Slideshow: The Circles of the Sun
A visual aid for illustrating how the sun crosses the sky in different parts of the world.
When I taught my students about the sun in different parts of the world, including the "Land of the Midnight Sun", and the southern hemisphere where the seasons are backwards, I wanted a visual aid to illustrate the differences in the sun's path through the sky in different seasons in different latitudes. I searched the web and found a few good time-lapse photos and movies of the sun in different parts of the world, but not many. Here are a couple of my favorites:
In addition to this photographic evidence, however, I wanted a more comprehensive and systematic visual aid, illustrating the sun's motion in all latitudes, from pole to pole. In the end, I created this slideshow.
The Sun in Every Latitude
This download file is a PDF slideshow depicting graphically the path of the sun through the sky, at the solstices and equinoxes, and in the various latitudes of the world. There are 23 slides in total, representing 23 different latitudes, from the North to South Poles, including the Arctic and Antarctic Circles, the Equator, and the Tropics of Cancer and Capricorn. Each slide contains a map of the world with a black line marking the latitude in question, next to a graphic depicting a local observer's "sun wheel", i.e. the apparent path of the sun in the local sky, in the various seasons. Each "wheel" contains three circles, with the central one marking the apparent path of the sun in the sky at the equinoxes, and the circle on either side marking the path of the sun at the solstices. Equally spaced around each "wheel" are 24 hour marks, depicting 1/24th of a full circle, or how far the sun travels in an hour. Note that the "axis" of the wheel points to the North and South Celestial Poles, with the North Star effectively marking the North Celestial Pole in the night sky.
In northern mid-latitudes, unlike other latitudes, the sun rises and sets every day, but never touches the zenith (i.e. it is never straight overhead). The equinox sun rises due east and sets due west, and the circle of the sun is cut into equal halves, one up and one down, causing day and night to be equal at the equinoxes. (The word equinox derives from latin and literally means "equal night".) At the June solstice, the sun rises on the north side of east, sets on the north side of west, and climbs higher in the sky. The longer hours of daylight and stronger sun near the zenith create summer. At the December solstice, the sun rises south of east, sets south of west, and stays lower in the sky, where its light and warmth are weaker, thus creating winter.
As you travel north or south, the "tilt" of the wheel changes, with the "axle" standing up straighter as you head north. When you reach the Arctic circle, the wheel has tilted so far sideways that the solstice circles touch the horizon, meaning that the sun never sets on the summer solstice, and the sun never rises on the winter solstice. Inside the Arctic circle, there are extended periods of time in summer when the sun never sets, and in winter when the sun never rises. At the North Pole, the "wheel" lies horizontally, with a vertical axis, and the North Star directly overhead (at the zenith). The sun turns in permanent clockwise circles above the horizon in summer, and below the horizon in winter.
If we return to northern mid-latitudes and travel south, we will eventually reach the Tropic of Cancer, where the wheel has tilted up enough so that the sun can finally touch the zenith (once, at noon on the June solstice). In the region between the two tropics, the sun can touch the zenith, and outside the tropics, it never does. At the equator, the sun's circles stand up straight. Every day has 12 hours of sunshine and 12 hours of night, and the sun passes near to the zenith every day of the year, meaning that there is no difference between summer and winter. It is always hot. There is no difference in the weather between June and December, except that the sun is a little more north or a little more south.
In the southern mid-latitudes, the sun behaves exactly as it does in the northern mid-latitudes, except everything is reversed. Instead of traveling through the southern side of the sky, the sun travels through the northern sky. Instead of being high and up for many hours in June, the sun is high and up for many hours in December, and vice-versa for winter. The seasons are reversed in the southern hemisphere, because the sun's behavior is reversed. (Not as many people get to experience this, however, because as you can see from the map, the southern mid-latitudes consist mostly of ocean.) At the south pole, the sun behaves as it does at the north pole, except reversed. It travels in counterclockwise circles, always above the horizon in December, and always below in June.