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Edmund shook his head. "And it isn't like that," he added. "There's nothing particularly exciting about a round world when you're there." –The Voyage of the Dawn Treader, cap. 15

"Well, that's not quite fair," said Eustace. "I mean, it is rather a lark to think that the people in the Southern Hemisphere are having winter when we're having summer."

Caspian turned to him, startled. "How's that?"

"Oh, of course," said Eustace, brightening at an opportunity to show off his knowledge. "Because the Earth is tilted, you see, so that…"

"Tilted?" Drinian interrupted. "I thought you said it was a ball. How can you tilt a ball?"

"On its axis, I mean," said Eustace. "The bit it rotates about."

"Your world rotates?" said Caspian.

Eustace blinked. "Well, of course," he said. "That's why we have day and night. Day when our part of it is facing the Sun, night when it's not."

Caspian frowned. "Perhaps I shouldn't like to visit your world after all, then," he said. "I don't think I'd care to feel the ground continually moving under my feet."

"Oh, you wouldn't feel it," said Eustace. "You'd be moving with it, so you'd feel as though you were standing still. Like when you're in an automobile."

"A what?" said Drinian.

"Never mind," said Caspian hastily, as Eustace opened his mouth to respond. "But how does tilting your world make it summer in one place, and winter in another?"

"Oh, that," said Eustace. "Well, you see…" And then he paused, suddenly baffled – for, though he knew as a formula that the tilt of the Earth was responsible for the variation of the seasons, he had never been clear on the details. "Er…"

It was Reepicheep who rescued him. "If Your Majesty will permit me," he said, "I believe I can suggest a solution for this riddle. Eustace has told us that his world moves about itself; I wonder if it might not also move in space – about its sun, perhaps?"

"Of course it does," said Eustace.

"Then surely the matter is plain," said Reepicheep, and he drew his sword and began to swipe the wood of the ship (provoking a cry of protest from Drinian). "If the world from which Their Majesties hail is indeed spherical, different regions of its surface must necessarily receive sunlight more or less directly. Consequently, they will be warmed thereby to a greater or lesser degree – just as a blow from a cudgel injures more grievously when it strikes directly than when it glances. Now, if, as Eustace tells us, his world does not rotate in a plane parallel to that of the solar rays, then each region of it will stand in a different relation to those rays at various points during its circuit of the sun. When the northern end faces toward the sun, the southern end must face away, and vice versa. Thus, it is quite natural for the two regions to experience the seasons at differing times."

He sheathed his sword again and turned to the others – most of whom were staring dumbstruck at him, for Reepicheep's warlike side was so familiar to each of them that they had never quite thought of him in terms of a knight's other accomplishments. Lucy, however (who, through several months' worth of conversations over the chessboard, had become quite well acquainted with the Mouse's mastery of what he called "the Quadrivial arts"), came forward and frowned at the rude illustrations he had carved with his sword. "I'm not sure I see it, Reep," she said. "Do you think you could give a demonstration of some kind? I might understand that better."

"Gladly, Your Majesty," said Reepicheep. He scampered away towards the galley, and was back in under a minute with a tomato from Aslan's Table. Through this he stuck his sword, which he then held out before him at an angle of about 15°. "Supposing that Your Majesty is the sun – which is hardly implausible," he said with a bow, "and that this tomato is your world, then, when it stands in this position, the northern portion is facing towards Your Majesty and experiences summer, while the southern portion experiences winter. But, as it continues on its path –" and he suited the action to the word, walking in a circle around Lucy until he was directly behind her "– a time must come when these positions are reversed. Then the South will be facing towards Your Majesty, and the North into the sunless void."

Lucy turned around (cautiously, for this is difficult to do on a moving ship) and looked over Reepicheep's shoulder to see his sword still pointing outward in the same direction. "Oh, I see," she said, and laughed. "Silly of me. I was thinking that the axis rotated, too – like when you spin a pocket-watch on its chain, and it's always pointing the same direction. But of course that wouldn't be how this works."

Caspian, meanwhile, was staring at Reepicheep's impaled tomato with interest. "What about at the very top, though?" he said. "Even with the world rotating, it seems that there would be a period when those people never saw the sun at all."

"Oh, it does," said Edmund. "That's why Norway's called the Land of the Midnight Sun: they have those whole days in summer when the sun never sets, and the others in winter when it never rises."

Caspian looked properly impressed. "Where on the ball do you come from?" he asked.

Edmund came forward, studied Reepicheep's tomato for a moment, and then pointed to a spot on its skin not far below the lowest-hanging of the remaining leaves. "About there, I'd say," he said. (This was fairly close to accurate, although the latitude he identified was actually closer to that of northern Scotland.)

"So far to the north?" said Caspian in surprise. "Good heavens, I never guessed you came from such a cold country. You didn't seem to be dressed for it when you arrived."

"Oh, it's not that cold," said Lucy. "It gets a bit nippy in the wintertime, maybe, but no worse than in Narnia. It's because we have warm winds coming off the ocean, I think."

