(OOC note: While this is written by a GM, it is not officially canon, since a lot of it is based on ideas Mike had after Eternity was published. In particular, some of this contradicts what's on the terraforming brief, mostly due to me thinking things through a little more since then. The science is also pretty soft and rule-of-cool here, so please don't complain too much.)

Firstly, we need to define a few terms.

Terra Libre orbits Eta Helion in the orbital plane. The period of the orbit (i.e. 1 Terra Libre year) is a little under 270 Earth days. Since a Terra Libre day is a shade over 28 hours, this makes it around 230 Terra Libre days to a Terra Libre year. For the rest of this, “year” and “day” mean Terra Libre day and year, respectively.

With 28 hours to a day, there is an average of 14 hours of daylight - the axial tilt of Terra Libre is pretty small (i.e. the north pole points almost straight up from the orbital plane), so this only changes by about 30 minutes from the longest to the shortest day at mid latitudes.

So far, so simple, but then there's a complexity which we don't need to deal with in the solar system.

Eta Helion is in a nebula. The nebula is a massive cloud of dust, charged particles, and general stuff. And this stuff isn't just sat there, it moves. Which means that Terra Libre is affected by a nebular¹⁾ wind.

Let's call the direction this wind blows from windward, and the opposite direction leeward. Actually, since windward is at a slight angle to the orbital plane, let's call the projections of those directions onto the orbital plane windward and leeward. When facing windward, with your head directed towards Terra Libre north (i.e. the direction extending upwards from the north pole), to your left is port, and to your right is starboard. Terra Libre orbits from windward, to starboard, to leeward, to port.

OK, got that? A diagram might appear here later just to make it clearer.

The thing is, the magnetic field on Terra Libre isn't particularly strong. On Earth, we're also being bombarded by charged particles (from the solar wind), and our magnetic field focuses these towards the poles. When they eventually hit certain levels of the atmosphere, they dump their energy and we get the Aurora Borealis (in the north) and Australis (in the south). Since the focusing effect is not as strong on Terra Libre, there is nothing stopping the charged particles of the nebular wind from reaching the atmosphere all over the planet - so, whatever side of the planet is facing windward is lit up by spectacular aurora.

Now, if that was all that was happening, you'd expect about 14 hours of aurora every day, taking the whole night when the planet was windward of the star, and happening all day when the planet was leeward. Two things stop this.

Firstly, while there's not much magnetic field, there's enough that particles which reach the atmosphere at an angle get funnelled to the magnetic poles (which are pretty close to geographical north and south). So the aurora actually only last about 9 hours when the planet is directly windward.

Secondly, other objects in the system cast a shadow. While Terra Libre isn't all that magnetic, the asteroid belt between Terra Libre and Eta Helion²⁾ is. This means that when Terra Libre is windward of Eta Helion, the nebular wind which reaches it is a lot less strong. The aurora is weaker, and only visible for a short time before sunrise and after sunset (and only then due to the gravity of Eta Helion making some parts of the nebular wind curve to hit Terra Libre slightly side on).

So, in a typical year, the times of dawn(DN)/dusk(DK)/aurora start(AS)/aurora finish(AF) might be as follows. Just as a reminder, the day is 28 hours, so times go from 0000 to 2800

• Terra Libre Windward of Eta Helion
  • DN: 0700
  • DK: 2100
  • AS: 2330
  • AF: 0430
• Terra Libre Windward-by-Starboard³⁾ of Eta Helion
  • DN: 0658
  • DK: 2102
  • AS: 2100
  • AF: 0200
• Terra Libre Starboard of Eta Helion
  • DN: 0653
  • AS: 1630 (visible from now)
  • DK: 2107
  • AF: 2530
• Terra Libre Leeward-by-Starboard of Eta Helion
  • DN: 0650
  • AS: 1200 (not visible to naked eye until close to dusk)
  • DK: 2110
  • AF: 2400
• Terra Libre Leeward of Eta Helion
  • AS: 0400
  • DN: 0645 (aurora not visible after this time)
  • DK: 2115 (aurora becomes visible again)
  • AF: 2400

Alright, got that? No? Well read it again.

OK, so that's for a typical year - but what makes a year atypical, I hear you ask? Well, put simply, an eclipse. but nothing so tame as a moon eclipsing the sun, no. When the asteroid field eclipses the nebular wind completely (about once a century), the aurora goes dark. This generally lasts a couple days. And this would be fine, except that when you call something “the long night”, and one of the light sources that has glowed above humanity's head for decades is going to go out, people get a little twitchy… And when people get twitchy, BAD THINGS HAPPEN.

The Long Night (fic to come)