They congratulated each other on their lucky escape, and then busied themselves about various duties aboard the air-craft. The rest of the day was spent in making minor adjustments to some of the machines, oiling others, and in planning what they would do when they reached the moon. In this way three days and nights passed, mainly without incident. They slept well on board the _Annihilator_, which was speeding so swiftly through space--slept as comfortably as they had on earth. Each hour brought them nearer the moon, and they figured on landing on the surface of that wonderful and weird body in about three days more. It was on the morning of the fourth day when, as Mark and Jack were taking their shift in the engine room, that Jack happened to glance from the side observation window, which was near the Cardite motor. What he saw caused him to cry out in surprise. "I say, Mark, look here! There's the moon over there. We're not heading for it at all!" "By Jove! You're right!" agreed his chum. "We're off our course!" "We must tell Professor Henderson!" cried Jack. "I'll do it. You stay here and watch things." A few seconds later a very much alarmed youth was rapidly talking to the two scientists, who were in the pilot-house. "Some unknown force must have pulled us off our course," Jack was saying. "The moon is away off to one side of us." To his surprise, instead of being alarmed, Mr. Roumann only smiled. "It's true," insisted Jack. "Of course, it is," agreed Mr. Henderson. "We can see it from here, Jack," and he pointed to the observation window, from which could be noticed the moon floating in the sky at the same time the sun was shining, a phenomenon which is often visible on the earth early in the morning at certain of the moon's phases. "Will we ever get there?" asked Jack. "Of course," replied Mr. Roumann. "You must remember, Jack, that the moon is moving at the same time we are. Had I headed the projectile for Luna, and kept it on that course, she would, by the time we reached her, been in another part of the firmament, and we would have overshot our mark. So, instead, I aimed the _Annihilator_ at a spot in the heavens where I calculated the moon would be when we arrived there. And, if I am not mistaken, we will reach there at the same time, and drop gently down on Luna." "Oh, is that it?" asked the lad, much relieved. "That's it," replied Mr. Henderson. "And that's why we seem to be headed away from the moon. Her motion will bring her into the right position for us to land on when the time comes." "Then I'd better go tell Mark," said the lad. "He's quite worried." He soon explained matters to his chum, and together they discussed the many things necessary to keep in mind when one navigates the heavens.