# Is it possible for a Halo array to have a 24-hour day/night cycle?

The Halo series features ringed worlds called Halos that are roughly 10,000 kilometers in diameter. You play the game across a variety of locales on the inner surface of these ring worlds, but also across different times of day.

Is it geometrically possible for a Halo-like ring world to have a 24-hour day-to night cycle, where the sun that the Halo orbits is up for about 12 hours, then sets and remains unseen for another 12?

• Certainly. Now, if you want more than 0.2% of Earth gravity, that's a different matter.
– Mark
Jul 24, 2020 at 2:17
• @Mark not necessarily Jul 24, 2020 at 3:54
• @NickS Plug a 5000km radius and 0.000694 rotations per minute into SpinCalc and you find that your ring's only rotating fast enough for about 0.27% of a gee. Jul 24, 2020 at 19:29
• So, with 1 g, you could still have different times of day as seen in the game... but you would have a (roughly) 1 hour and 15 minute day-and-night cycle. Jul 25, 2020 at 0:20

Yes. A rotating angled mirror can be used to reflect sunlight into the ring to match the day/night schedule of your choice. This is common for Bishop Ring habitats, which a Halo qualifies as:

• Minor technical addon: the mirror would have to rotate at almost the same speed as the ring itself. A 5000km radius ring spinning to provide 1g would rotate in about 75 minutes, and you'll want a cycle longer than that. Jul 24, 2020 at 19:32
• Added that detail for clarity.
– rek
Jul 24, 2020 at 21:36

Yes

And it's rather simple.

Have the ring's disk plane point almost at the sun but slightly off. (/) ^sun The backslash is the halo seen from the perimeter. Then the rotation could simply be once every 24 hours and you have half the time where the inside surface is pointed at the sun, and half when it's pointed away.

In another method, you could spin the ring end over end in addition to rotation about the hollow axis, but that probably wouldn't produce as even of a day/night cycle.

• Is that rate of rotation enough to generate gravity? Your second suggestion would not work without constant acceleration, the axes would average.
– rek
Jul 24, 2020 at 17:31
• One rotation every 24 hours of a 10000-km ring will produce 0.2% of Earth-normal gravity.
– Mark
Jul 24, 2020 at 19:58
• Hmm. Both are good points. I didn't do the math when I made my answer. I believe you are right. Do you think I should edit or remove my answer? Jul 24, 2020 at 22:10

Make the rings bigger and it works. Reference to the Culture Series Orbitals by Iain Banks. Wow those are huge!

Put shades that dim or get transparent in any color and time schedule you like.

Put the ring into a shaded area and apply light by mirror.

Close the ring to avoid atmosphere loss and turn the lights off and on. My daughter loves to do this, but she does it faster than in a 24 hours rhythm.