There are 2 reasons why boats and planes might
disappear from sight.
Reason 1: The biology of the eye. How good
are our eyes at seeing? Eyes are good at
seeing things only to a certain distance. If I am
at one side of a football field, I can see a ball
very well if it's on my side of the field, but the
further and further away it gets, it is harder to
see. But if I was a hawk, I would be able to see
this ball even if it were placed 2 football fields
away. A hawk can probably even see a small mouse
at that distance. Why can some animals see
things better than others? Our eyes have
evolved and developed over thousands of years but
some animals adapted better eyesight than ours
because they need it to survive.
Reason 2: If we are looking at a flat
landscape, things like boats and planes may
disappear over the horizon. This is not due to our
poor eyesight, it's because the earth is a sphere
(like a ball). In general, the curve of the earth
is so small that we don't usually notice that we
are living on a curved surface. In fact, it was a
major discovery in science when the earth was
found out to be round, not flat, because it's not
that obvious! It's only when we are looking over
very long distances that we notice, such as
watching a boat on the horizon. The crew on the
boat will not notice that the earth is curving,
but the people watching will see the boat slowly
dip down below the horizon as the boat follows the
curve of the sphere.
I am not sure what you mean by 'disappear'.
Usually when a plane 'disappears', it crashes, and
nobody has found the crash site or knows where it
went down. Ditto for a boat that sinks. Airplanes
and boats do not simply vanish, though - we just
don't know their fate, but whatever it was, it
wasn't good (if it were, then the plane or boat
would have reached its destination instead).
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