Measure sunlight in yard to determine new garden spot

August 23, 2008 7:12 AM

Is there a way to measure how much sunlight different parts of my yard get over the course of a day?

For comparisons sake (not an absolute number) to determine where the best spot for growing vegetables is, which part of the yard gets the most sunlight over the course of a day. There are a lot of trees and so the sun comes and goes often, depending on where in the yard, and time of day as the sun moves.

posted by stbalbach to home & garden (14 comments total)

A sunlight meter will do the trick -- the one I'm familiar with is called a SunCalc. It works.
posted by vers at 7:22 AM on August 23, 2008

Put your camera on a tripod and select a view that covers your yard, set a fixed exposure. Place pieces of white or gray paper where you think a garden might go. Take a picture once an hour, sunrise to sunset.

In Photoshop, take the first picture, blend in %50 of the second, to that add 33% of the third, 25% of the forth, 20% of the fifth, 16.6% of the sixth and so on.

Blur the result a bit to remove noise, and use the eyedropper to read out the values for each location.
posted by StickyCarpet at 7:27 AM on August 23, 2008

They do make a gizmo for this purpose.
posted by weapons-grade pandemonium at 8:05 AM on August 23, 2008

...as vers pointed out.
posted by weapons-grade pandemonium at 8:06 AM on August 23, 2008

Something worth checking on: one of my local garden centers will let you borrow a SunCalc for free (you put down a deposit of the purchase price, they refund it if you bring it back).

You can do the camera thing (or a webcam) and just look at it to see what parts of the yard get the best sun all day. One thing you'll have to mentally adjust for is the angle of spring sun, which is going to be a little different.
posted by Lyn Never at 8:08 AM on August 23, 2008

Not an overly economical suggestion, but Campbell Stokes Recorders are used by many meteorological offices to records sunlight levels accurately.

And they're also possibly the closest thing there is to weather instrument porn . . .
posted by protorp at 8:56 AM on August 23, 2008

I've attempted to make similar calculations in my own garden. The first thing to realize is that, yes, as you say, the sun moves. It doesn't just move throughout the day, but throughout the year. So, making observations today will yield different results than making observations next week or next month.

Depending on your timeline, I recommend making observations near your last frost date, at midsummer, and near your first frost date. These will give you some idea of the variation in sunlight on your chosen spot. You can actually Do Math to get some approximations, figuring the altitude of the sun, the height of the trees, etc. But it's easier just to look.

The SunCalc thing looks cool, but it costs $30.

My main point here is that a single day may not give you the data you need because the elevation of the sun changes over the course of the year, as does its time at rising and setting.
posted by jdroth at 9:20 AM on August 23, 2008

There is also some really awesome instrument for this -- I spent awhile looking for the name, to no avail, but hopefully someone else will know what this is called.

It's a reflective half-dome that sits on a tripod. You have to level it carefully. Then, you can look in it to see what it reflects (eg, treetops; blue sky). You can then put a solar path chart on top (?) of it and, by looking at what parts of the chart are covered by those treetops, you can know what months those trees will be shading that space. In other words, it's like that gizmo linked above, but to know the answer for the entire year, not just a given moment in time.

Anyhow, it's an awesome thing that I believe costs at least a couple hundred dollars, but if anyone out there knows the name of it, you could potentially borrow one from a local library or solar center or landscape architecture grad school or ag extension or something.
posted by salvia at 12:54 PM on August 23, 2008

I think what salvia is trying to think of is the Solar Pathfinder. I use one to determine the best place to put greenhouses, solar collectors, etc. Works great, but as he said they're a couple of hundred dollars. They're designed to show exposure throughout the year; I doubt that you really care how much sun your garden gets in the middle of the winter, but I could be wrong. You might be able to find a place that will loan/rent you one, though.
posted by Death by Ugabooga at 4:32 PM on August 23, 2008 [1 favorite has favorites]

How about this off-the-cuff low-tech solution?

Buy a bunch of paper cups and a big bag of unpopped popcorn. The night before your test, cover the lawn in paper cups in a grid of whatever resolution you want [every two feet?]. Take the next day off.

At your start time, walk around the yard and drop a kernel of corn in each cup that's in what you consider to be "sufficient" sun. Repeat every half hour, or hour.

At the end of the day, walk the yard and record the kernel counts on a piece of graph paper. There you go. The parts of your yard that get the most light should be apparent.

Repeat as desired over the course of the year. You could use lettered slips of paper or colored popcorn to add time to your data[where is the sun at noon?, etc.].
posted by chazlarson at 7:59 PM on August 23, 2008

The popcorn solution is clever, but very susceptible to squirrel interference.

Similar but different solution: fill the cups with water and leave them alone for the day. Check how much water is left at nightfall. More sun == more heat == more evaporation == less water.
posted by tkolar at 8:36 PM on August 23, 2008

You could use small nails [or hex nuts, or marbles] to eliminate the squirrel factor. Popcorn just came to mind because it was a small hard thing you can buy thousands of for a buck or two. I kind of imagined the squirrels would be kept at bay by the person walking through the yard all day.

Actually, the first version I thought of was to use dowels and spring clothespins. Cover your yard with a 2-foot grid of dowels. Every half-hour, go clip a clothespin to the dowels that are in the sun. Count the clothespins at the end of the day. You could mark the dowels off by time and clip the clothespins on at the appropriate spot to track that dimension as well.

On first blush that seemed more wasteful, though, since you probably wouldn't have a use for all those dowels and clothespins after the sun-tracking effort. If you did it repeatedly over the course of the year, though, it might make sense to use the Clothespin Sun Tracker.
posted by chazlarson at 8:54 PM on August 23, 2008

Death by Ugabooga, that's it! Thanks. (This corn clothespin nails water discussion is really interesting and probably a better solution.)
posted by salvia at 11:25 AM on August 24, 2008

pennies instead of popcorn? a small hard object that will weigh the cups down, and you probably already have a big jar of them somewhere.
posted by twistofrhyme at 12:37 PM on August 24, 2008