Look on any globe or map, and you’re bound to find a mysterious figure-eight pattern. It’s called the “analemma,” and it really isn’t that mysterious. It outlines the position of the sun in the noontime sky throughout the year.
Most of us know that, during the year, the sun’s midday position seems to change. At noon, on the first day of summer in the Earth’s Northern Hemisphere — which occurs on June 20 this year — the sun appears very high in the southern sky, and on the first day of winter, it appears very low.
This occurs because the Earth’s axis is tipped by about 23.4 degrees to its orbit around the sun. In the summertime, we who live north of the equator are tipped more in the direction of the sun; in the wintertime, just the opposite is true.
In addition, we all know that the sun rises in the east and sets in the west. At midday, it crosses an imaginary line that runs through the sky from due north to due south; this is called the “meridian.” In the morning hours, the sun is “ante (before) meridian,” or “a.m.” In the afternoon, the sun is “post (after) meridian,” or “p.m.”
So, if you were to go outdoors every day for a year and plot the sun’s position at exactly noon (ignoring daylight saving time, to keep things simple), you might expect to see the noontime sun migrate up and down along the meridian throughout the year and to reach its highest point, of course, next Sunday.
But nature occasionally doesn’t behave as we expect.
First, our planet orbits the sun not in a circular path but along an elliptical course, and it moves at varying speeds throughout the year. This occasionally causes the sun to appear “ahead of” and “behind” schedule.
In other words, the actual sun seems to run slightly faster or slower than our clocks. And this also means that the sunrise and sunset times are shifted from what we might expect throughout the year.
Second, the Earth’s axis is inclined to the plane of its orbit and makes the sun appear at its highest point in the sky on the first day of summer and at its lowest on the first day of winter.
So, recording the sun’s noontime position for an entire year (or even at another time altogether) shows that not only does the sun’s annual movement not migrate north and south along a perfectly straight line, but it actually traces out this distorted figure-eight pattern.
Equally interesting is that each body of our solar system displays a different analemma in its sky; some are very simple, and some appear quite complex. Click here to learn more about this phenomenon from some great text, illustrations and animations.
So, what does this all mean to us regular folks? Anyone who pays close attention to such things will find that the longest and shortest days of the year do not come exactly when we might expect, and those readers who keep track of sunset and sunset times will find that these seem to behave oddly as well.
— Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at mammana@skyscapes.com and follow him on Twitter: @dennismammana. The opinions expressed are his own.