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I'm working on a story set in an Earth-like world but with rings (say, proportionately similar to Saturn's). I'm trying to get a solid handle on how these rings would affect the sky. For the purposes of this post, let's focus on navigation.

For a little more info, let's assume a location in the Northern Hemisphere, such as in the Midwest USA (Cincinnati, Nashville, Indianapolis, etc.). It's a generally medieval-type setting with no glow from modern city lights affecting the night sky. There are one sun and one moon. Assume similar axis tilt to Earth. There are 425.2 days per year (so a leap year every 5 years) and 28 hours per day. Moon rotation is every 24 days (a multiple of 8 was recommended for the calendar I did at fantasy-calendar.com; compare this to our own moon of about 27 days). ...Just in case any of this is helpful.

QUESTION: How would a person navigate at night in a world like this? For instance, would they be able to use something like a "North Star," or would the ring-glow block out most of the stars? Or maybe the rings would always appear in the south sky? Stuff like that. I'm interested in possible navigation tactics both by land and sea.

Brythan
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Luminocity
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  • Related: https://worldbuilding.stackexchange.com/questions/44924/ discusses how the rings themselves will look from the surface at various times and places. – Cadence Nov 08 '18 at 20:50
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    https://physics.stackexchange.com/questions/238653/why-do-planetary-all-rings-seem-to-be-on-the-equatorial-plane Rings are usually (but not always) above the equator, so navigating is real easy: if you are facing the rings, you are facing south (as long as you are in northern hemisphere). – Bald Bear Nov 08 '18 at 20:56
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    oh, and how wide the rings are tells you how far north you are: http://www.planetary.org/blogs/jason-davis/20130626-earths-skies-saturns-rings.html – Bald Bear Nov 08 '18 at 20:58
  • @BaldBear - thanks for the comments. That's kind of what I thought, about being in the southern sky. However, wouldn't the rings band across the entire sky? I just wonder if it would be difficult to know if you're going due south vs southeast, etc. Same for the other way. I just wonder how feasible it would be to ensure you're going true north just by putting the rings to your back? – Luminocity Nov 08 '18 at 21:08
  • Looks at pics in the second link: rings have edges, and stripes in them, so you can tell if you are heading directly into them, or at an angle. You do mention nighttime navigation, and it will be harder as good portion of the rings will be shaded. But you can still use Polar star, or some constellation near the pole (like Southern cross) – Bald Bear Nov 08 '18 at 21:45
  • @BaldBear - what do you mean by the rings being shaded?

    Also, does the season matter? I've seen it suggested that the rings wouldn't be visible at all during the winter. Can anyone verify?

    To any extent someone would have to use the stars, I just wonder how visible they would be with those rings in the sky.

     – Luminocity Nov 08 '18 at 22:07
  • @BaldBear you should really consider turning your comments into an answer, rather than just comments, so that the OP can select your answer. – Cadrac Nov 09 '18 at 14:24

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Navigation is super easy: as long as you can see the rings, their center is directly south of you. Moreover, height of the rings in the sky tells you how far south you are.

Look at these pictures.
They assume Earth has Saturn's rings. Furtherthe rings are above the equator, i.e. their axis of rotation is same as for earth (and Moon), which is how rings usually are. I believe you assume the same. And I do assume Northern hemisphere.

Rings will not take up the entire sky. Polar regions of the sky will be still visible. They will cover up half the zodiac constellations.

Rings will be visible from temperate and tropical regions. They will not be visible from poles and polar regions. As you travel north, rings will be lower and lower in the sky, and will eventually disappear below the horizon. Exact point where it happens will depend on size of the rings and the planet.

Rings will be visible even at night. , part of the rings will be dark(er) b/c Earth casts its shadow at them, but you can still see portions of the rings are lit by the sun (see last picture). The shaded portion of the rings could be slighly illuminated by light reflected from moon or earth (just like shaded portion of the moon still gets some light). And in any case, rings will block starlight behind it.

As a bonus, you can tell time from position shadow on the rings -- it will move much like the hour hand of the clock.

There will be no seasonal variation, or daily variation in height or location of the rings. For that, you need rings rotating around a different axis from the the planet, which is rare but possible.

A large rock in the rings will likely move, kinda like our moon. It might also grow over centuries.

Bald Bear
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Not an expert by any means, but I did a bit of Googling around because this question is really interesting. I found this snippet in an article on the NASA website, discussing the rings of Saturn:

Researchers have discovered that while most of the ring particles are as small as dust and pebbles, there are a few chunks as big as mountains, and even some small moons several miles across embedded in the rings.

This suggests that within the rings there could well be some (much) larger fragments, like moons. Perhaps on your planet, there's one such chunk of rock in the rings which always sits over a specific spot on the equator. If it's big enough and doesn't move relative to the planet's surface, it could be used to navigate.

Also if you're in the Northern Hemisphere, the rings would always be to your South. So the 'peak' of the rings in the sky (like the top of a rainbow) would lead you directly South from wherever you currently are if I'm not wrong.

Sources:

NASA article about Saturn's rings: https://www.nasa.gov/centers/goddard/news/topstory/2009/rings_equinox.html#maincontent

user43712
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    That would require the ring to be in geostationary orbit; geostationary orbit is significantly out there, and wouldn't produce a produce a ring proportionally similar to Saturn's. – Gene Nov 09 '18 at 16:37
  • Is it feasible that such a large or uniquely identifiable rock within the rings could be in the same position consistently, such as every night? It brings to mind a concept of the "South Rock," a huge rock in the ring that appears every night at true south. Put this to your back, and you get true north. Of course, I suppose that would only work in northern hemisphere...and I also wonder if this would hold true in different eastern and western positions. – Luminocity Nov 09 '18 at 17:12
  • @Gene I assumed geostationary orbit, which isn't feasible. Thanks for correcting me! – user43712 Nov 10 '18 at 17:15
  • The altitude of geo-stationary orbit for Saturn is 53,811 km. The altitude of Saturn's rings are 7,000 km to 80,000 km. So, yes, a very thin slice of Saturn's rings are in geosynchronous orbit. So this should be possible on ringed-Earth also. The question is whether a tiny ring-embedded moon with a diameter of merely several km would actually be visible with the naked eye. How white/reflective the ring and moon are would affect this. – cowlinator Dec 18 '20 at 01:47
  • A geostationary moon would be unnecessary for determining north, because the apex of the rings would always be directly south (or north); however, the moon would be very helpful for determining your longitude. – cowlinator Dec 18 '20 at 01:51
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Day or night the rings would always be visible to the planet (assuming they full rings like Saturn)

The only point at which the rings may not be visible is if in heavy overcast or you were at a point on the planet whose position perpendicular to the ring's planar origin. (worded this way incase the planet somehow rotated different with respect to the ring) In this case they would be hidden in the horizon.

Since in just about every possible situation the rings position can be anticipated it can therefor be used for navigation. Similar in function to how the "sun rises In the east and sets in the west".

anon
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