The Tilted Sunrise
How to make beautiful time-lapse photographs of a sunrise, and what you can learn from them.
As I say to students: “Wouldn’t it be nice if we could reach out with a gigantic marker, and draw on the sky, and trace the paths of the sun and moon and stars? Wouldn’t it be cool if we could see a recording of trails on the sky showing where everything came from and where it all goes? Well…if we can’t do that, what could we do?” One thing we could do would be to find a nice open space to observe the horizon, and then place stones in the ground around us to mark the locations of sunrise and sunset, making a sort of Stonehenge…or a Royal Observatory. Another thing we can do in the modern world — almost as good as drawing on the sky — is to make time-lapse photographs of what goes on in the sky.
Every object in the sky, however, presents its own unique challenges to photographers. In the case of the sun, the sheer brilliance means that, unless you do something special, it will be extremely overexposed in your photograph. All you can see, with your eyes or with a camera, is just a big puddle of glare. If you want to get a good look at the size and shape of the body of the sun itself, you need help. Normal sunglasses are not good enough, but for your eyes, you can buy very inexpensive “eclipse glasses”. These are in effect "super sunglasses", and they cut down the light levels dramatically, eliminating the glare, and letting you get a good look at the size and shape of the sun itself. They are sold mainly for watching solar eclipses, but they work just as well with the everyday sun. In my experience, just giving them to children on a normal day and having them look at the normal sun still produces a lot of exclamations. It’s dramatic to see in person that perfect glowing circle without the glare, if you’ve never seen it before. For your camera, you can buy “solar filter film” made of the same material as the eclipse glasses. (I had a good experience buying bulk eclipse glasses from the Rainbow Symphony Store, but you can easily find both eclipse glasses for your eyes, and solar filters for your camera — or your telescope — by searching Amazon.)
A photograph of the sun, with and without a solar filter.
With these “sunglasses” on the camera, however, we have a new problem. We get a nice picture of the sun…but we can’t see anything else. The rest of the picture is black. However, if we want to see both the sun and the landscape together, it is not too difficult to stack multiple photographs on top of each other. We can start with a nice background photograph without the sunglasses, perhaps taken just before the sun rises, and then overlay several subsequent photographs of the sun with the sunglasses on, treating black as "nothing" or "transparent". When we add the photos together in this way, we make a composite photograph of the sunrise that looks like this:
This is a time-lapse photograph I made of a September sunrise in Aliso Viejo, California. Here we finally get to see a beautiful (if I do say so myself) trail of the sun across the sky. Now, with such a photograph, we can notice a few things about the sun’s motions. For one thing, the sun does not rise straight up into the sky. It crosses the horizon on a diagonal. How tilted is it? It inclines from vertical by about a third of the way over (i.e. about a third of a right angle, or about 30°). When I teach astronomy to children, I like to follow this up later by showing them that the moon and some of the stars rise in the east too, and at the same angle, and that the North Star hovers in the northern sky at the same angle above the horizon, and we wonder why everything in the sky seems to be tilted by this same angle of around 30°. I then show them photos and movies from around the world, and we discover that the angle is not the same everywhere, but varies with how far north or south you go. The circles of the sun stand up straight at the equator, and lie almost flat near the poles. In short, the sunrise in the photograph above tilts to the right by about 33°, because Aliso Viejo, California lies at 33° North latitude.
Another thing we can notice is that the sun does not speed up or slow down, but marches pretty steadily along its path. My filtered sun photographs were taken at 8-minute intervals, from which you can deduce that the sun travels its own diameter every couple of minutes.
This picture shows us nicely how the sun rose on one particular day. But what about other days? Is the sun's sunrise motion always the same, or does it rise in different ways on different days? Having made one such photograph, can we make more sunrise photographs on other days to see whether the sun’s path changes as the weeks and months go by? Before I show students my evidence, I usually ask them to guess how it might change. They often guess that the tilt angle will change. What do you think?
