Anyhow, last year a section of the wall collapsed and wiped out the only road, making access difficult, and possible only by day. Plans were under way for a cable railway, but work was halted when it was decided to transfer the station down below in a year’s time. At night the station is cut off from the outside world. All radio communication is suspended. Why is that?”

“Sir?”

“Why does all radio communication cease?”

That was the commandant for you. What had begun as a harmless briefing on his Mission had suddenly been turned into an exam! Pirx broke out into a sweat.

“Since the Moon has no atmosphere or ionosphere, radio communication is maintained by ultrashortwave frequency… A network of relay stations, similar to TV transmitters, was constructed to—”

The commandant, his elbows propped on the desktop, twiddled his ball-point in a display of forbearance as Pirx went on expounding on things any schoolchild would have known. He was venturing into territories where his limited knowledge left much to be desired.

“These transmission lines”—he hurtled on, coming upon more familiar waters—“have been installed on both the Far Side and the Near Side. Eight are located on the Far Side, linking up Luna Base with Sinus Medii, Palus Somnii, Mare Imbrium—”

“You can skip that,” the commandant suddenly interrupted in a fit of magnanimity. “Nor is it necessary to hypothesize on the origin of the Moon. Proceed.”

Pirx blinked.

“Radio interference occurs when the relay network enters the terminator… when one half of the network lies in darkness and the other half in light—”

“I know what a terminator is. There’s no need to explain it,” the commandant said benignly.

Pirx coughed and blew his nose. Still, he couldn’t go on coughing or blowing his nose forever.