Know all the Angles | |
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Contrary to what David Percy says, there are other ways to alter a shadow besides blowing on it. As we shall see, it's extremely rare for a photograph--terrestrial or lunar--to show a shadow of the proper length. For example, during the Apollo 11 EVA, the sun's angle was between 13 and 15 degrees, which means the astronauts cast shadows with lengths four times their height. Yet this annotated picture from Dark Moon has a shadow that seemingly shows a sun angle of 26 degrees: Since Aldrin's shadow goes off the picture, it's tempting to suspect that David Percy ended his line a little too soon. He didn't. If you look at a higher-resolution version of AS11-40-5872, you will see that all the shadows show nearly the same steep sun angle. | |
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David Percy drew the diagonal correctly, but he made one small mistake: he assumed it meant something. Drawing a line from top of head to tip of shadow will give you a true sun angle if, and only if, the shadow was photographed broadside-on. View it from any other angle and the shadow will be foreshortened (that perspective thing again), and the results will be utterly meaningless. ![]() So let's do it. Let's see if the angle at which Neil Armstrong took AS11-40-5872 really will give the kind of shadow we saw on Buzz Aldrin. And let's see if that foreshortened shadow length really will match a 14° sun angle. This partial map (which you can find over here or on page 35 of Dark Moon) shows the relative positions of Armstrong, Aldrin, and their cameras. The shadows were pointing nearly due west (give or take a couple of degrees). But the cameras were aimed either southeast or northeast. ![]() The astronauts' shadows were viewed at angles between 35° and 40°, enough to make them look nearly 50% shorter. A shadow can be compared to a triangle, with its height representing the object making the shadow, its length being the shadow itself, and the hypotenuse formed by the angle of the sun. ![]() Now let's turn our triangle to the 35° angle at which Armstrong had actually photographed Aldrin, like so: ![]() It's still the same triangle, but now perspective makes its length appear only 2½ times its height, while its angle measured against the horizon (the far edge of the table) is nearly 30° (which corresponds closely with the angle of the lens flares). And then we place the tilted triangle over Percy's annotated image: It's a pretty good match, but not perfect. (Nothing is perfect in the real world.) My rough math suggest that a 35° viewing angle of a 14° sun angle should give an apparent angle of 23.5°. These extra few degrees could be caused by Neil Armstrong taking this picture on higher ground--a likelihood further borne out by the fact that the far horizon appears above Aldrin's helmet. Finally, there are my own errors in cutting and turning that triangle. (As for the ragged point, that resulted from an over-reliance on Photoshop's Magic Wand.) So with these caveats in mind, let's turn to Percy's second picture. In the TV image, Neil's shadow was pointing away from the camera at an angle of 40°. Here's how our triangle looked when we turned it away at a 40° angle and pasted it between Percy's line and Armstrong's shadow: |
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| Once again the triangle's hypotenuse represents the sunbeam travelling from the astronaut's head to the end of his shadow. And once again a 14° angle can be raised to an "impossible" steepness by viewing it off-kilter (and perhaps with a little bit of help from slightly unflat ground). | |||
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