As we approach the summer solstice, it can become difficult to design to your yard's patterns of sun and shade: more specifically, it's very easy to over-estimate just how much sun a given site receives. I've certainly planted vegetable beds in "full sun" locations this time of year, only to find them in permashade as the equinox rolls on by.
If you're only planting tomatoes or other summer annuals, such an impending lack of sunlight may not be a big deal. But if you're designing a comprehensive landscape -- and particularly if you haven't lived at the site for a full year to see the change of light and seasons first-hand -- it pays to figure out when, and where, the light will retreat.
To the rescue comes the U.S. Navy, which kindly publishes a calculator to determine the altitude (angle above horizon) and azimuth (angle east of true north) of the sun at any minute of any given day. Combined with a shadow length calculator, this data should help you determine the boundaries of your shady areas throughout the year.
Thus, if you know that you live in a 20-foot tall house in Palo Alto, CA, you can calculate that on June 21 of this year (the summer solstice), your home will cast a 5-foot deep shadow at solar noon. However, on the winter solstice six months later, that shadow will extend about 36 feet!
It's most useful to calculate shadow points for the most massive things on your property -- usually structures like houses and detached garages, but also evergreen trees, fences, and neighboring buildings -- on the two solstice and two equinox dates. If you're really persnickety, you can forecast shadows not only at high noon, but also three or four hours on either side of noon, since "full sun" plants typically want 6-8 hours of direct sunlight per day (don't forget to shift the shadow angle to the west or east appropriately).
If you map all these points onto your site plan (I find using different color pencils for the different dates helps), you can get a pretty clear picture of where to plant what (or where to site that winter sun-pocket bench). Plus, you'll have a sense of what you might need in the way of lighting or heating throughout the year, and create separately-switched circuits based on what the darkest/coldest zones will be during the times of year you plan to be outside.
I know, it's all very geeky... yet somehow strangely exciting, no?
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