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What's "overhead"?

Looking straight up may be harder than you think.
OMearaStephen
Recently, I went outside for a walk and casually noticed that the Sun was about one-third of the way up the eastern sky. Or was it? Out of curiosity, I noted the time and later used software to determine that the Sun was actually only 17° high at that time. How could I have been so far off?

Part psychology, part physiology?

In Light & Color in the Open Air, Marcel Minnaert helps us to understand why it is common for us to overestimate the heights of objects in the sky. It stems from a psychological effect in which we naturally perceive the hemisphere of sky as a flattened vault that appears about twice as close overhead as it does to the horizon. If you try to estimate the midpoint between these two points, the result will not lie at a height of 45° but generally somewhere between 20° and 30°.

“It is very important that unprejudiced observers be found” who must divide “not the angle but the arc” into two equal parts, Minnaert says.

The illusion intensifies under cloud cover and lessens on clear and crisp starlit nights. Generalities also come into play. Some of us use the word overhead loosely. This is especially true for those living at mid-northern latitudes, when, say, we see the noonday Sun sailing “high overhead” in summer, or a “midnight” winter Moon. In reality, the Sun never passes overhead from these latitudes but 20° to 30° from it (depending on the latitude from which we observe).
ASYOM0318_01copy
Determining a specific star’s altitude, or even finding the overhead point, is not as easy as you might think. A star-filled sky helps.
Stephen James O’Meara
Part pain in the neck

Typically, when looking straight ahead, the eye can see objects about 60° above its center point. So a Sun 60° high will appear at the summit of the eye’s field of view. Tilting the head back to see a 70°-high Sun or Moon only seems to affirm that it is overhead — a location most observers will admit can literally be a “pain in the neck,” so little bother is given to the accuracy of the observation. “Overhead,” in the most casual sense, means an object within 30° of the zenith.


Of voids and optic moths

Like Minnaert, I also found that casually estimating the midpoint of the sky during the day was difficult. It was easier on a starlit night for the simple reason that it’s hard to be accurate in a blank sea of sky, but easier when it’s full of stars serving as guideposts. Accuracy in the perceived midpoint increases if the Sun or Moon is near it — especially if you take the average of three observations (similar to what visual Jupiter observers do when making transit estimates). The accuracy also increases dramatically the more you make these observations over the course of days, which speaks to Minnaert’s warning of having only unprejudiced observers make the initial observations.

For instance, when I made my first observation of a waning daytime Moon midway up the sky from the horizon, I was off by 15°. Later, I refined the observation to within 3°, and finally to the correct value by taking the average of three observations over an hour.

At night, I also began by misjudging a star’s altitude but was able to refine it to an acceptable value. I also found that I misplaced “overhead” when a bright star was within 20° of the zenith. One night, however, when no bright star was available near that point, I accurately selected a faint star to within a couple of degrees of true overhead.

The reason? I believe that our eyes are attracted like moths to the brightest object near our invisible target destinations (midpoint or zenith), drawing our attention away. When only faint stars are visible, we spend more time scrutinizing the sky, which sharpens our accuracy. Bright stars may serve well as “immediate” signposts, but they actually lead us astray.

As always send your thoughts and comments to sjomeara3@gmail.com.

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