"It was, of course, only a very small operation from their point of view, but to Earth it was the biggest thing that had ever happened.
There had been no warning when the great ships came pouring out of the unknown depths of space. Countless times this day had been described in fiction, but no-one had really believed that it would ever come.
Now it had dawned at last: the gleaming, silent shapes hanging over every land were the symbol of a science that Man could not hope to match for centuries to come.
For six days they floated motionless above 'his' cities, giving no hint they knew of his existence. But none was needed: not by chance could those mighty ships have come to rest so precisely over New York, London, Paris, Moscow, Rome, Cape Town, Canberra...
Even before the ending of those heart-freezing days, some men had guessed the truth. This was not the first tentative contact by a race which knew nothing of man. Within those silent, unmoving ships, master psychologists were studying humanity's reactions. When the curve of tension had reached its peak, they would act..."-- Author C. Clark,EARTH AND THE OVERLORDS, Chapter 2