Introduction: Capturing the Solar Analemma With Pinhole Photography
This Instructable is about capturing the figure eight shape traced by the sun in the sky over the course of a year. This is called the solar analemma. The figure eight is caused by two things. First, the Earth has a tilted axis. This causes the sun to appear higher or lower in the sky throughout the seasons. Second, the Earth's orbit around the sun is not perfectly circular but slightly elliptical. This causes the sun to shift slightly east or west in the sky at different times of the year. Put the two effects together and you get the figure 8.
People have traced the analemma for centuries by noting the position of the shadow of a pointer or the projected image of the sun at the same hour each day. More recently, people have used film and digital cameras to record the sun's position and then composite the various exposures into a single image. Both the historical and modern techniques have the disadvantage of generally requiring a person to be at the same place and time each day, or at least every few days at a minimum, over the course of a year. This is problematic for most of us!
I had been taking solargraphs, pinhole pictures showing the streak of the sun across the sky each day for extended periods, for some time when I ran across a technique for using the pinhole camera to capture a solar analemma. What you see here is a record of my attempts. I assume the reader is somewhat familiar with the concept of pinhole cameras and solargraphs. If not, please check out this link for a quick intro to the topic.
Step 1: Capturing a Solargraph
With most pinhole solargraphs, the camera is fixed in position for days, weeks or months and left open 24/7. The daily track of the sun across the field of view makes a streak on the photo paper inside the camera. At the end of the period, the paper is removed in a darkened room and scanned on a flatbed scanner. The intense sunlight changes the chemistry in the photo paper, leaving a faint visible negative image. After scanning, you invert the colors and apply a strong contrast stretch to bring out the image. The paper chemistry reacts differently to different temperatures, humidity, and light levels. The colors seen here are not added; they are what came out of the original image after inverting and stretching.
Step 2: Creating the Analemma Camera
My analemma camera is very simple. It's just a long box made of 1/4" foam board. Inside is a half cylinder focal plane in the "back" of the camera big enough for a standard 8"x10" piece of black and white photo paper. I like matte paper as it seems to be less prone to stray reflections bouncing around inside the camera. As you can see, the inside is painted flat black. In the front of the camera, offset high, is my pinhole. You need to consider how high in the sky your summer sun will get and place the pinhole accordingly so that the summer sun will project through the pinhole and still hit the photo paper. If you live in the tropics (between latitude +/- 23.5 degrees), your sun will reach or go past vertical. In that case, you'll need to angle your camera back to capture the whole image.
What makes this camera different from a standard solargraph camera is the rotating mask on the top. I used a cheap electric clock with an AC - DC adapter / battery to power it. Most of the time it runs off the house current but if there is a power outage the rechargeable battery keeps it running. I made a disk of light card stock and marked it in 5-minute intervals on the back so I could keep rough track of the time of day. starting at the noon hour, I cut a notch that was 2-minutes wide. I glued the mask to the minute hand of the clock so that once an hour the sweep of the minute hand carries the notch in front of the pinhole and creates a 2-minute exposure. I had to play with the balance of the mask at first since the cheap electric clock wasn't strong enough to turn it with a slight imbalance in its weight distribution.
Step 3: Considerations on Pinhole Aperture
The analemma camera requires a very wide field of view to capture the full vertical sweep of the sun's path over a year. I use a bit of an aluminum soda can with a pinhole punched into it. My first attempt, however, didn't account for the thickness of the aluminum relative to the size of the hole. As you can see in my image, I only caught the bottom of the analemma because the thickness of the aluminum cut off the higher-angle light rays. For my second attempt, I sanded the aluminum very thin using fine sandpaper. It is much more like aluminum tissue now and very fragile. However, it gives a much wider field of view.
Step 4: Waiting...
Once you have your camera built, loaded, sealed and placed (I set mine in a south-facing upstairs window in a spare bedroom), it's a waiting game. I started my image on New Year's Eve 2015 and let it run for a full year, opening it up on New Year's Eve 2016. The hardest part of waiting was trying to make sure the camera didn't get bumped during house cleaning!
Step 5: Opening Your Camera
After the year's time, I opened my camera. The image here is what the photo paper looked like as it came out of the camera. Each figure 8 marks the position of the sun at a particular hour of the day. Each dot is the 2-minute exposure at that hour. On one of the equinoxes i stopped the mask in the open position to capture the full streak of the sun on that day. You can see a scatter of points in the lower part of the figures where my clockworks malfunctioned and didn't keep correct time. After a couple of weeks I noticed the time was off and reset it for the rest of the year. The tallest figure is the noon hour. The camera flips the image so the right side of this negative represents the dawn hours from 7:00 A.M. onward. My final image takes the negative, inverts the colors and flips it right to left. I thin applied a strong contrast stretch to it. I captured 9 complete or partial analemmas in this shot.
Step 6: Final Thoughts
I hope this Instructable inspires you to try your hand at making a solargraph and ultimately to try and capture an analemma.
Some considerations to keep in mind: You need someplace where your camera can see the sun, won't be disturbed for a full year, and is weather protected. Having a way to check the mechanism periodically is helpful to ensure you don't waste much of a year on a broken clock drive. You might consider doing a partial exposure, say a month or so, to check the process and make sure your camera is working correctly before you go for the year-long effort. No matter what you end up with, have fun with it! And when someone asks you how long your exposure was, you can honestly tell them it's on the order of 17,500 minutes!
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