Planet E’s orbit was .55 AU in radius, closer to Tau Ceti than Venus was to Sol, but Tau Ceti emits only 55 percent of the luminosity of Sol, so E and E’s moon were receiving 1.71 times as much stellar radiation as Earth, while Venus receives 1.91 times as much. E’s moon, now called Aurora by everyone, orbited in a tidally locked, nearly circular orbit of E, at an average distance of 286,000 kilometers. E’s mass created a gravity of 3.58 g; Aurora’s was .83 g. This was the main reason they were going to try to occupy Aurora rather than E, which, though it fell in the class called “large Earth,” was too large, or to put it more precisely, had too strong a gravitational pull at the surface, for their rockets to launch off it, let alone for them to feel comfortable or even to survive.

Aurora received light both directly from Tau Ceti and by way of a powerful reflection of Tau Ceti’s light from the surface of E. This reflected sunlight (taulight?) was significant. Jupiter, for comparison, reflects about 33 percent of the solar radiation that hits it, and E’s albedo was almost as great as Jupiter’s. The sunlit part of E was therefore quite bright in Aurora’s sky, whether seen by day or night.

The surface of Aurora therefore experienced a complicated pattern of illumination. And because it was tidally locked to E, as Luna is to Earth, the pattern was different for the E-facing hemisphere and the hemisphere always facing away from E.

The hemisphere facing away from E had a simple pattern: its days and nights lasted nine days each, the day always full sunlight, the night fully dark, being starlit only; and never a sighting of E at all.

The hemisphere facing E had a more complicated pattern: its nine-day-long solar night included a very considerable amount of reflected sunlight from E, which always hung in the same spot in the sky, different spots from different parts of Aurora, but always fixed, while going through its phases. Nights on the E-facing hemisphere of Aurora saw E go from quarter phase (circle half illuminated) to full moon around midnight, waning to quarter phase again near dawn. Thus there was always significant E light during this side of Aurora’s solar night. The darkest time in this hemisphere actually came at noon during its solar day, when E eclipsed Tau Ceti, so there was neither taulight nor E light on the part of Aurora that was eclipsed, which was a very broad band across the middle latitudes.

There were also narrow libration lunes at the border of the two hemispheres of Aurora, in which E, while it went through its phases, rose and dipped just above or below the horizon. This libration rocking happened everywhere of course, but it was not so easy to see when it was high in the sky against the always shifting background of stars.

Possibly a diagram would make this regime clearer, but the analogy of Luna to Earth may help to make things comprehensible, as long as it is kept in mind that from Aurora, E seemed much larger in the sky, bulking about ten times larger than Earth seen from Luna; and because its albedo was high and it received 1.71 of Earth’s insolation, it was much brighter as well. Big, bright, and from wherever one stood on the E-facing hemisphere of Aurora, always fixed in the same spot in the sky, granting a slight libration shift. At their landing site, this spot was almost directly overhead, only a bit south and east of the zenith: a big glowing ball of a planet, waxing and waning slowly. “When we learn the phases, we’ll be able to use it like a clock,” Euan said to Freya. “A clock or a calendar, I don’t know which to call it. The day and the month are the same thing here. Whatever we end up calling it, it’s not a unit of time we had on the ship.”

“Yes we did,” Freya said to him. “Women’s periods. We brought the months with us.”

“Ah yes, I guess we did. Well, now they’re back in the sky again. But only eighteen days long. Wonder if that’s going to mess things up.”

“We’ll find out.”

They had chosen to land on Greenland partly because it was in the hemisphere facing E. Someone said that if one were standing on E looking up at Aurora, Greenland would have been located on the disk of Aurora about where a tear would leave the left eye of the man in the moon, on Luna as seen from Earth. A nice analogy.

The complicated regime of light on Aurora created very strong winds in its atmosphere, and the waves on its ocean surface were therefore often very large too. These waves had a very long fetch, indeed in some latitudes they never encountered land at all, but ran around the world without obstruction, and always under the pull of .83 g, so that they often reached a very large amplitude, well over 100 meters trough to crest, with the crests a kilometer apart. These waves were bigger than any that ever occurred on Earth, except in tsunamis. And since they never went away, during the nine-day nights the ocean surface froze over only in certain bays, and in the lee of various islands. When the time came for people on Aurora to take to the ocean, a time many spoke of enthusiastically, the seafaring would involve considerable challenges.

“So, now we’re about to go outside the station,” Euan said into his helmet speaker. Within the ship, 287 people were listening to his feed, while 1,814 people listened to other members of the landing expedition now leaving the station. 170.043, A0.3.

“Suited up, the suits are pretty flexible and light. Good heads-up display in the faceplate, and the helmet’s a clear bubble, at least all of it I can see, so it feels fine to have it on. The g feels just like on the ship, and the air outside is clear. Looks like it might be windy, although I don’t know why I say that. I guess I’m hearing it pass over the station buildings, maybe the rocks too. We’re far enough from the sea that it isn’t visible from here, but I hope we’ll be taking the car west until we get to the bay west of here and can have a look at the ocean. Andree, are you ready to go? All right, we’re all ready.”

Six of them were going out, to check on the robotic landers and the vehicles that were ready to drive. If the cars were good, they would take a drive west to the coast, five kilometers away.

“Ha-ha,” Euan said.

Freya settled down to listen to him and watch the view from his helmet cam.

“Now we’re outside, on the surface. Feels the same as in the ship, to tell the truth. Wow, the light is bright!”

He looked up, and the camera view of the sky flared with Tau Ceti’s light, then reduced it with filters and polarization to a round brilliance, big in the royal blue sky—

“Oh wow, I looked at it too long, I’ve got an afterimage, it’s red, or red and green both at once, swimming around. Hope I didn’t hurt my retinas there! I won’t do that again. I thought the faceplate would do better at filtering. It’s going away a little. Good. All right, lesson learned. Don’t look at the sun. Better to look at E, wow. What a giant round thing in the sky. Right now the lit part is a thick crescent, although I can see the dark side of it perfectly well too, I wonder if that comes through in the camera. I can see cloud patterns too, just as easy as can be. It looks like a big front is covering most of the dark part, sweeping into the part that’s lit. I’ve got a double shadow under me, although the shadow cast by E light is pretty dim—

“Wow, that was really a gust! It’s very windy. There’s nothing to show it here, the rocks are just sitting there, and I’m not seeing any dust blowing. The horizon is a long way off.”