (night) truly equal hours of light (day) at the equinox.

Daily, the sun seems to travel on an east (sunrise) to west (sunset) arc. At noon, the sun is at the midpoint of this east-west arc. When the yearly north-south arc and the daily east-west arch cross, we have high noon and shadows are the shortest. The one place in the world where the sun is directly overhead at noon is the equator and there are only tiny shadows or none at all.

The second part of this article shows how observations of the sun and its positions help me to decide on which plant groups belong in the tricky ranges of shade.

The introduction to this series of articles is found at Environmental Factors 1.

©

Text and photograph by Georgene A. Bramlage, [March, 2006]]. Reproduction without permission prohibited.