February 26, 2009
As many people know, the Sun’s position in the sky changes throughout the year. In the Northern Hemisphere, it reaches its highest point in the sky around June 21st and its lowest point around December 21st. This is due to the tilt of the Earth on its axis. I decided to track this by marking the tip of the shadow from a sundial at the same time throughout the year -- at 12:00 (noon) Mountain Standard Time or 1:00 p.m. during daylight savings time (19:00 GMT). I expected to see a relatively straight line moving up and down, back and forth, year after year. As my experiment progressed, to my amazement, my mark was not moving in a straight line. After checking my clock again I researched further and found out about the analemma.
I was aware that the Earth didn’t travel around the Sun in a perfect circle, but rather in an ellipse. However, I never hypothesized that this would affect my shadow marking experiment. As it turns out, because we travel in an ellipse, the Earth doesn’t move at a constant speed. This effect creates the shadow at noon to be either ahead of or behind actual solar noon. Over time, this creates a ‘figure 8’ as seen here. The top is December and the bottom is June -- the lower Sun position results in longer shadows. As you can see from the scale, next time I should use a bigger sundial.