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IS IT ME OR IS THAT A REALLY BIG MOON?
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From The Backyard Stargazer
An Absolute Beginner’s Guide to Skywatching with and without a Telescope
Have you ever watched a summer full Moon as it rises over the horizon? Perhaps it's rising over the edge of a hill or framed by a familiar fringe of trees As the last rays of the setting sun fade to a copper glow, you take a second look. The Moon seems huge!
Later that evening, you happen to catch sight of that same full Moon, now high in the sky. You glance away, then back. Wait a minute - it's much smaller! How did that happen?
You've just experienced the Moon Illusion, an enigmatic phenomenon that people have been puzzling over for thousands of years. Aristotle was the first to come up with an explanation for it, suggesting that the Earth's atmosphere, acting as a magnifying lens, was the cause of it. In fact, the Earth's atmosphere visually squashes the Moon, which if anything, should make it appear smaller. Then, in 1,000 A.D., an Arabian physicist named Ibn Alhazan suggested that familiar landmarks, such as trees and houses, make the Moon seem large by comparison. After all, we know the Moon is a huge object - it should be enormous, compared to objects on Earth.
Alas, though he came close, Alhazan didn't quite explain it, either, as you still see the effect if you're out in the middle of the ocean with no landmarks at all against which to compare the Moon.
And here's another enigma - if you measure the size of the Moon (with the eraser on the end of a pencil, for example) on the horizon and then high in the sky, you'll discover it's exactly the same size. (Try it - see the Projects section.) Multiple-exposure photographs of the rising Moon also show no change in its size from horizon to zenith. Yet, studies have shown that people estimate the size of the horizon Moon to be as much as twice the size of a zenith Moon.
So, what is going on here? Theories are still being hotly debated. Suffice it to say, what your eyes see and what your brain thinks it's seeing are two different things. It's a purely psychological effect, not a physical one.
Oddly enough, you can dispel the Illusion just by looking at the horizon Moon from a different perspective. If you bend over and look at it from between your legs, the Illusion vanishes. Clearly, it's simply a matter of your point of view.
Reprinted with permission of the publisher, Quarry Books, from page 31 of The Backyard Stargazer.
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Projects
Project 1: Observe the Full Moon
Check your calendar, monthly astronomy magazine, or astronomy website for the date of the next full Moon (your local newspaper will also carry this information). Go outside just after sunset and look in the eastern sky. The Moon will be rising.
Things to do:
• Observe the maria; how many can you name?
• Can you see any craters with the naked eye? How many? Which ones? Tycho is the most obvious crater: can you see its rays? How about the rays around Copernicus? If you’re keeping an observing log, you might want to sketch what you see.
• Now take a look at the Moon through binoculars (if
you have them). How much more can you see? Check the Map of the Moon and note th features you observe in your observing log.
Project 3: Study the Moon Illusion
Check the usual sources to fin out when the next Full Moon will occur and where it will rise. Find an unobstructed view of that horizon and note the Moon as it just begins to peek over the horizon. Does it look abnormally large?
Now, measure the horizon Moon with a small disk, something the size of a pencil eraser or a small mint, perhaps - something that just covers it. Try this again later in the evening, when the Moon is higher in the sky. Is the Moon the same size? Does it look smaller than wen it was on the horizon?
You can try looking at the horizon Moon through your legs. Does the Illusion disappear?
Note: These projects appear on page 32 of the Backyard Stargazer, a 176 page guide to the heavens, which includes full-color photos, star maps, and many, many projects for you and your children to enjoy. Quarry Books. Available at most bookstores or from Amazon.com.
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The Backyard Stargazer
 An absolute Guide to Skywatching With and Without a Telescope
by Pat Price
People have been stargazing for thousands of years, but only recently has it become a mainstream hobby, one you can practice in your own backyard! With The Backyard Stargazer we discover:
• Seasonal star maps to help spot constellations - and more - year round
• Things we never knew about the Moon, Sun, and planets
• What causes an eclipse and how we can travel to see one
• How stars are born and where comets come from
• How to photograph auroras, meteors, and constellations
• What can be seen with the naked eye, binoculars, and a small tele scope
• Over 250 color photographs and illustrations
• Hands-on projects
• Complete bibliographies
Astronomy as a hobby is gaining widespread interest among the general population. Planetariums in every city are drawing record crowds and "astronomy trips" to view the northern lights or an eclipse are the hot new vacation packages.
The Backyard Stargazer opens up stargazing for everyone from as close as their own backyard. Knowing how to look at the sky and what to look for is a challenge. Backyard Stargazer provides readers with the information and instruction they need to enjoy an enthusiastic joy of astronomy without having to invest in their own observatory.
Astronomy as a hobby is nothing new, but with today's "back to nature" hobbies on the upswing, this book is just what those who look to the heavens are searching for.
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