A science-fiction writer may, of course, speculate about things that science has not yet discovered. But whenever he deals with what is already known, he should get his facts straight.

That’s what I tried to do in this story. The planet Mercury was depicted as accurately as possible by me, according to the best available data and theories, as of 1957.

The trouble is, scientific “facts” won’t stay put. In the spring of 1965, radar and radio observations indicated that Mercury does not eternally turn the same face toward the sun and that the dark side—even in the course of a very long night—does not get especially cold.

So perhaps this story should not now be reprinted, or perhaps it should at least have been rewritten. But information is still coming in; we are not quite sure that the new data mean what we think they mean; surely all our ideas are due for another upset or two before we get to Mercury and see for ourselves. It is not yet impossible that the older picture may turn out to have been right after all. Be that as it may, theory at the moment is in such a state of flux that one can’t say with any confidence what Mercury, or any other part of the universe, really is like.

Let this older story remain unchanged, then, as an intellectual exercise if nothing else.

—Poul Anderson

“Well, all right! I’ll go to their temple myself!”

“You must be crazy even to think of such a tonteria,” said Juan Navarro. He sucked hard on his pipe, decided it was finished, and knocked out the dottle. “They would tear you in pieces.”

“Quicker than starving to death on this hellhound lump of rock.”

“Very small pieces.” Navarro sat down on a workbench and swung his legs. He was a Basque, medium-sized, longheaded, dark-haired, with the mountaineer’s bony independence in his face. He was also a biologist of distinction, an amateur violinist, and a hungry man waiting to die. “You don’t understand, Joe. Those Dayside beings are not just another race. They are gods.”