Night sky with no stars

In My Humble Opinion
1

In Iain M Banks Against a Dark Background, the story takes place in a solar system that’s not in a galaxy. This is not a spoiler, since it’s never discussed in the book and is not important to the plot. There’s a few hints, such as the title, but most readers probably miss them. (I don’t even recall where I learned this factoid about the book; it wasn’t from reading it.)

Anyway, there are some stars out there in intergalactic space. My understanding is that they’re all former galactic stars that got ejected. In fact, we also know of a few high velocity stars that are on their way out. They’re going too fast to stay in a galactic orbit. I was wondering what would be the effect if this had happened to our Solar System.

So consider this hypothetical situation: The Sun was ejected very early in its life so that now it’s so far away from anything that there’s nothing outside the Solar System that’s visible to the naked eye. I think that would be somewhere around 4 or 5 million light years away from the Milky Way, but perhaps somewhat further. And it would likely have to go towards the Local Void so that no other galaxy is visible.

Other than this ejection event, nothing else changed for the Solar System. That is, none of the planets’ or other bodies’ orbits were affected. However, since long period comets are thought to be the result of other stars passing through the Oort Cloud and disturbing the orbits of objects there, once the Sun leaves the Galaxy, there’ll be no more generated. The Sun would leave the Galaxy in a few million years at most, so virtually none are left by the time it’s a couple billion years old. But we’ll assume that that didn’t make any significant changes. So, for example, the Chicxulub impactor was either not a long period comet, or if it was one, it started off so large and in such a long orbit that it survived 4 billion-plus years.

The stars’ primary use has historically been for navigation. GPS has largely made this use irrelevant, but there are still some times when they are needed. For example, they are still used for navigation of spacecraft. But before GPS and even before the invention of the compass, would nighttime navigation have been impossible without stars? I really don’t know.

So what’s visible to the unaided eye in this hypothetical night sky? The Moon and five planets (Mercury, Venus, Mars, Jupiter, and Saturn) will be the main things. Would those collectively be enough to determine which direction is north?

There would at times be other things visible to the naked eye. Uranus is visible when the seeing is good, but it looks like a very dim star. Without the competition of a couple thousand other very dim stars, it would be discovered in prehistory. It probably could help navigation when visible. It’s above the horizon about half the time, but seeing is often not so good so there’ll be lots of times when it can’t be seen even though it’s above the horizon.

A few asteroids make brief non-periodic appearances. Vesta becomes naked-eye visible for a month or two every three years on average. It’s only visible at favorable oppositions, so not every orbit. On especially favorable oppositions, it’ll get to 5th magnitude, which is significantly brighter than Uranus. Near-Earth asteroids (Apophis, for example) make occasional close passes where they’re naked-eye visible for short periods of time (anywhere from a few hours to a couple days). Short period comets will also be visible at various times. But none of these would help with navigation.

Now what would be the effect of this on navigation before the invention of the compass? Would the Moon and planets be enough to determine which way is north? Would navigators risk sailing away from coasts when they can’t determine which way they’re going at night? Or maybe they can, but only when the Moon is up, so half the time they restrict their sailing to coasting only.

How might this have affected history? For example, there are a number of places in the Mediterranean where, even before the compass, ships took shortcuts out of sight of land. Also there’s a number of islands which required such sailing to get to, and many of those were inhabited in classical times. If they can’t tell which way is north at night, would they still take those shortcuts or go to those islands?

As I understand it, Norse navigators sailed (roughly) along lines of latitude by keeping astronomical objects at a fixed altitude (after adjusting for time of day/night). Would they be able to discover Iceland and Greenland without stars?

Polynesian navigators also used the stars along with various other clues. Would they have been able to expand the way they did without stars?

Another effect of no stars is that the science of astronomy will have a much different development. I’m not sure how or even if it would develop, since there would be no reference constellations to measure the movements of the planets.

This is a big topic, covering science Fiction, astronomy, navigation, and history. I asked some specific questions, but don’t feel restricted to discussing only those. Feel free to discuss any aspect of it. I may have made some invalid assumptions along the way and I’m sure someone will point those out.

2

They may not have seen individual stars, but maybe they were able to see other galaxies, and used them instead?

3

You’re talking about Banks’ book, I assume. I’m not sure if they could see galaxies with the naked eye, perhaps as faint smudges, or not. At the time of the book, they’ve developed interplanetary travel, so they’re way beyond just naked-eye astronomy. The few hints that I remember don’t help.

