“You didn’t have too many times, Dad,” his son had said, “but there were enough. It ties in with other things, anyway. The intervals between signalling and the arrival of the trading torpedo have been varying in a period of just about a hundred and twenty days, taking several years into account Of course, a lot of those ‘periods’ didn’t have any trading occur, but the period is there; first two days, then three. That hundred and twenty days is the synodic period of Mercury — the length of time it takes that planet to catch the Earth up on successive trips around the sun. I remembered Mercury’s position when we studied it this spring, and did some figuring; your short times came when it was closest to us, the long ones when it was on the other side of the sun, about twice as far away. Those torpedoes seem to be coming from there at about one and a quarter G’s of acceleration.” Mr. Wing, though no physicist, understood this clearly enough. The concept had been publicized sufficiently in connection with airplanes.
He had looked over Don’s figures, which were easy enough to follow, and agreed with his results; and the boy had, at his request, drawn a diagram of the orbits of inner planets of the Solar System showing the current positions of the planets themselves. This he now had in his pocket.
The word “home” had just been under discussion, more or less as a result of chance. Mr. Wing had made the concept reasonably clear, he believed; and it seemed to him that the time had come to put one of his cards on the table.
He began by waving an arm to encompass the whole horizon. “Earth,” he said. The Sarrian repeated the word, but without any gesture of his own suggesting that he understood. The man repeated the word, stamping on the ground as he did so; then he took a new page in the notebook and made a sketch of the planet as he thought it would appear from space. As a final illustration, he molded a sphere from a lump of modelling clay which had been found in the playroom and had already been put to good use. Then he pointed to the sphere, drawing, and the ground, repeating the word after each in turn.
Ken understood. He proved it by scratching a picture of his own on the ground, reaching as far as he could over the side of the oven and using his strip of metal. It was a perfectly recognizable drawing of the sun and orbits of the first three planets. He knew he might be exceeding the local knowledge of astronomy, but the fact that the native seemed to know the shape of his world was encouraging.
Mr. Wing promptly pulled out Don’s diagram, which was substantially the same as Ken’s except that Mars’ orbit and position were shown. He spent some minutes naming each of the planets, and making the generic name clear as well. Then they spent some more time in a sort of game; Ken added Jupiter and Saturn to the diagram, in an effort to find out how much astronomy the human being knew. Mr. Wing named those, and added Uranus, Neptune and Pluto; Don, who had made no contribution up to this point, made a correction in the orbit of Pluto so that it crossed that of Neptune at one point, and began adding satellites at a furious rate. They took the burst of Sarrian speech that erupted from the speaker as an indication of the alien’s surprise, and were gratified accordingly.
Ken was surprised for more reasons than one.
“Drai, if you’re listening, these folks are not any sort of savage. They must have a well-developed science. They seem to know of nine planets in this system, and we only knew about six; and there are an awful lot of moons one of them is busy telling me about right now — he’s even put two with Planet Four, and we didn’t notice any. They either have space travel or darned good telescopes.”
“We haven’t seen a space ship here in twenty years,” Feth’s voice reminded him. Ken made no answer; Mr. Wing had started to talk again. He was pointing to Planet Three on his own diagram, and repeating the name he had given it.
“Earth — my home.” He indicated himself with one hand to emphasize the personal pronoun. Then he moved the finger to the innermost world. “Mercury — your home.” And he pointed to Sallman Ken.