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I imagine a planet similar to ours, except for two details:

  1. The presence of everlasting clouds
  2. No yearly seasons

Explanation: In this world clouds are very high and thick, so that sunlight (or starlight, for that matter) only comes through like on a cloudy day here on Earth. There are clouds over deserts (but no rain there) and above the highest mountains. Despite these conditions, enough light and energy reach the planet's crust and allows for life to exist.

The planet's rotation axis is not tilted, so that there are no seasons.

When humanoid civilizations start to appear, they develop myths and religions - usually based on animal figures whom they considered to be gods. Later, after the development of writing and accounting, philosophers start to ponder about the origin of their planet.

My question is threefold:

Without ever being able to see stars or planets, how would such civilizations figure out anything about the universe?

How would the state of their planetary weather shape their development of physics?

What would it feel like when they managed to send their first rocket beyond the clouds?

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    $\begingroup$ We consider for all matters that clouds are everywhere - far higher than mountains (15km +) I'll edit the question. $\endgroup$
    – Daniel
    Oct 12, 2016 at 13:33
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    $\begingroup$ I would recommend Douglas Adams' Life, the Universe, and Everything for an example of a similar culture. Though I recommend it more because it's a good book, not so much because it'd be useful. $\endgroup$
    – DaaaahWhoosh
    Oct 12, 2016 at 13:44
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    $\begingroup$ Not quite the same idea, but possibly useful: Greg Egan's novel Quarantine $\endgroup$
    – John Dallman
    Oct 12, 2016 at 13:44
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    $\begingroup$ The excellent novel Nightfall by Isaac Asimov & Robert Silverberg has an interesting take on a scenario similar to this, in which a planet has multiple suns, and therefore it's almost never dark enough to see the stars. $\endgroup$
    – tex
    Oct 12, 2016 at 15:35
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    $\begingroup$ Does radio astronomy count? $\endgroup$
    – Beta
    Oct 12, 2016 at 16:07

14 Answers 14

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There are a few things they could learn before they get above the clouds.

They can discover that their world is rotating, because pendulums rotate, at a speed dependent on where you are on the surface. They should be able to relate this to day and night, which they do have.

They can observe that the tides have two components, one which matches the length of the day, and one that changes over a period of about thirty days. It would take an enormous leap to come up with the correct solution, and they'll probably have a wrong theory about this, but when they get above the clouds, they'll be able to relate what they see to these factors.

Their development of physics and some kinds of mathematics will be slower, because they don't have a bunch of quite simple things to be explained in the sky. Their biggest problem will be navigation. They can gradually discover latitude, but it will be slow work. Edit: They have to notice that the transition from night to day (sunrise and sunset, though they don't know that) takes longer in some places than others, and that travelling north or south (magnetic compasses will work and will be very important) changes the time taken for the transition. They should be able to figure out from that, and the rotation of the world, that it is spherical.

Once they discover infra-red radiation, they will be able to detect the sun, and just possibly the moon, and this will increase their desire to get above the clouds.

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    $\begingroup$ Without inclination, day and night will be of exactly the same length at all latitudes. $\endgroup$
    – Sir Cornflakes
    Oct 12, 2016 at 14:13
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    $\begingroup$ Every time a ship leaves a harbor, and slowly sinks beneath the horizon as it travels outward—that's easy evidence for a spherical world. $\endgroup$
    – Greg Martin
    Oct 12, 2016 at 16:26
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    $\begingroup$ @GregMartin if they were under perpetual cloud, would the visibility ever be good enough to see all the way to the horizon? $\endgroup$
    – Pyritie
    Oct 12, 2016 at 16:55
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    $\begingroup$ Possibly not, but the OP writes "In this world clouds are very high", so perhaps yes after all. $\endgroup$
    – Greg Martin
    Oct 12, 2016 at 18:14
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    $\begingroup$ Now add the clouds back in. At the equator, you’ll have (roughly) twelve hours of bright light, then a fairly sharp transition to twelve hours of near-complete darkness. Near the poles, you’ll have a bright-ish twilight during most of the day, and a long transition to a dark-ish twilight at night. You’ll also still be getting much less light overall at higher latitudes, so they’ll be colder than the equator, just like here only without the seasons. So I suspect the concept of latitude/North–South will still be recognised early — though it may be harder to measure precisely than on earth. $\endgroup$
    – Peter LeFanu Lumsdaine
    Oct 12, 2016 at 19:59
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OK, let's look on the remaining observables:

