Sky's Up July - September 2018 | Page 60

So why do observers see canals on Mars ?

Surprise ! It ’ s complicated . Everyone ’ s eyes + brain have had tens and maybe hundreds of thousands of years to develop pattern recognition networks so we can recognize the various parts of scenes in the world around us . It mattered a lot when your ancestor was out on the savannah in Africa trying to see camouflaged lions , leopards , and cheetahs hiding in the tall grass while looking for their next meals . It still has its uses in the modern world ( finding that contact lens you dropped on the bathroom floor , for instance ). We also tend to make patterns where there are none . You can see some examples on Google images , including : 1 . The Man in the Moon ( both faces and a whole-body shape are suggested ). 2 . The rabbit in the Moon ( several different interpretations of shape , orientation , and placement are suggested ). 3 . The “ face ” on Mars . It was a claim based on a soft image of a hill with a fiducial mark from the camera ’ s sensor making a faux “ eye .” The image was taken by a Mariner spacecraft camera . No surprise , with better optics and camera sensor in a later orbiter it turned out to be a rocky hill . 4 . Vermont ’ s Old Man of the Mountain ( RIP ; check your quarters minted in 2000 ). 5 . The reclining Menehune in profile on a mountain ridgeline near Nawiliwili Airport , Kauai . The patterns our minds construct can also be lines and geometric shapes when such regular patterns are simply not there . So , borrowing from a television show demonstration I remember from decades ago , I challenge you to print out Figure 5 ( click the image for a larger format ) and tack it up using the recommended distance from the table below the image . Now study Faux Mars left and right through your binoculars ( or finder telescope ) and especially your full-size telescope with magnifications from about 50x to 200x or more . Make sketches of them . We ’ re cheating a bit on this since you will be printing the images and know what ’ s on them . Or have someone make sketches by hand or in your computer and post them without showing you . Astronomers of the past didn ’ t know what to expect . But do your best to make honest sketches of whatever you see through the telescope . Look at the table of drawing diameter and distance from the telescope . The challenge is getting them a sufficient distance away to make the outer circle have an angular diameter of 25 seconds of arc . That ’ s a small angle and Sky ’ s Up

the bigger the circle diameter , the farther away the circle has to be from you . A football or soccer field has sufficient room ( and convenient markings of distance ) but a schoolyard or a straight stretch of road ( WATCH OUT FOR CARS !) can also be used .

Printed Circle Outer Diameter [ IN ] [ CM ] 0.5 1.3 1 2.5 2 5.1 3 7.6 4 10.2 6 15.2 8 20.3

If you prefer to see what your eye + brain can do without taking out your telescope , just enlarge the picture set fully on your computer screen . The originals were made with a presentation program and the large circles were drawn 3 inches in diameter . The small circles are direct reductions in size to ½ inch , which immediately reduced their contrast . You can see what your eye + brain does right on your screen . Now walk as far back as you like and observe how both large and small pairs change with distance . Pretend you ’ re sitting at your telescope on a cold night sketching Mars . It ’ s easy to imagine misinterpretations of what was seen by the astronomer-artist . If you want to check my numbers , 25 arc seconds is fractionally greater than Mars ’ maximum angular diameter ever gets . The scale factor I used included most of the decimal places ( 6.060165321 x 10^-5 ) but they were lost in rounding off to integral values of the distance for the table . For these observations , exact values are not as important as the distance being close to the quoted value and , most important , paying attention to the view in the telescope and on the printed sheet or on your screen .

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