Sunlight analysis

March 25, 2006 7:10 AM

Is there a way, possibly using Google maps or some other app, to determine the path of the sun over my property for the coming summer months? Specifically, I am trying to determine if a particular region near some oaks will get enough sunlight for a small vegetable garden.
posted by docpops to home & garden (12 comments total)

Docpops, I don't know of an app that will do it for you, but I can tell you that with a little bit of work, you can figure this out by hand. How do I know? Because I did the same thing last year. We wanted to start a vegetable garden on our property line, up against a fence where the neighbors have some tall arborvitae. Since you live a mere 37 miles from me, I'm relatively certain some of the same assumptions that I made for my experiment apply to your situation.

Essentially, I reasoned that since we live near the 45th parallel, and because the sun varies 23 degrees from solstice to solstice, then it sits 22 degrees above the horizon at the winter solstice and 68 degrees above the horizaon at the summer solstice. (I admit that these assumptions could be wrong, but I don't think they are.) You can find charts indicating the sun's path over the year, and from this extrapolate the approximate elevation of the sun on various days.

Next you need to determine the approximate height of your oaks. After you've done this, you can calculate theoretical shadow lengths, etc.

This might sound complicated, but it's not very. I did it in an hour one day last spring, and my calculations were pretty darn accurate. (I checked them throughout the summer.)

I'll check today to see if I still have my notes and data. If I do, I'll e-mail them to you. As I say, since you and I essentially live in the same place, many of my assumptions should apply to your circumstance, too.
posted by jdroth at 7:38 AM on March 25, 2006

jdroth - thanks. I'd love to see more of your calculations. There are a few truly perfect locations on the land that have more to do with proximity to the house than possible sunshine. Oregon summers are so freaking sun-intense as well that I sometimes think even semi-shaded areas do OK. My main concern is I am on a hill that slopes down to the north. I'm curious, since you mentioned in your blog that there were some Arbor Vitaes shading the garden, did you get a good crop?
posted by docpops at 7:55 AM on March 25, 2006

I haven't used this particular software, but it's a consumer grade solution made by the same people who make the very slick Chief Architect. It might do the trick.

Alternatively, maybe you can model your site with another nice program, Sketchup. I've done some time lapsed animations of how the sun traces shadows troughout the day.
posted by tfmm at 7:59 AM on March 25, 2006

Docpops, I'll check my garden notebook when I go outside. (It looks like we have a sunbreak in which to do some yard work.) I'll send you any notes I find.

I'm curious, since you mentioned in your blog that there were some Arbor Vitaes shading the garden, did you get a good crop?

Yes and no. We got exactly what we expected, actually. The stuff we planted up against the fence didn't do well because of a combination of too much shade (which we had calculated) and being too close to the arborvitae plants. We had to tear out a six food wide swath of sod a couple weeks ago to give us more useable garden space. (And we've got tomatoes under grow lights just itching to be planted in the newly tilled soil.)

Ah, spring. It's good to watch things grow.
posted by jdroth at 8:12 AM on March 25, 2006

Well, my wife tells me we chucked all my careful notes when we decided to just physically expand the garden. So, I've tried to do some on-line research for you. It looks like this solar concepts page probably has all the calculations you need, but the page is a little overwhelming. I also found two solar calculators: the Grok solar calculator is AWESOME. I didn't try this SunPath applet, but you might. I'm going to go play with the solar calculator...
posted by jdroth at 8:59 AM on March 25, 2006

There was a similar question not long ago, which may add some light.
posted by Kirth Gerson at 9:11 AM on March 25, 2006

Thanks everyone.
posted by docpops at 9:16 AM on March 25, 2006

Okay, so here's my recommendation: use the solar calculator that I linked to above (and which I've been playing with for the past fifteen minutes -- it rocks). Enter your longitude and lattitude (which should be available via your Mefi user page, if nowhere else). Choose -8 for the time offset. Ask for the data to start on January 1st 2006 and end on December 31st 2006, and to be generated at one day intervals. (You can choose smaller intervals, but then you end up with a lot of data. Once you get used to the app, you can decide what date range and time range you're interested in.) Generate the numbers.

The resulting data doesn't compensate for daylight savings, of course, but I don't think that's too important. You'll get a rough idea of what you want even without adjusting. In the tables that the app generates, the number you want is "apparent solar altitude", which tells you how far above the horizon the sun is at any given time.

All of this, of course, only really gives you data about the sun, and doesn't tell you how much light your garden spot is going to get. For that, you'll have to apply some trig, probably, which (for me) can be difficult. But still, I think this sun calculator will give you a good start.

One last thing: I should mention that the best way to determine whether a spot is good for a garden is just to try it. If you have a small yard, that can be problematic, because you don't want to just go digging up patches of dirt, etc. every year. If you have a big yard, it's probably no big deal. If you don't want to dig, you can always just spend a year watching the play of sun on that particular patch of ground. Last year was our first summer in this new house, so we picked a spot for the vegetable garden, worked out the aforementioned sun chart, and gave it a go. We learned a lot, and were able to make adjustments so that this year we should have a kick-ass vegetable patch.

Good luck!
posted by jdroth at 9:19 AM on March 25, 2006

I'd skip the calculations and borrow a Solar Pathfinder. In California, I think some energy companies like PG&E will lend out tools like this.

It's a dome mounted on a tripod. You take it out to the garden. It's shiny, so you'll see your oaks reflected in it. It has months & dates on it, so by seeing where the reflection of the oaks falls on the time/date chart, you can see during what times of day and what times of year the garden area will be shaded.
posted by salvia at 10:42 AM on March 25, 2006

Another tool is Sun path chart program at U of Oregon. Also this PDF explain some details. I have used the Solar pathfinder and it is great , however a bit expensive for a homeowner who only needs to do it once (per property).
Salvia is right that Pg&e has some loners, however last i heard, only in San Francisco.
posted by blink_left at 6:47 PM on March 25, 2006

OK, I checked out the Sun Path Chart program - very cool. I feel like a twit, though. Say I'm facing due east with the help of my cheap REI compass. How in hell do I figure out a 10 degree elevation and a 120 degree azimuth? Do I need a sextant? If I'm using a sextant, should I be in full nautical regalia?
posted by docpops at 8:09 PM on March 25, 2006

Get yourself some astronomy software. I use Starrynight Backyard (I am sure there is a demo here). You set where you are at, what way you are looking, and the date, and press run. The stars will appear and the then the sun will come up and cross the sky. There are even little trees showing the shadows. Plus it will help you with your moonplanting. OK, I have gone weird.
posted by priorpark17 at 10:08 PM on March 26, 2006