  • There is yet day and night
  • The are climate zones on the cloudy planet from the poles to the equator
  • They can measure air pressure in dependence of the altitude coming up with Torricelli's law

At some point in their history, the intelligent beings on the cloudy planet will realise that it is spherically shaped. They will also realise that they receive light and energy from a source beyond the clouds. They will have some model that either the outer source of light rotates around their planet or that their planet rotates and the source is in some fixed direction.

At some point they will decide between those two hypotheses using Foucault's pendulum.

They will probably conclude by extrapolating Torricelli's law to high altitudes that there is a vacuum high above their planet.

They will wonder about the distance of the external source of light and at some point they will try to send up balloons to break through the clouds and make better observations.

After some bad experiences (the first researchers were blinded by the unfiltered light) they will establish observatories on balloons watching the sun and the sky at night.

From there on, the further development of their astronomy will not be too different from the astronomy on a planet with clear skies.

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    $\begingroup$ Actually observing the stars from balloons or even planes is much harder than from earth because you'll have a hard time to keep your telescope pointing in the same direction. $\endgroup$
    – celtschk
    Oct 12, 2016 at 19:15
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    $\begingroup$ @celtschk, just take a fixed star as reference. Balloons don't shake like planes do, so you can adjust the telescope while the balloon gently veers. $\endgroup$
    – Turion
    Oct 14, 2016 at 9:55
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Until they achieve flight or mastery of electromagnetism, the space between the earth and the clouds is the universe

Consider that primitive religions are always built around what the people can actually see or sense. Assuming a similar technological progression, they can learn a great deal about their world without being able to look skyward to the stars. In a more modern comparison, a rural person's view of the world was incredibly limited before the Internet. Everything they knew or talked about came from their tiny sliver of the planet. Now, with the Internet, they can walk around Rome, even if they'll never be able to afford to go there. Rome was always there, just now that poor farmer can see it.

There's gonna be somebody who decides that they want to see what's above the clouds and builds a machine to do it. When they do get above the clouds, their observations will provide a huge(!) kick in the pants for exploration of space.

Flying above the clouds isn't strictly necessary to see outside the clouds. Radio telescopes can easily pierce the clouds to see beyond them. Early radio telescopes can image the skies, perhaps before airplanes or rockets can penetrate it.

Everyone's view of the universe is inherently limited, by any number of factors. The Universe is always there, just they can't see it. Human culture always expands to the limits of the knowledge available (then tends to keep pushing and get even more knowledge.)

