which provided the motive power for the chariot of Mars, the war god."
"Which is which?" Mayer asked.
"The one that looks about the size of a tennis ball is Phobos," Brink told him. "Deimos is the one that makes you think of a marble."
"I expected them to look bigger and brighter than that," Mayer remarked. "They are both quite close to Mars aren't they?"
To which the older man replied, "That's correct. Phobos is only 5,826 miles from the center of Mars, Deimos is 14,600 miles away—about six one hundreths of the distance between the Earth and Luna."
"They must be awfully small," was Mayer's comment.
"Deimos is about five miles in diameter. Phobos is approximately ten miles across. The apparent diameter of Deimos is about one-fifteenth that of Luna and Phobos, looks as if its diameter is about one-fourth the diameter of the earth's moon. The reason that they are relatively less brilliant than Luna is that they are nearly fifty million miles further away from the sun. The light reflected by Deimos is about 500 times as weak as that of Luna."
"Even if the chariot horses of Mars are smaller and less brilliant than our Earth-moon, I still think they are beautiful," Mayer rejoined. "You don't mind if I watch them, do you?"
"Watch them until you're looney for all I care," the Captain grumbled and went on with his work.
After a few minutes of silence, Mayer said: "Excuse me, Chief."
"Yes?" was the patient response.
"Those two moons, Deimos and Phobos, seem to be getting closer to each other."
"Naturally," Captain Brink answered gruffly. "They can't very well help it."
"But—"
"Don't you understand? They are revolving around Mars in opposite directions. Phobos rises in the west and sets in the east, and it takes only eleven hours to circumnavigate the planet. Deimos rises in the east. It takes 132 hours to complete its diurnal circuit—if that means anything to you."
"Thank you Chief. But how in the universe can Phobos behave in such an erratic, backward manner. I thought every satellite had to revolve in the same direction as the rotation of the mother planet."
"Most satellites do," Brink conceded. "But there are other exceptions. Since we are stationed in Ganymede, you should know that both the eighth and ninth moons of Jupiter have retrograde motions. Phoebe, the outermost satellite of Saturn, also moves in the opposite direction from the other moons. This erratic behavior, as you call it is usually explained by the theory that the moons with retrograde motions were originally independent asteroids which wandered afar and were captured by their respective planets."
"Sounds reasonable," Mayer rejoined. "Much obliged for the lesson in astronomy."
AS Mayer continued to watch the heavenly chariot steeds he was somewhat surprised to see the round disc of Phobos shrink to a lovely, miniature crescent, while the orb of Deimos which previously had been shaped like a distorted eclipse became almost full.
He was interrupted in his observations by the half stern, half good humored voice of Captain Brink:
