Map the Sun's course through the year

Posted by Sarah Scoles
on Tuesday, September 03, 2013

Tor Arne Holm has created a device for tracking the Sun's position throughout the year.
Astronomy can feel a bit abstract. You can’t feel the heat of a supernova, hover inside a star cluster, or set foot on an exoplanet. You can collect photons with your eyes or a camera, measure positions, do calculations — and that’s all great. But there’s something satisfying about getting your hands a little dirtier. Tor Arne Holm, a teacher from Skjetten, Norway, wrote to us about an astronomy project that produces the kind of concrete results that give students the physical grounding needed to understand a complex geometrical and gravitational concept: how the Sun’s path across the sky changes a little every day of the year.

Anthony Ayiomamitis took these images of the Sun as it appeared over the Temple of Zeus in Athens over the course of a year. // Anthony Ayiomamitis 
I teach mathematics and physics in an upper secondary school, where the students are ages 16 to 19. I never studied astronomy in my courses at the university, but I have read a lot about the topic in my life. Still, I am very much an amateur in astronomy.

A few years ago, I came across images of the Sun taken throughout a whole year on exactly the same time on the clock each day. These images were taken by Anthony Ayiomamitis. Before I saw his images, the analemma — the shape the Sun’s position traces in the sky over the course of a year — had passed me by.

You can see it in the first picture. The topmost image of the Sun was taken July 21, and the image at the bottom was taken December 21. In the years since, I have been fascinated by this phenomenon, the analemma.

Last winter, I got the idea of making a device to help me mark the Sun’s position on the wall of my house. The device is about 1 meter long, and at the tip of it I have soldered a small disc with a hole (third picture).

When the Sun shines on the disc, its image appears on the wall. Once a week at 1 p.m. sharp, I mark the Sun’s image by hammering a nail in the wall. In the course of one year there will be the pattern of an analemma.

Tor Arne Holm created this device that projects an image of the Sun onto the south wall of his house. // Tor Arne Holm
Each day at the same time, he places a nail where the Sun appears. This image shows where it was at 12:53 p.m. local time on August 9. Tor Arne Holm
On the summer solstice, the day the Sun rises highest and is up the longest, the weather was very cloudy and there was lots of rain, so I hope the Sun will shine at 1 p.m. June 21st  the coming year! I shall update you when I have a complete analemma at the end of May next year.

I often wonder if the great thinkers of Greece, Babylonia, and Egypt 2,000-3,000 years ago knew about the analemma? What would they then think of the heliocentric system vs. the geocentric system? I give this challenge to the reader.

You can find out more about the Sun’s analemma at www.analemma.com. My site is  www.sarufi.com
— Tor Arne Holm

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