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  • $\begingroup$ Given radios are easier to make than spaceships, I think your bolded statement is not necessarily true. Once you have radio telescopes you can explore the stars without having to get above the clouds. $\endgroup$
    – Shufflepants
    Oct 12, 2016 at 19:38
  • $\begingroup$ @Shufflepants, that's a good point. I've amended my answer. $\endgroup$
    – Green
    Oct 12, 2016 at 19:42
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    $\begingroup$ @Shufflepants You don't need a spaceship to get to space. You just need an airplane that flies very high. Staying in space requires entering orbit, which does require a "spaceship". Radio astronomy early success in the 1930s (after airplanes!) only made sense due to non-radio astronomy information. The development of radar then gave radio astronomy a big boost. By the 1930s airplanes where already 13 km up, balloons 16 km up. Flight limits are mainly pressure based; so are clouds. 16 km is above any reasonable cloud level, a similar record on this planet would be as well. $\endgroup$
    – Yakk
    Oct 14, 2016 at 15:35
  • $\begingroup$ @Yakk OP never specified how high the clouds on his planet were. Venus has clouds as high as 60-70 km. Granted, air pressure on the surface of Venus is way higher than on Earth, and in that atmosphere a regular plane would be able to fly much higher than normal due to the increased air density at higher altitudes. But especially in a sci-fi setting, any of these factors might be different than expected and depending on the circumstances, radio astronomy might be far more likely to develop before craft that are capable of breaching the top of the clouds. $\endgroup$
    – Shufflepants
    Oct 14, 2016 at 15:47
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    $\begingroup$ "When they do get above the clouds, their observations will provide a huge(!) kick in the pants for exploration of space." You forget the Krikkits who Due to the dust cloud, the sky above Krikkit was completely black, and thus the people of Krikkit led insular lives and never realised the existence of the Universe at large. Upon first witnessing the glory and splendor of the Universe, they casually, whimsically, decided to destroy it, remarking, "It'll have to go". $\endgroup$
    – TemporalWolf
    Oct 14, 2016 at 22:32
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The logical outcome of perpetual cloud cover is a world that is exceedingly wet. This would have a number of effects, but I'll just list 3 as I'm guessing this is not exactly what you meant.

1. Disease Spreads more Rapidly / Medicine Advances Faster (When Possible)

The very wet environment would help transmit a huge number of pathogens. As a result, people are very motivated to solve medical questions. When advances are possible, those advances occur quickly.

But note that not all advances are always possible. Without the microscope, for example, microbiology would not be an available field. Of course the microscope would probably take longer to invent since there was no need at all for anyone to work on improving lenses for Telescopes except for basic navigation or warfare.

2. Cities Form More Slowly

Due to the wet climate breeding parasites people learn fairly early on that living in close quarters with other people is much less advantageous. In fact, it seems like every large community has a massive disease outbreak every year or two that you might better avoid by living further apart and interacting less.

The only large/successful cities would be those that evolved following fairly strict practices which - as they would discover many hundreds or thousands of years later - happen to be more hygienic.

3. Shipbuilding & Exploration Occurs Faster

Being very wet, everyone needs ships to go any notable distance. Rivers everywhere. So ship building takes a big leap forward. Exploration of new lands occurs much faster as a result of both increased numbers of ships and faster improvements in ship building techniques.

As for how people feel when they do get above the clouds...

In awe, I would suspect. We were pretty surprised when we got into space and found out it wasn't made of ether, but as we'd been staring up there for a long time we had plenty of evidence to help us get over it. Your people would have no idea about what to expect at all.

Send up a plane in the day: The sky is blue and there is a big yellow thing in the sky!

Send up a plane at night: No, the sky is black now and there are stars everywhere. And a moon! Only this civilization literally no words for "stars" or "moon", so they have to describe them in other ways.

Send up a plane at twilight: No, the sky is reddish! I can see the moon, but no stars???

The huge conflict in the day/night cycle and the discovery of all celestial bodies would be significant drivers to motivate people to build more planes and send them up day and night. Space exploration would probably happen at the first opportunity.

Oh, and surely all sorts of religious heck would break loose.

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    $\begingroup$ I like your answer a lot, though I don't agree 100% with the statement that diseases necessarily spread faster when it's wet. It is true that pathogens benefit from it, yes, but I guess in a "wet world" people would naturally adapt to those conditions? Just as people who live in rainforests, for example? $\endgroup$
    – Daniel
    Oct 12, 2016 at 13:54
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    $\begingroup$ This answer seems to focus far too much on the effects of living on a wet world, and not on answering the question $\endgroup$
    – Drgabble
    Oct 12, 2016 at 15:10
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    $\begingroup$ I don't think the day/night cycle would be an issue. Even under cloud cover they'd still have night and day, so it wouldn't be that difficult to figure out that the sky changes depending on how much light the ground is receiving. $\endgroup$
    – Pyritie
    Oct 12, 2016 at 16:54
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    $\begingroup$ @Pyritie - Imagine you go up in the day. The sky is blue. Why blue? I'm not sure they would have a reason to expect the sky to be blue at all. Without any better knowledge, why wouldn't it simply be a dark blue at night? Or really any given color, just without the big light source to illuminate it. Being a pure black void would probably not be the majority expectation. Until we went into space ourselves the consensus was that space was made of something - so it's fair to assume they would think the same, and that it would probably have a color associated. $\endgroup$
    – GrinningX
    Oct 12, 2016 at 17:52
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    $\begingroup$ Point 2 seems to apply very strongly to our own planet, but that didn't stop people living in cities, even before we had sewers or water on tap. $\endgroup$
    – trichoplax is on Codidact now
    Oct 16, 2016 at 13:33
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Short answer: They would, eventually

