When we look at the sky, we can see the Sun, the moon and the stars which are millions of miles away from us. But if we stand on the seashore, and look at the horizon, we may not be able to see beyond 4 kilometres. Why is it so? We know that our Earth is spherical in shape, i.e. it has a definite curvature. Though it is flat at the poles and bulges out at the equator, yet the curvature is everywhere. If we stand on the seashore, the higher we stand, the farther we would be able to see. The horizon would appear further away with every increase in height above the sea level. At a height of 6 metres, we can see up to 10 kilometres. From the top of a 90 metres cliff, we can see up to 34 kilometres, while at the summit of 1050 metres on a mountain, we would be able to see up to 130 kilometres. From an aircraft flying at a height of 4800 metres, we might see up to a distance of 265 kilometres. So it is due to the curvature of the Earth, that we cannot see beyond a certain distance.