The Moon At Moonset

Hey Kids Stop Kidding Yourselves

In recent CED issues, the issues of light and optics have graced our pages. A new one came to the CED mailbox. Why is the moon so big at moonset? I thought this to be an easy one. The biggest moon that I had registered in my memory banks was a sighting at the end of the Susanville Valley in northeastern California. It was a hot day and the moon at moonset was oooh and ahhhh in bigness quality. I concluded that the moon is big in proportion to steepness of the temperature lapse rate. But my mind played tricks on my eyes. My education in this matter was faulty and I must give credit to Bob Davis, one of my down-the-hall colleagues. He convinced me I might be wrong by telling me that if you take a sequence of pictures as the moonsets and measure the diameter of the moon in each picture, it just does not change! My neural networks crafted the change I had seen. I had been brain-duped. What follows is a discussion of why sometimes your neurons just can't be trusted.

It’s In Your Mind Not Out There

That optical-illusion moon on the horizon arises because your mind (and mine) subconsciously relates the distant sun or moon to the nearby horizon. Here we need the concept of the heavenly vault to get anywhere in thinking about all this. The heavenly vault is the hemispherical "shell" we see when we scan the heavens, above us and to the horizon. When frolicking in the great outdoors, your mind tells you that the distance to the roof of the vault above you are less than the distance to the roof of the vault at the horizon. Or, let’s put it this way -- you think the stars above you are closer than the stars on the horizon. You have tricked yourself like this from the beginning. So, the sun, by way of example, from noon to sunset, goes from a place you just know is close to a place you just know to be farther away. With our agile neuron-gymnastic, we perceive the setting orb to get bigger and bigger as it approaches the horizon. To us, the orb gets
Bigger because we "know" it is farther away!

Hey, this is serious. You believe one thing that is clearly wrong [the stars above are closer than the stars on the horizon] and as a result you swear that something else that is flat-out-wrong is right and that your senses tell you it is so [the moon or sun gets bigger as it approaches the horizon]! In political campaign lingo you would be said to have "failed senses." Ouch! Are there ecological things we perceive to be true but are?
Wrong, perhaps never tested, but cause us to have illusions about and err on other ecological things? Oh, bother! Do false beliefs have consequences? Does one wrong make a right wrong? Well, even though this sounds like a conclusion, I am not done yet.

The View from the Short Grass Steppe SGS Formerly CPR

How about height of the sun in the sky in winter? At noon, in the pre-dead of winter (Dec. 21), the sun seems high in the sky. Yes? No! Consider CPR at say 45 deg. N. Latitude. The sun at noon is 45 deg - 23.5 deg above the horizon, i.e. 21.5 deg above the horizon. Man, that's low. I thought it was higher than that! The heavenly vault tricked me again. We overestimate heights of things near the horizon! Mountains and hills at the horizon are too high and the steepness of slopes we see before us are judged steeper than they really are. What are we to do? Squint! See the world with half-closed eyes. It helps minimize the illusion.

Heavenly Vault Metrics

The Heavenly Vault is an illusion and so we cannot measure its dimensions. But, we can ask our minds-eye about the ratio of the distance of (eye to horizon)/(eye to zenith). Estimates by fooled observers range from 2 to 4! Try this. Look up and find the zenith then look to the horizon. Bisect this distance and mind mark-it with a star you can find again. If the vault were spherical, the angle to the bisect point would be 45 degrees.
Now take a big protractor and measure the angle. For most of us the actual angle would be between 20 or 30 degrees! The vault is really, really flat in your round head when it is cloudy and at twilight. The vault is least flattened, most spherical, when the sky is crisply clean, in the dark of night when the stars are very bright! The bisected angle averages 22 degrees at night and 30 degrees at mid-day. The mountains you climb seem higher at dusk than mid-day. This helps you make better decisions when to camp for the night before pushing on. If you look at the vault through large piece of red glass the vault at the zenith seems low and through an equally large piece of blue glass the vault at the zenith seems high.

Height Divided by Distance

An alternate way to do this geometry is with a loved-one. As above, look to the zenith. Look to the horizon. Do this several times until you can bisect that arc into two equal arcs. Looking to that mid-point of the full arc. Now, have your loved-one walk away from you until the top of their head is at this mid-point. Now measure the distance to your loved-on and measure the height of your loved-one. The angle you need to calculate is valued as the height of your loved-one divide it by the distance walked. This fraction equals the Tangent of the angle. Whip out your pocket-trig tables and look up the angle. Else wait till you get home and can do this in full privacy.

How to Kid Yourself

Well, this is more than I intended to say when I started this piece but I am still not done. The heavenly vault is more nearly spherical when viewed from flat on your back than when you standing up. So, if the full moon on the horizon bothers you vampires, prostrate yourself. In the recumbent posture, sometimes called the “suds” position, the moon will not seem so threatening. Find a day when a bit a cloud is near the horizon and note its apparent distance. Now take a piece of newspaper and hold it up so you can't see the terrestrial part of the horizon and the cloud will suddenly seems closer than before! The vault has become more spherical. Remember a lot of this stuff is in your head. How you learn all this, as a little kid without classroom instruction in incorrect things to remember is not at all clear. Somewhere along the way we become corrupted observers of things near and far, low and high.

Well, you now have a lot of work to do. When you get done checking the things above, try hanging from the monkey bars or your gravity boots and retest all these observations in the up-side-down position. That finished; watch the sunset or moonset from window seat 4a at 35,000 feet. Is the heavenly vault effect greater and merit a big "Oh, boy!" or less and win only "ho-hum." Happy eyeballs and calm neurons!

Harvest Moon, Baby Moon & Coyote Howler

CED readers know something about backscatter and manure piles. This little offering is about sunlight backscattered to Earth by the moon. Moon gazers know how very bright it is on the night of the full moon. The moon is brighter than the days before and brighter than the days after. Brighter than the very-slightly-larger, illuminated moon-disk at full moon compared to just before and just after when the journey to crescent form begins. Here we go back to recent Science magazine findings [Hapke, Nelson and Smythe (1993). 260: 509-511]. The cause offered has to do with the size class of the backscatter granules on the moon surface (near the wavelength of the light itself). It isn't green cheese or Tinkerbell dust. Very nice wavelength coherent backscatter (rather than destructive) interference gives that extra wallop to your retinal rods. What a kick. You just get all romantic and gushy. Given a random model, one out of thirty or thirty-one teenage mothers earned their titles under coherent solar-lunar backscatter. Young love concentration wanes before the moon does and the rest is history.

The Man in the Moon

As conditions vary and our eyes train on the moon, images are crafted out of the light and dark patches and forms that are the visible surfaces of the moon. At last count the following wonderland images have been reported: human face side view, human face three-quarters, human full face, a woman’s figure, an old woman with a bundle of twigs, a hare, and a lobster. The image of the moon provided will permit you to expand on the list.


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