Wouldn't they have figured out that something (apparently) rotates around their planet and warms up the environment in intervals? A faint glow perhaps like you would see the sun on a cloudy day. Until one day they try to figure out what this glow is. They would have noticed the clouds are only a short distance away from the land, thus they would assume that the clouds are obstructing their view of this ball of fire.

They then invent something that would allow them to see past the clouds, radio telescopes. Note that the discovery of radio isn't quite inspired by astronomy. They would then focus on the sun at first, seeing flares and whatnot. But then they see tiny specs scattered among the darker regions of the sky. This is when they discover that perhaps there's much more beyond the clouds than just the Sun.

Soon after, they begin to document the behaviour of these specs, noting that most move in a linear direction (stars) while some on curves (planets). Using complex math, they eventually figure out the solar system even without seeing it. The difference is that, they only figure this out after the invention of the radio telescope, compared to back then when Galileo discovered Heliocentrism by observing planets and stars, and the Sun.

Some years later perhaps, they would invent the rocket to send people beyond the clouds. Might be something disastrous, but eventually they'll see the beauty of space, and in no time, they would be sending telescopes like the James Webb space telescope into space. fin

About what they would feel going past the clouds? Scared. The feeling you would get if you find out aliens were real. It makes you doubt a lot of your understanding of the universe.

This is a fascinating topic. And correct me if I'm wrong, I'm a programmer, not a scientist.

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  • $\begingroup$ Exactly, some electromagnetic wavelengths can penetrate the clouds and they will be able to "see" the stars with the right technology. (Earth's atmosphere actually does block some lightwaves, if our eyes worked in slightly different ways we may not be able to detect the stars either...) $\endgroup$
    – Matthew Wilcoxson
    Oct 12, 2016 at 16:25
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    $\begingroup$ Well said. I guess the problem would be the nudge to get them to be curious as to what is past the clouds. Planets like Venus are isothermal because of the atmospheric composition, and the thick clouds could possibly hide the Sun completely, BUT, perhaps an asteroid would get them thinking "Where the hell did that come from?" $\endgroup$
    – Brian
    Oct 12, 2016 at 16:44
  • $\begingroup$ This is how we have accurate maps of Venus' surface. Some of our radio telescopes on Earth are strong enough to see through the clouds and get maps of the surface, see newatlas.com/venus-radio-radar-image/36560 This technique works really well for close-ish and large-ish objects, and with better technology it can be improved for more accurate details and smaller objects. $\endgroup$
    – Cody
    Oct 12, 2016 at 19:08
  • $\begingroup$ What about equipment measuring X-ray, gamma or gravity waves? Would there ever be an incentive to even consider such things, or do they require knowledge about stellar remnants? If there were the possibility, one could make the clouds opaque toward any form of EM radiation. $\endgroup$
    – MauganRa
    Oct 12, 2016 at 21:03
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    $\begingroup$ @MauganRa Baby steps. The important thing is to have the people curious enough to wonder what's beyond the clouds really. If radio won't work, they would continue to develop means of looking through clouds: Shufflepant's balloon idea, asteroids passing through clouds, etc. Technological advancements aren't necessarily influenced by Astronomy alone. $\endgroup$
    – Brian
    Oct 13, 2016 at 14:19
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Nephology (The science of clouds) would make a nice starting point. By analysing the clouds with remote detectors (radio/radar for instance) they may find that they receive other radio transmission from beyond the clouds, i.e. the stars.

