Parking using your shadow
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Pilotforhire587
Lycra Man
Never heard of that trick but your student is half right. The trick would work all the time if the wings or any part of the aircaft that may impact another object/aircraft are the same height. But think of this, If I am 6'5" and you are 5'9" and we stand next to each other my shadow will be longer. That all being said the trick should work fine if you are a cessna parking next to another cessna but thats about it
Hacker15e
Who am I? Where are my pants?
Guess you've never done a landing from the back seat of a tandem aircraft....or a wheel landing in a taildragger with a big engine sticking out front.
In those situations, sideways and down is the only view of the runway you have!
Hacker15e
Who am I? Where are my pants?
I personally would not bet my job (since that's usually what it costs when a pilot bends his airplane during ground movement) on the relative placement of shadows on the ground, given all of the variables that can possibly change how that looks.
There's a reason that aviation invented marshallers, wing-walkers, paint centerlines, etc...and it's to keep from running airplanes into other objects. If shadows were that good of an idea, it would have been adopted instead.
There's a reason that aviation invented marshallers, wing-walkers, paint centerlines, etc...and it's to keep from running airplanes into other objects. If shadows were that good of an idea, it would have been adopted instead.
BeRich
Very Special Agent
fish314
Well-Known Member
The OP is correct. If the shadows of both objects are visible and both produced by the same source of light it doesn't matter what the sun angle is, nor what the angle or shape the objects are upon which those shadows are cast. If the shadows do not touch, the objects are guaranteed not to touch. The opposite, however, is not true. If the shadows ARE touching, the objects either may or may not be touching.
That said, this technique does pose some of it's own hazards, and I would use it primarily as a backup, or not at all. For example: except when the sun is directly overhead, the angle of the sun will cause a distortion in the shape you see. This angle will cause foreshortening of the shadows in the opposite direction of the sun... which means the parts of the shadow that are farther away from you will appear larger than the parts that are closer. A one inch gap between the two wingtips might appear as a 1 foot or 1 yard gap between the two wingtip shadows (and of course, the wingtips themselves will appear larger). That could cause you to over-judge the distance and not stop in time.
The shape of whatever the shadow is projected ON could potentially cause problems as well. For example, imagine the shadow of one of the stationary airplanes projected onto the side of a building, right near the corner of the building with the wingtip's shadow right AT the corner. The shadow of the moving airplane might be projected onto the ground, or onto the other face of the building where you can't see it easily. You might not be looking in the right place to see where the shadows come together.
And of course, there are plenty of scenarios where you might not get a useful shadow to work with. The airport might be next to a mountain or large building, and one or both airplanes might be in shadow THEMSELVES, and therefore not casting any shadows of their own. Or the ramp might be light by stadium lighting in such a way that the shadow of one airplane is being cast by one of the lights, but the shadow of the other is being cast by a different stadium light (in which case the two airplanes COULD touch without the shadows appearing to touch). Or you might not get a shadow of one of the wingtips, because it lies in the shadow cast by the fuselage. Or you may be taxing into the sun, which would put all of the useful shadows behind you, and therefore not easily visible from the cockpit, etc., etc.
In short, I think this little trick may be a pretty good backup, or a good trick for certain situations... but I wouldn't teach it as the end all, be all. I would use things like looking at the wing tips, or painted lines and marshallers, etc. etc. One trick we use on the KC-135 is where an object or a line appears to fall on the outboard engine... but we had painted taxi-line training strips painted at home station at a lot of bases. Basically they are just a set of lines that are measured 10' and 25' from the wing tip, provided the nose wheel is on centerline. That way, anyone taxiing out of parking on a training sortie (read: new co-pilots), can look out and get a very visual cross check of what 10' and 25' from the wing tip look like. Can be hard to judge when your wing tips are so far away.
On small airplanes, I would say just look at your wings.
That said, this technique does pose some of it's own hazards, and I would use it primarily as a backup, or not at all. For example: except when the sun is directly overhead, the angle of the sun will cause a distortion in the shape you see. This angle will cause foreshortening of the shadows in the opposite direction of the sun... which means the parts of the shadow that are farther away from you will appear larger than the parts that are closer. A one inch gap between the two wingtips might appear as a 1 foot or 1 yard gap between the two wingtip shadows (and of course, the wingtips themselves will appear larger). That could cause you to over-judge the distance and not stop in time.
The shape of whatever the shadow is projected ON could potentially cause problems as well. For example, imagine the shadow of one of the stationary airplanes projected onto the side of a building, right near the corner of the building with the wingtip's shadow right AT the corner. The shadow of the moving airplane might be projected onto the ground, or onto the other face of the building where you can't see it easily. You might not be looking in the right place to see where the shadows come together.
And of course, there are plenty of scenarios where you might not get a useful shadow to work with. The airport might be next to a mountain or large building, and one or both airplanes might be in shadow THEMSELVES, and therefore not casting any shadows of their own. Or the ramp might be light by stadium lighting in such a way that the shadow of one airplane is being cast by one of the lights, but the shadow of the other is being cast by a different stadium light (in which case the two airplanes COULD touch without the shadows appearing to touch). Or you might not get a shadow of one of the wingtips, because it lies in the shadow cast by the fuselage. Or you may be taxing into the sun, which would put all of the useful shadows behind you, and therefore not easily visible from the cockpit, etc., etc.