4

Forget ocean navigation, prior to controlling fire you wouldn’t want to leave your cave after nightfall. On moonless nights, it would be DARK! You could get lost an arms length from your figurative front door.

Wonder what that would do to culture. No stars to ponder, might as well sleep. The study of philosophy might be delayed thousands of years.

eta: it might be psychologically claustrophobic. As far as anyone can tell they are the sole inhabitants of creation. The invention of the telescope might actually make it worse. In what was previously a nearly empty universe, now there are these little things in the sky to ponder.

5

In the scenario you describe, I think we would see some neighboring galaxies as stars - tiny points of light. Through a telescope, they may reveal smudgy blurs.

6

You’re kind of fighting the hypothetical here. I’m not sure how far away the Sun would have to be for the Milky Way (and Andromeda Galaxy) to not be naked eye visible. Andromeda is naked eye visible, but it’s only 2.5 Mly away. So is the Triangulum Galaxy, and it’s smaller and slightly further away. NGC 55 is about 6.5 Mly away and is not visble. The Wikipage on IC342 says it would be visible if it weren’t for the obscuring gas and dust in the line of sight. Its distance is uncertain, but could as little as 7 Mly.

Anyway, for the sake of this thread, just assume no galaxies are naked eye visible.

7

The sun can be used for navigation. At local noon, when shadows are shortest, they point due north (or south in the southern hemisphere).

I’ve seen Venus cast shadows in the predawn hours. Jupiter and Mars can get pretty bright too, so there would be moonless nights when it isn’t completely dark. Life might evolve even better night vision than what exists now.

8

I think the phases of the Moon could be useful as well, after sufficient observation time. The direction of the terminator line is dependent in part on your position on the Earth. Might not be as accurate as using stars, but probably good enough for at least some uses.

Combine that with a decent ephemeris of the visible planets, and I think you’d be okay.

9

How do you create ephemerides with no fixed objects in the sky?

10

Navigation wouldn’t be a problem. At worst, you just don’t travel at night when the moon isn’t visible. Not really a hardship, since who wants to travel when it’s pitch dark?

11

If you’re out in the open ocean, you can’t just stop travelling when the sun goes down. Doesn’t work that way.

12

Then the prevailing wind should give you something of a clue until the sun, moon, or a planet rises.

13

Seems to me, fixed objects in the sky aren’t necessary.

Scientific ephemerides for sky observers mostly contain the positions of celestial bodies in right ascension and declination, because these coordinates are the most frequently used on star maps and telescopes. The equinox of the coordinate system must be given. It is, in nearly all cases, either the actual equinox (the equinox valid for that moment, often referred to as “of date” or “current”), or that of one of the “standard” equinoxes, typically J2000.0, B1950.0, or J1900. Star maps almost always use one of the standard equinoxes.

“right ascension” and “declination” are defined entirely with regards to the Solar System. It might be harder to decide on using such a system without other elements worth recording, but there’s nothing in principle to suggest that the people on this Intergalactic Earth could not come up with this system, given time and motivation.

14

How do they measure right ascension and declination without some fixed object to measure from? Also, this is about before the invention of the compass, which was way before the invention of the marine chronometer, so they have only a vague idea of the current time.

15

If you already have a good theory of the orbit of the Earth around the Sun, then you can define various celestial reference directions. However, in the story, how far has astronomy advanced with a paucity of fascinating objects in the sky to look at?

16

People were building things like Stonehenge that marked the position of the Sun on important dates like the solstice thousands of years before we had compasses. The fixed position was where we were standing on the Earth.

It wouldn’t be too hard to incorporate some observations of the visible planets, if they’re not being overwhelmed by all the stars we see. Heck, observing the planets would become even more important, since they are one of the few things we’d see, and not just be weird outliers mixed in with all the fixed stars.

Give a few thousand years for people to reconcile the different observations of the solstice and whatnot from various geographical positions, and I think we’d get it sorted out.

17

Dunno. The planet in Dark Background was already at an advanced tech level throughout the story. He never went into the details about how they got to that point.

Great. At Stonehenge they can measure right ascension and declination. How do they do it in the open ocean on a moving and rocking platform?

18

At least accurate enough to hit North America!

Plus, you’d have the Moon as a reference for at least half the month.

19

With no annually changing background stars, wouldn’t it be easier to believe the sun revolved around the Earth? Without background stars, would planetary retrograde motion be easily spotted? Maybe the universe really does revolve around the Earth!

20

From one night to the next, the movements of the planets (particularly the gas giants) would be quite similar to that of the sun. Retrograde motion should still be noticeable on longer timescales when measured relative to fixed objects on Earth’s surface.