On another side note, astronomy developed from astrology so perhaps the study of the clouds came from some other mystic concepts. I'd suggest the word "Nepholonomy" (a play on the astrology/astronomy naming convention).

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  • $\begingroup$ I hadn't thought of that. Thanks =) $\endgroup$
    – Daniel
    Oct 12, 2016 at 16:37
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All the other Many of the other answers seem to think that a trip above the clouds is necessary for astronomy to begin. This is not necessarily the case.

If people develop radio technology, radio waves penetrate clouds as if they are not there. It is possible they will develop a primitive radio telescope to track down the source of errant radio noise. And then astronomy can start with radio astronomy. Though I imagine a trip above the clouds may quickly follow as they discover with their radio telescopes that the source of light each day turns out to be a highly localized radio source that is not diffused by the clouds and it turns out there are innumerable fainter localized sources (stars).

And depending on how high the highest clouds on this planet are, it would make astronomy beginning with radio astronomy even more likely. If the clouds are only a couple thousand feet high, all it would take is some hot air balloon or WWI era plane to reach above them.

But if they are 60-70 km high as they are on Venus, it will take some seriously high altitude helium or hydrogen balloons or some really advanced aviation technology or rocketry if we assume that the atmosphere is still as thin at those heights as on Earth (if we assume atmospheric density of something like Venus, surface pressure would be almost 90 times as high as on Earth).

If the clouds were really that high, even if there are those who believe they can be breached, any early attempts to breach them will be met with failure and only fuel ideas that perhaps it is clouds all the way up forever and that attempting to breach them is as foolish as we would consider trying to reach the end of space.

Even if there are early attempts to breach the clouds before the invention of radio telescopes, it make take the invention of radio telescopes before more serious endeavors to breach them succeed if only because the radio telescopes would prove that there is something up there besides more featureless clouds.

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  • $\begingroup$ If they are as curious as we are, they will try to figure out a way to get a closer look at the clouds. They will discover that they can be penetrated, and then will want to look inside. This will result in eventually getting a peek out the other side. Final result, either panic and withdrawal, or further exploration. $\endgroup$
    – WGroleau
    Oct 15, 2016 at 0:19
  • $\begingroup$ @WGroleau My main point is just that depending on the height of the top of the clouds, it may be far easier to invent radio telescopes than to build something to get above them. $\endgroup$
    – Shufflepants
    Oct 17, 2016 at 15:50
  • $\begingroup$ Yes, it may be. If they have both technologies, of course listening is easier than going. On the other hand, if you have no concept of anything "out there," or if your radio technology is not directional, it might take a long time to come up with the idea that the static comes from a particular source instead of being "just something that is everywhere." $\endgroup$
    – WGroleau
    Oct 17, 2016 at 21:55
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It does not have to be a cloudy planet. Simulations of galactic collisions have shown that some stars get flung out alone into space devoid of neighboring stars. The only thing they can see in the night sky are faint smudges of far away galaxies. I have often wondered how beings on this world will interpret the universe. If their solar system has other planets/moons then they can develop astronomy up to a point. If you speculate that they are somehow in a nebula, (though that can imply star forming which changes my initial conditions), then even the distant galaxies can be obscured.
Clearly, there would be no evidence for the big bang, dark matter or dark energy, or even supernova to create elements.

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    $\begingroup$ Krikkit. $\endgroup$
    – gerrit
    Oct 12, 2016 at 17:21
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Short answer: You are describing Venus

Long answer: The world is vastly different, but should proceed astronomically in a similar manner

A canopy of any type of opaque gas will cause a vast warming of the planet. There was a theory with creation scientists that prior to the flood, there was a canopy of water vapor (or alternately hydrogen ice, frozen water and other possibilities) around the Earth that caused an increase in oxygen levels as well as increased atmospheric pressure and decreased ultraviolet radiation that would explain the Biblical timeline of longer lives and of the sudden flood.

