High Noon is at Twelve O'clock on only Four Days of the Year

Dennis Cowles is an old friend from Fairbanks, now transplanted to Anchorage, who for years has had the unsettling habit of calling me up and asking me questions I can't answer. Last week, Dennis called and put his latest query this way:

"I'm putting together information for a survival guide. In explaining how to use the sun as an indicator of direction, I found that the time between sunrise and noon is not the same as between noon and sunset. As a matter of fact, there's up to 16 minutes difference. Why is that?"

Asking around, I found that few of the people I work with were aware that this was so, but we soon learned that Dennis was right. Checking the Naval Observatory Ephemeris and various time charts showed that the times for sunrise and sunset are, indeed, skewed about the noon hour. (This is true noon we're considering here, please note--the time when the sun is at its highest point in the sky, unaffected by too-wide time zones or Daylight Savings.)

It took some time, but with a little help from my friends (especially Hans Nielsen), I began to think of a question I'd asked my teachers many years ago. The question was: What is that funny-looking figure-eight-shaped thing on the side of the globe?

I have always considered myself fortunate in having been able to attend one-room (all grades) schools until I reached high school. One really neat thing about my schools was that they always had a globe, each with the figure-eight thing stuck someplace out in the Pacific Ocean (few modern globes carry them any more). It seems to me that we used to go through an awful lot of teachers back then, but I never had one who could tell me what that figure was. As it developed, it turned out that Dennis' question gave me the answer to what it was that bothered me so many years ago.

We live with a 24-hour day, and with only extremely minor variations, that's how long it takes the earth to turn on its axis relative to the fixed reference of the universe. We go around the sun once a year, but--and here's the catch--we don't pass around it an equal number of degrees each day. We would if we went around in a perfect circle, but the earth's orbit is an ellipse. During our winter months (in the northern hemisphere), we're a little bit closer to the sun than we are during the summer. By the laws of physics, we therefore swing around a little faster, but the earth keeps rotating on its axis at the same rate.

This means that when we look due south at noon in February, the sun has fallen a little behind where it "should" be, and it is slightly west of the north-south meridian overhead. As summer approaches and the earth swings wider (and slower) in its orbit, the sun catches up, and by late April it has moved slightly ahead (to the east) of a north-south line passing through the zenith at noon. This apparent meandering performs a complete cycle each year, during which the sun is actually due south at noon on only four days. These days are not, by the way, at the equinoxes and solstices, as you might expect, but do fall close to them.

When the elevation differences of the sun in the sky due to seasonal variations are included, the resulting figure-eight-shaped path for the sun in the sky at noon is the same for an observer any place in the world (those living above the Arctic Circle will just have to imagine the figure passing below the horizon). This path is called an "analemma," and it is this figure that is found on the old globes. (There's not enough room here to go into the reasons why it is smaller at the top than at the bottom, or why it is not exactly centered on the noon meridian.)

The main reason that old globes stressed the importance of the analemma is that relating the sun's (or a star's) location at a known time was the only way that early navigators were able to determine their longitude, and minor variations like this counted.