In short, I think this little trick may be a pretty good backup, or a good trick for certain situations... but I wouldn't teach it as the end all, be all. I would use things like looking at the wing tips, or painted lines and marshallers, etc. etc. One trick we use on the KC-135 is where an object or a line appears to fall on the outboard engine... but we had painted taxi-line training strips painted at home station at a lot of bases. Basically they are just a set of lines that are measured 10' and 25' from the wing tip, provided the nose wheel is on centerline. That way, anyone taxiing out of parking on a training sortie (read: new co-pilots), can look out and get a very visual cross check of what 10' and 25' from the wing tip look like. Can be hard to judge when your wing tips are so far away.
On small airplanes, I would say just look at your wings.
inventor
Well-Known Member
Yes.
No. The astonishingly minor variance of the angle of sunlight at 90 million miles is not perceptible to the eye. Regardless of the time of day, or angle of the sunlight to the ground. It makes no observable difference, morning, noon, or night.
fish314
Well-Known Member
I don't think you've thought this through. The variation of the angle of the sun to a particular point on the earth is based on the Earth's rotation (primarily), and slightly on the orbit of the Earth around the sun....but the big variation is the Earth's rotation.
And of course it makes a difference on a shadow you see. For example, the easiest proof of this is the bold, above. You don't even SEE a shadow at night, from sunlight, anyway. What you see from other lights would depend of course on whether there are any other lights. The big deal, though, is that the angle between the incoming light and the object upon which the shadow is cast changes drastically throughout the day.
Now, in one way you are correct. Shadow's from the sun only change in 1 direction, because since the sun is so far away, the light rays coming from it can basically be considered parallel. So, in other words, the shadows get longer but they do not get wider away from the object casting the shadow. When the shadow is being cast from a light source that is closer, like a lightbulb, the shadow cast on the wall gets larger in both directions, based on the distance between the object casting the shadow and screen it is being cast upon, as well as the distance between the source of the light and the object.
But an interesting thing happens when you are looking at your OWN shadow cast by the sun, especially at low sun angles. The shadow is stretched in one direction... basically it is lengthened. But because one end of the shadow (your head) now falls so far away from you, but the other end (your feet) falls right at your feet, it appears as though the shadow of your feet is much larger than the shadow of your head. In other words, to you, it appears as though the shadow of your head has "shrunk" compared to the shadow of your feet. That's stretching in the width direction as well as the length direction. It's due entirely to the same phenomenon that causes everything to look smaller the farther it is away from you, but it can look a little weird if you don't know what you are looking at.
Check out the attached pictures. I've named them "Shadow1," "Shadow2," "Shadow3" and "Shadow4" to demonstrate what I'm talking about.
Look at the difference between Shadow1 and Shadow2. Both show a moderately low sun angle, taken probably around the middle of the afternoon. The foreshortening is obvious, as both shadows are a little bit "stretched." In "Shadow 1" however, we are looking at two people's shadows from the perspective of someone away from the scene. Compare that to "Shadow 2" were the camera is taking the picture from very near the two people who are casting the shadow. Notice how in "shadow 2" the upper body and head appear "shrunk?" In Shadow 1, the people's feet are farthest away from the camera and hence appear smaller than they should, and in shadow 2 the people's heads are farthest away. Of course, in Shadow 1, the difference in distance between the shadow of the feet and the shadow of the head to the camera is not very big. So the shadows appear basically proportional. You can't really see that the feet are actually a little smaller than they "should" be. But in Shadow 2, you can clearly see that the heads are smaller than they "should" be.
Shadow 3 shows what happens when the source of light is NOT the sun. Because the sun is millions of miles away, it's rays of light come in basically parallel to each other, and you don't get the growth effect that you see here. This picture used some other source of light... like a spot light. Anyway, what has happened is the shadow of the hand has grown far larger than the actual hand casting the shadow. This doesn't happen in sunlight.
Shadow 4 shows how important the angle of the surface on which the shadow is cast is to the way the shadow appears. The sun is at a low angle, so the wall is approximately perpendicular to the rays of the sun. Notice, the part of the dog's shadow on the wall is not really stretched. This is approximately the same thing that happens directly under an object lit by the sun's rays at noon. At noon, the ground is perpendicular to the sun's rays, and the objects shadow falls directly under it... and stretched very little.
But check out the dog's LEGS in this picture. They are VERY stretched. This is how an object's shadow appears against the ground with a low sun angle... and it matches the effects of "shadow 1" and "shadow 2." In this case, we have a little of the foreshortening effect that we saw in Shadow 2, because the camera is taking the picture from near the dog.
One last thing to check out is the dog's body and head on the wall. Because the source of light was the sun, notice that the size of the dog's body and head are the same as the dog's ACTUAL body and head. You don't get the growth effect we saw in Shadow 3.