This canopy theory has been all but abandoned due to the massive warming that would be evident from the sun warming the clouds, them radiating the heat to the Earth and then trapping it there in a massive blanket. (details in link).

With regard to astronomy, having nothing to see will cause nothing surmised. Depending on the thickness of your canopy, your population will either have no light or a change from brighter to dimmer through the day. Seeing a change in brightness will cause someone to find out why and will spur some to brave the unknown to find out what makes the clouds bright. With increased atmospheric pressure, flight would be easier but dangerous to ascend through the clouds. After solving this they would probably proceed as all other planets we have observed. :)

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  • $\begingroup$ The "canopy theory" shouldn't be relevant if the planet's orbit is out maybe closer to Mars. "Greenhouse" heating might then simply warm the planet to comfortable levels. Titan is too far out, but it illustrates the orbit possibility for perpetual clouds. (Titan's mass could possibly also keep atmospheric pressure reasonable.) $\endgroup$
    – user2338816
    Oct 13, 2016 at 10:31
  • $\begingroup$ Assuming a distance out to mars, the gas used to make the clouds needs to be something other than water. Titan's clouds are methane, ethane and perhaps other compounds. Perhaps I made a bad assumption when reading OP stating "rain" to always be water, but for there to be water vapor in the atmosphere we have a pretty narrow inhabitable band around a sun with which to work. Anywhere within that band would cause some pretty severe heat on the planet. $\endgroup$
    – Keeta - reinstate Monica
    Oct 13, 2016 at 12:29
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    $\begingroup$ So you're saying that Mars never had an atmosphere that allowed a large amount of liquid surface water? AFAIK, current evidence strongly disagrees. But this thread isn't about "Mars"; it's about a fictional planet that probably was formed with far more water than Mars and that kept a strong global magnetic field for longer. $\endgroup$
    – user2338816
    Oct 15, 2016 at 10:14
  • $\begingroup$ No, I'm saying that Mars never had an atmosphere that would support a complete canopy. The two situations are very different. The stabilization point for there to be something that is a continuous "cloud" in an upper atmosphere is very tenuous and it couldn't be water. I am not the one that brought up "Mars" specifically, you are. I am refuting the idea that at our level of star radiation, given YOUR example of "Mars", water vapor is not sustainable as a complete canopy, which is a requirement of the OP. $\endgroup$
    – Keeta - reinstate Monica
    Oct 17, 2016 at 12:27
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Helium filled derigibles with cameras attached. Eventually someone playing with helium (or hydrogen) will think it is neat to fill a balloon and watch it soar. Then one day, they hook a wireless camera to it -- even if they think it'll be lost forever in the clouds, they still want the view of ground. So up it goes. And then it breaks through the cloud layer, and then all the scientists are killed for being heretics and preaching that the world is not the only thing in the universe. But eventually enough people try this and see sun and stars and they move past the "kill the messenger" stage.

Heck, they might do it before wireless just by retrieving the camera after it comes back to the ground. I forgot about the low tech route. :-)

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    $\begingroup$ The low tech route is to send a human up, not a camera. $\endgroup$
    – gerrit
    Oct 12, 2016 at 17:20
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    $\begingroup$ Attaching a human to a helium balloon and letting them crash back to Earth safely is a tech we still have not managed. There was nothing low tech about human-passenger statosphere air flight. $\endgroup$
    – SRM
    Oct 12, 2016 at 19:33
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    $\begingroup$ Who said the human would survive? The only way to realise it's not safe is by trying. Countless of humans in history have died trying to fly... $\endgroup$
    – gerrit
    Oct 12, 2016 at 19:35
  • $\begingroup$ Depending on how tall the cloud layer is you might not be able to send a balloon up high enough before rocket-flight is achieved $\endgroup$
    – dot_Sp0T
    Oct 13, 2016 at 5:28
  • $\begingroup$ Human on helium balloon is a LOT of weight. All sorts of hyperlight weight materials needed compared to camera weight. I'm extremely doubtful of the human approach, even with a suicidal dare devil. $\endgroup$
    – SRM
    Oct 13, 2016 at 13:18
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Other than the light/dark change that occurs with day and night, one small link they would have to space is that comets/meteors would still come in on occasion. The second thing that could give them a lot to ponder is the effect of distance from the center of their planet on gravity and atmospheric thickness.