Rather than have students make an extensive series of observations every week or every month for many months, I normally just “give away” the fact that the sun’s path changes in a rhythmic pattern every year. The sunrise swings back-and-forth along the horizon each year like a swing set, reaching a northerly extreme on the day we have named the "summer solstice", picking up speed and passing quickly through due east on the "fall equinox", then slowing down and rising at a southerly extreme on the "winter solstice", before turning around and heading back north again. Could we overlay three time-lapse photographs of these three sunrises, to show the annual back-and-forth drifting of the sunrise? I tried, and made this:
This is a view from the city of Aliso Viejo, California, looking east over Saddleback Valley at the Santa Ana mountain range in the distance. Saddleback Mountain is visible at the left, with the shorter of the two peaks marking northeast (azimuth 45°) as seen from the viewing location. This photo is a blend of three separate time-lapse photographs of the sunrise on Aliso Viejo — one sunrise each at the summer and winter solstices (on the left and right), and one at the fall equinox (in the middle), with the fall equinox sunrise marking due east (azimuth 90°).
We can see clearly in this photo that the location where the sunrise crosses the horizon is what changes significantly with the seasons. We can see that the summer sun rises to the north of east, and heads higher into the sky, while the winter sun rises to the south of east, and stays lower in the sky, thus changing the duration and intensity of warming each day, and giving to us our seasons.
Does the tilt angle change? We can see in the photo that the tilt of the sun circle does not change. Or does it? Keen-eyed students will often notice that the equinox path appears to be a straight line in the photograph, while the two solstice paths appear to curve outwards a little. When I first noticed the curve in the summer photo, I casually assumed it was a projection effect or wide-field distortion of some kind, and I ignored it. But when I finished the winter sequence and finally combined all three photos, it struck me that I was actually recording the sun’s different declination circles in the sky. The sun’s apparent path through the sky on any given day is always a circle, but we view the circle from different perspectives in different seasons. At the equinoxes, we view the circle from the inside, from the circle’s exact center, so we don’t notice the curve at those times, but during summer and winter, we view the sun’s circle off-center, from a little to one side or the other, so at those times we perceive the curve of the circle. I noticed a similar effect when I looked up at the curved ceiling rafters of the Mission Viejo public library. The rafter immediately above me appeared straight, and those on either side appeared to curve outwards, just like the circles of the sun.
Curved ceiling rafters appear straight or curved depending on perspective.
As a side note, perhaps I should admit that I cheated just a little with the winter sunrise. I wasn’t happy with the view of the winter sunrise from the camera’s original position, and I also wanted to see if I could catch a few photographs of the sun passing behind a tree, so I shifted the camera by about 15 feet from where it had been for the summer and fall photographs. I’m pleased with the way the winter sunrise composite turned out, but this also meant that the winter and fall photographs didn’t overlap well. I tried to hide this when I smoothed the two photographs together. I’m currently working on an analogous sunset panorama, and I made sure to plan all of my angles ahead of time this time.
If you want to try making similar photographs of your own, I can offer a few suggestions. You could probably use the camera in your mobile phone, but you would need to find a way to hold it steady for an extended period of time, and a way to add and remove the “sunglasses”, and find an app that allows you to take photographs at timed intervals. I wouldn't recommend it. I own a Canon Rebel, and I used it for all of my solar photographs. For the “sunglasses”, I started out with “eclipse glasses” made especially for cameras–they are basically circles of solar filter film in a press-on cardboard lens cap. I found mine to be unwieldy, however. I kept shifting the camera when I put it on or took it off. Instead, I bought a sheet of solar filter film and an empty metal filter ring and made my own screw-on filter for my DSLR, and it works much better. The manufacturer of the film also claims you can just wrap it over the lens and hold it in place with a rubber band, but I have never tried that. You can also buy special solar filters made specifically for your camera (or for your telescope), but they are more expensive. I also recommend a versatile but sturdy tripod without any wiggle room in the mount. You want to be able to easily adjust the camera’s angle in multiple directions before you start, but also make sure the camera doesn’t move between the beginning and end of your sunrise sequence. I ruined one or two sunrise sequences because my discount-brand tripod had too much wiggle room and the camera shifted position as I made adjustments. I bought an Amazon Basics tripod for about $25 and for the price, I’ve been extremely happy with it. For making sure I take photographs at regular controlled intervals, I have used both my laptop computer, with software that came with my camera, and an external camera timer that I bought for about $25. The computer worked fine, but the timer was far more convenient. For blending the photographs, I used computer scripts that I wrote myself, but there are several commercial and open-source software options available. I have used the free software StarStax and found it to be helpful and easy to use, but you could also use Photoshop or Gimp if you are familiar with those.