The image of the Sun

Today at Moses Lake the sun went down in cloudless skies. Naturally, my mind drifted to the subjects of religion, time, astronomy, sunspots, optics, history, and climate, and of I ran into the fermentation section of our ethanol plant to see if I could image the sun. The west wall of our plant has a lot of little (maybe 1/8″ = 3mm) holes in it and a good view of the setting sun. The sun shines through each of these holes, and they produce images of the sun on the opposite wall. These are, literally, sunspots in the sense of “spots of sun”:

Images of the sun like these were used by the Catholic Church as a very accurate sun dial. They drew curves on the floor to indicate noon, summer and winter solstice, etc. These are called Meridian lines. When one specifies a time as “AM” or “PM”, this is an abbreviation for “ante meridian” or “post meridian.” The moment at which the disk is perfectly centered on the meridian curve indicates high noon. High noon occurs in slightly different directions depending on the time of year, so the meridian figures are curved in a distorted figure eight, called an analemma. The very interesting history of the meridian lines inside churches is the subject of the book The Sun in the Church: Cathedrals as Solar Observatories by J. L. Heilbron.

With the improvement of time keeping, the necessity of making solar observations by sunshaft decreased and some of the beautiful meridian curves have been removed from cathedral floors. The Catholic Church, however, still supports astronomy. The tradition is carried on at the Vatican Observatory in Arizona.

You can get a better sun image at home if you use a piece of paper. After adjusting contrast and brightness, here’s my image of the sun:

The above is a nice clean image of the sun. You can tell that it is in pretty good focus by noticing how sharp the border of the sun’s disk is. If there were sunspots visible at that time, I would have seen them.

Here’s a sunspot photo I took a few years ago at our warehouse in Woodinville. I’ve enhanced the contrast to show the spot better. The grid pattern is from the piece of cardboard I’m displaying the image upon:

These are a lot easier for the eye to see than for the camera. The clearer skies here in Moses Lake make for sharper pictures.

So that you will know what sunspots look like if you happen to see one, here’s a good sunspot at sharp focus from the professionals (aka your tax dollars):
Big Bear sun image showing spot from Sept 15, 2005
The above image is from the GONG observatory Big Bear from 3 years ago, September 15, 2005. The corresponding image from today is as bald as a billiard ball:

Sun Observations

Because it is so easy to see sunspots, speculation is that the first observers were Arabs, who would have had small holes in their tent walls that should have thrown good images. However, it is not at all obvious why these images should look like the sun. When a human sees a spot on the wall produced by the sun this way, the natural instinct is to assume that the imperfections in the image are due to imperfections in the hole, rather than in the sun. Somewhere I read that the ancient Greeks had difficulty with the concept and an unsolved problem for them was the fact that a fence made of rectangular boards, with square holes through which the sun shines, produces round dots.

In fact, the details of the hole in the wall mostly effect the brightness of the image of the sun. So long as they are not too large. This is because the difference in dimensions do not get magnified when the image gets magnified. Immediately after a 2mm x 3mm hole there will be a shaft of sunlight that is 2mm x 3mm in size, but as this expands (and it expands due to the fact that the sun is not a point source of light), the dimensions of the shaft of light become more and more regular. One eventually gets a 12mm x 13mm shaft of light, then 22mm x 23mm, etc. And the size of the whole determines the smallest sunspot you can see. As you move away from the hole the image grows. You’re looking at it from the same distance, so this increases the magnification.

The Camera Obscura

This evening the skies were fairly clear all the way to the horizon (and this is “big sky” country). So I was able to see the outline of the horizon imprinted on the sun’s disk. Turns out that there was a tree between me and the horizon. And there were two holes close enough together that I could get two images of that tree, and the red setting sun:

Of course the image is upside down. This is the basic principle of the camera obscura a technique of throwing an image on a wall. A great article on the subject is Inside the Camera Obscura – Optics and Art under the Spell of the Projected Image, which also includes a description of the meridian lines at the Vatican.

The idea of the camera obscura dates at least to 400 years BC (Chinese), but it was explored completely by the Basra-born optician Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn al-Haytham ابو علی، حسن بن حسن بن هيثم
He lived around 965 to 1039 AD, and is more succinctly known as Alhacen.

Alhacen was placed under house arrest after he feigned madness in order to avoid complying with an unreasonable order to use his understanding of mathematics to regulate the floods of the Nile. Basra probably looked good to him. While under house arrest, he wrote the prescient and influentional Book of Optics. Some say he was the father of the scientific method. But despite all this, he did not describe sunspots.