"Off a southerly current, you mean?" said Drinian. "Aye, I can see how that would warm a place." Then he frowned. "Though how you get prevailing winds on a round world is more than I can imagine…"

"Because the Earth rotates, of course," said Eustace instantly. "The air stays more or less still, but the Earth is always moving, so we feel the air moving in the opposite direction from the way we're going."

Caspian shook his head. "Astonishing," he murmured.

"Half a second, Eustace," said Edmund. "That can't be right. The Earth goes from west to east; that's why the Sun rises in the east and sets in the west. If the air was just going the opposite way, then the wind from the Atlantic would blow onto America, not onto England."

"Oh." Eustace blinked. "Um… yes, it would, wouldn't it?"

There was a moment's puzzled silence. It was broken by Reepicheep, who had been thoughtfully rotating the tomato on his sword. "A thought strikes me, Your Majesties," he said. "Perhaps the solution may lie in the differing rates at which the lands of your world travel?"

The others stared blankly at him. "What?" said Eustace.

"Consider," said Reepicheep. "If a round object moves, every part of it must move along therewith. But different parts must travel a greater or a lesser distance. If a group of Talking Ants, for instance, were to stand on the wheel of the Dawn Treader as the Captain veered her to port, an Ant standing near the centre of the wheel would travel a far shorter distance than an Ant at the edge. And so it must be on your world, as well: the further off one is from the centre (where any sphere is widest), the shorter the path one must travel in a day – and, consequently, the slower it must be traveled if one is to move in synchronicity with the rest of the world."

"Then the ground in England actually moves faster than the ground in Scotland?" said Lucy.

Reepicheep cocked his head. "Of course, I do not know these lands," he said, "but if by Scotland Your Majesty means a region that is further north yet than your home country, it is certainly so."

"It makes sense, Lu," Edmund said. "Remember how Uncle Charles used to tell us stories about Kenya, and how the sun rose and set so quickly there? He could never explain it, but it fits with what Reepicheep's saying. If England moves faster than Scotland, imagine how fast Kenya moves."

"But how does that affect the winds, Reepicheep?" said Caspian.

"Suppose," said the Mouse, "that there were some means by which air from the South could be brought northwards. In that case, it would be moving in the same direction as the land beneath it, but more rapidly, for it has just come from a place where the land itself moves more rapidly. Those who lived in the southern regions would thus feel it as a breeze traveling in that direction."

Drinian slapped his thigh. "Right you are, Reepicheep," he said. "And there would be a way of getting that. You've been talking of how Their Majesties' sun strikes the middle part of their world harder than the tips, making it hotter; of course, that would cause the air to go up at the middle and down at the tips. Everyone who's ever lit a fire knows that heat sends things upward."

"I see what you mean, Drinian," said Caspian slowly. "And Doctor Cornelius once said that air was like water: it always flowed to the place where there was least of it. So up in the sky, where the clouds are, the air would flow from the middle to the tips – but near the ground, where one could feel it, it would flow from the tips to the middle again."

"Precisely, Your Majesty," said Reepicheep. "A great, circular River of Wind – or, rather, two Rivers, for there must be one flowing to the southern tip as well as to the northern."

"I think I've heard of that circle of wind," said Lucy thoughtfully. "That's what causes the aurora, isn't it?"

"Well, that's more about magnetism than wind," said Eustace, "but it's the same idea."

"The aurora?" said Caspian, puzzled. "The dawn, you mean? I thought Eustace said that your world's rotation caused that by itself."

"No, not the dawn," said Edmund. "There's this thing that happens up in the far north of our world – up in those places that have the day-long nights – where the night sky is filled with these beautiful patterns of coloured light."

"It's because of the magnetic poles," said Eustace. "You see, the inside of the Earth is all full of hot, melted iron…"

"Like a blacksmith's forge," Lucy added.

"Yes, and as it moves, it creates a force that draws certain of the sun's rays towards the poles. You can't see them usually, because they're a different kind of ray from the kind that lights up the Earth and makes it warm, but when they get into the atmosphere in the Arctic, they mess with the gases in the air and make colours. I saw it once, when Augusta took me to visit her cousins in Ullapool," said Eustace proudly. "It's a ripping sight."

Caspian let out a deep breath. "Truly, you three come from a world of wonders," he said.

"All Aslan's works are wonderful," said Reepicheep. "Blessed be He."

Which seemed to sum up the matter nicely.

Author's note: Before anyone writes to correct me, I realize that I should have added a discussion of the Ferrel cells; as it stands, Drinian's model still predicts that the prevailing winds should blow westward between 30° and 60° N (and eastward between 30° and 60° S). I've let the characters fail to notice this, because (a) I know from personal experience how easy it is to lose track of which motion means which wind pattern in these discussions, and (b) as far as I can tell, no-one really knows why we have Ferrel cells, and we can hardly expect a Narnian seaman to succeed where the whole edifice of Earthly meteorology has hitherto failed. But, apart from that, I'm pretty sure everything's accurate.