Re: Helicopter or Shadow?

Message posted by JB737 on October 19, 2007 at 15:54:11 PST:

Very likely, Joerg is 100% correct, that you are seeing the copter itself, right in the same place as its own shadow, hence on the ground or very very low.

But there is another possibility, namely that the copter is high enough up that its shadow is not visible anywhere, due to being a penumbral shadow rather than umbral. In that case, the copter is not on the road, but somewhere in the air, along the line between its apparent road/ground location, and the satphoto camera in orbit.

You can calculate the maximum distance from an object to the easily visible umbral part of its shadow. If you've seen a diagram of the shadows which cause solar and lunar eclipses, that is a perfect illustration of what I mean.

Beyond about 115x the size of an object in its smallest dimension (e.g., width of rotor blade in this case to see blade shadows, or the helicopter body to see a shadow of just that, or the diameter of the moon if talking about a solar eclipse, or diameter of Earth if talking about a lunar eclipse) as viewed from the direction of the sun (a light source of 0.5 degrees angular diameter), the shadow becomes penumbral and essentially invisible.

At 57x distance, the umbral shadow width would be half the width of the object, etc.

As the helo goes up, it goes from having an accurate shadow of itself, to losing the finer details, to losing the rotor shadows and leaving only a body-blob shadow, to where even that disappears at a certain copter-to-shadow distance.

Another thing to think about is whether the shutter speed is fast enough to "stop" the blades at flight rpm well enough to see them. If not, then it is obviously on the ground with blades stopped. If only marginally so, then the blade tips will have a thinner/blurrier shadow than the blade roots, due to their greater speed.


In Reply to: Re: Helicopter or Shadow? posted by Joerg (Webmaster) on October 19, 2007 at 15:05:46 PST:


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