After I took the above image of the tree, I ran outside and climbed up the stripper tower to photograph the setting sun. The view is from above and to the north of the building. The tree concerned is the tiny bump that, from this height, just touches the horizon. It is alone, the first tree to the left of the point of the setting sun:

Before the image was reduced to 500 x 400 resolution for this post, I could see the structure of the tree enough to identify it. In the above picture, taken by the same camera, you can see no detail in the tree whatsoever. Amazing how well the camera obscura magnifies.

Sunspots and Climate

So the discovery of sunspots was likely several thousand years ago. Someone seeing the image of the setting sun could have realized that their tent was actually an observatory and conclude that sunspots were on the sun. But they didn’t publish, and so Galileo got the credit in 1613. (Kea, don’t let this happen to you.) And from that time on, observations of sunspots have been regularly made by astronomers. From the wikipedia article on sunspots, the number seen per year goes up and down regularly, but not quite stably:

In the above chart, there are two periods where the sunspot count was reduced. During the Maunder minimum, from 1645 to 1715, sunspots were exceedingly rare. And there was a period called the Dalton Minimum, 1790 to 1830, where sunspots were just more rare than usual.

The funny thing is that the Maunder minimum corresponded to a time of very cold temperatures in Europe; it was the time of the so-called “little ice age”. The effect of sunspots on the earth is to change the amount of cosmic radiation that the earth recieves.

When cosmic radiation hits nitrogen atoms in the upper atmosphere, it sometimes converts them to carbon. The cosmic radiation makes neutrons. The nucleus of the nitrogen atom absorbs a neutron and spits out a proton. This converts the atom to being an atom of carbon. But instead of the usual carbon it is slightly radioactive, carbon-14. Therefore, when sunspots come and go, it effects the amount of carbon-14 that is created, and this effect, that the amount of carbon 14 being created changes from year to year, is compensated for when carbon 14 dating is done.

It’s also known that cosmic radiation can enable clouds to form. This is the basic principle of the “cloud chamber” that was used so much in elementary particle physics of the mid 20th century. And clouds effect the climate. For a more complete description of the science, see Lubos Motl’s Sunspots Correlation With Temperature blog post.

The previous solar cycle, number 23, was very active, and general expectations were that the next solar cycle would also be very active. However, the new cycle is getting off to a very slow start. All this has got a few scientists worried that we may be in for another little ice age. The problem with global cooling is that it decreases the amount of food that we can grow. For the scary view of sunspots, see Sorry to ruin the fun, but an ice age cometh, which talks about Napoleon’s retreat from Moscow and that various parts of the planet have seen record cold weather recently, such as the first snowfall in Baghdad in centuries.

The last glacial maximum covered the place from which I now write under thousands of feet of ice. That would be real bad, but at least it would take some time to build up. On the other hand, it doesn’t take much cool weather to disrupt our crop growing. This past spring, there were some worries that the US corn crop would be severely reduced due to the cold wet spring.

So let’s hope that the sunspots come back, and we can return to worrying about global warming.


Filed under History, physics

6 responses to “The image of the Sun

  1. It would be interesting to see the Thames freeze — one used to be able to ice skate across sometimes, rather than using London Bridge.

    But if Seattle goes under thousands of feet of ice, you’ll have Moved On Elsewhere by then.

    The planet moves so slow.

    Thanks for the sunspot info.

  2. Matti Pitkänen


    a question definitely out of topic. I saw that had written and article about nature of time to fQXI. I sent also my contribution yesterday but it has not appeared to the page. I wonder whether this due to immediate rejection or do they read it before they possibly put it on the page (or reject as I has as a scientific dissident full reason to expect).

    Matti Pitkanen

  3. Pingback: Global Warming » The image of the Sun

  4. Great post, carl. Sunspots and Earth’s climate are indeed an important topic. I was hoping that more cities would have ice skating and skiing too! We should try to get our predictions published, somewhere, so someday it will be possible to see who found it first.

  5. P.S. September 22 was Autumnal equinox, when in ancient observatories (like Chichen Itza or Stonehenge) the Sun’s image would line up with the stones.

  6. carlbrannen


    I have no idea what is going on inside the sun, so I can’t publish anything on it. I look forward to reading your theory on this. Cycles on cycles…

    Back in 2006, a couple of sunspot people, William Livingston and Matthew Penn, measured the temperature and magnetic fields of sunspots over a cycle and a half. They concluded that sunspots would disappear around 2015.

    Their paper, “Sunspots may vanish by 2015” was rejected by publishers, but they got the last laugh as we seem to be descending into a very deep and prolonged sunspot minimum.

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