why does an aeroplane or a bird flying at a great altitude not cast a shadow on the earth ?

Answer 1

First, take a look here:

http://en.wikipedia.org/wiki/Penumbra

Now, look at the lower illustration. Notice that the umbra only extends so far, and where it ends the antumbra begins?

Since an airplane or bird at great altitude does not completely cover the sun, their shadow has an umbra and an antumbra. But the umbra does not reach all the way to the ground. The umbra is the part that casts a visible shadow on the ground. So there is some dimming of light caused by the airplane, but not enough to cast a shadow. When it flies lower, the tip of the umbra reaches the earth, and you see a shadow on the ground.

Answer 2

It does. When flying in an aeroplane you can see the plane's shadow on the earth

Answer 3

Light scattering in the atmosphere causes the shadow to become more and more diffuse ("fuzzy") the further the light rays have to travel. After a certain distance the shadow becomes too diffuse to detect.

Answer 4

The Light splits around the object and then refuses before it hits the ground, impossible if too close to the ground, Like humans.

Answer 5

It is not big enough to eclipse the sun.