Friday, June 7, 2013

The Sun and the Garden

 Obviously the Sun plays a most significant role in the success of a garden. We chose a spot out back where we pretty much have unimpeded exposure from 8 in the morning until 7 at night. It changes a bit as the summer rolls on - more shade from the one close tree as the Sun dips lower, longer shadows in the fall when it falls below the mesa earlier. But in general we are guaranteed the 6+ hours that sun loving plants purportedly crave.

I doubt many of us really consider where the Sun happens to be in the sky at any given time in the year. We know it's low in the winter, particularly if your southern exposure is ringed with trees. We know it's high in the summer, baking our brains when we get out of the car at midday. But generally, the Sun is "just there." While that true, the real story is far more compelling.

Last June I decided to shoot an Analemma, a multi-shot project that traces the path of the Sun through the sky. The word comes from the Greek ἀνάλημμα which translates as "pedestal of a sundial." Using conventional cameras, it is extremely difficult to do because it means leaving your camera hard-mounted outside somewhere and taking a photo 24-50  times on the same frame of film using a solar filter. It's been done a dozen or so times, the first by a gentleman named Dennis DiCicco between 1978 and 1979. When using film, the roll is developed at the end of the year and then printed on some interesting background that was not part of the original shot (Greek hills with temples on them are very popular choices.)

The advent of digital photography greatly simplified the project however, requiring only your willingness to dedicate a camera and tripod for a year, some time positioning the shot twice a month and a copy of Photoshop. The process I used was pretty simple - I began with a nice morning photo of my backyard with no sun in the frame of reference that would serve as my baseline. Then on the 6th and 21st of each month I went out, aligned the camera to the top of a fence post and the eave of my bike shop and took a picture at precisely 12:18:32 (I messed up and lost 18 minutes during my set-up for the first Sun picture.) From there, I loaded them into the computer, overlaid the latest photo on the baseline photo, carefully aligned my two reference points and cut the center out of the Sun and then pasted it "in place" onto the baseline and then deleted the now useless bi-monthly shot. The result after 28 photos and a year of work is this -

You can see a couple of interesting things here. The crossover does not take place on a significant date, like an Equinox or Solstice. The fact that one of the dates is My Lovely Wife's birthday (April 2) is probably cosmologically significant but how I am not sure. Another is the size of the upper and lower node. Because I live at latitude 35° North, the Sun is never directly overhead going no higher than 78°. At the Equator where the Sun reaches 90°, the Analemma would be directly overhead and equal between top and bottom. In the Southern Hemisphere, at my equivalent latitude, it would be the same shape but with the smaller node on the bottom. The difference in shape is due to the tilt of the Earth and how that affects our virtual position on the planet, relative to the Sun.

There are two components to the Earth's journey around the Sun that cause this Figure 8 to be formed. The first is the tilt of the Earth's axis (23.4 °) relative to the Sun and the second is the elliptical shape of our orbit. If the Earth stood straight up and down and orbited the Sun in a perfect circle, and you went out each day at noon and took a picture, the resulting Analemma would look like this -

 24 Suns superimposed on each other and honestly quite boring.

Now we all know that the Sun moves up and down the sky during the year, lower in the winter, close to overhead at the height of summer and this is due to the tilt. It's what changes the lengths of our days, gives us our seasons, creates temperate zones and allows for ice at the poles. As the Earth moves through its orbit, your position on the globe effectively changes and that is where we get Solstices and Equinoxes and all the variation in between. If the Earth orbited the Sun in a perfect circle but had its tilted axis, the resulting Analemma would look like this -

 The path of the Sun would be vertical from high to low, up and down from a central point.

The effect of the second component - orbit  - would not be as obvious if you simply went outside and looked at the Sun at noon everyday. In fact without a photo, you probably wouldn't notice it at all. This effect manifests itself in a difference in the Sun's position relative to the time on your watch - instead of being in the same place, it's going to be a bit further ahead or behind due to the Earth moving more quickly through the pointy ends of its elliptical journey. Three photos during the year  from an Earth without a tilted axis but with an elliptical orbit would look like this -

The Sun would lag or shoot out ahead of a centered point depending on where the Earth happened to be in its yearly trek through outer space.

But because we are blessed with both components, we end up with a habitable planet and a cool shape in the sky if we take the time to capture it.
The work was pretty interesting in the end. In addition to the lesson in celestial mechanics, I also learned something that I think I already knew - New Mexico has some pretty nice weather. Of all the photos I took, only a few had cloudy skies. And even then, they were not cloudy enough to stop my progress. Here are the raw shots presented in a grid -

And as a added bonus, I was able to create a video like the one I did above of the seasons. It shows trusty Sol making his way up and down and around, this year, next year and for the rest of the time he hangs in the sky. 

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