Well below cloud level it is easy to notice that the density of the air on earth varies a lot; a one mile change in elevation is noticeable in its effect on your stamina just walking around. Additionally, they might have mountains/balloons that get them far enough from the center of their planet to measure gravitational differences from the surface. These two facts could suggest a 'nothingness' beyond the clouds. In contrast, the occasional appearance of meteors/comets as well as the existence of sunlight would show that there had to be a fair amount of stuff beyond the clouds.

You might be able to roughly work out the distance to the sun by solving a system of equations related to light/heat output of a body of a given size at a fixed distance, the gravitational impact of the interaction of their planet and the sun at a fixed distance. If they can see a faint outline of the sun, this would further constrain where it could be. Once they realize how big the sun is, then you can learn that comets/meteors are either orbiting the sun or flying in from some more distant part of the nothingness. Even if the latter is true, some should wind up orbiting instead of hitting the planet; this might let them imagine other planets in the solar system. You can see the effects of planets in each other's orbits, and I can imagine that a sufficiently detailed and really long term study of the tides of their own planet might produce data showing the impact of nearby planets on their moon, assuming they have one.

All this study of things orbiting would probably draw some interest into the question of whether or not something was orbiting them. They might reasonably come to look at tides as a potential source of information about celestial bodies orbiting their planet.

As others have mentioned, the possibilities grow substantially once you can study light outside the visible range.

People have mentioned "awe" and "wonder" as possible reactions to discovering the universe. I'd like to suggest that terror is at least as likely. The sudden realization that the universe is unimaginably vast would probably create a lot of concern about who and what else is out there.

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    $\begingroup$ I find it highly unlikely that they would come up with any theory of gravity other than "things go down" without other planets to observe. What Kepler and Newton did was amazing and they had decades of highly accurate planet motion data and a long history of people looking at the heavens to go on. Without the heavens to look at and without distinct shadows (due to the diffuse light coming in through the clouds), even the fact that the world is a sphere instead of flat may take thousands of years longer to deduce. $\endgroup$
    – Shufflepants
    Oct 12, 2016 at 19:32
  • $\begingroup$ Meteors would be incredibly rare events for people to witness and gather any information from or about. Very few meteors make it to the ground. Most burn up entirely in the atmosphere and it is almost entirely those that people see. $\endgroup$
    – Shufflepants
    Oct 12, 2016 at 19:34
  • $\begingroup$ @Shufflepants Meteors are rare for us on Earth. That tells us almost nothing about this world. But even rare, it's certainly plausible that they're seen and found. A meteorite streaking through the cloud layer before an air burst shouldn't be unheard of, and just a few resulting meteor fragments would be enough to drive interest for generations at the least. Long enough, perhaps, for the next recorded event. $\endgroup$
    – user2338816
    Oct 13, 2016 at 10:43
  • $\begingroup$ @user2338816 I think you're underestimating how rare that's going to be. Halley's comet come around every ~75 years and it's visible to nearly everyone on the planet. And it still took thousands of years of recorded measurements stretching back to at least 240BC until 1704 for some one to even figure out all those measurements were even the same object. $\endgroup$
    – Shufflepants
    Oct 13, 2016 at 15:09
  • $\begingroup$ When was the last time you heard of some one you know, or some one who knows some one you know who saw a meteorite that actually hit the ground? Never? Well, that's going to be even more true for people throughout a history of people who can't see the sky. Especially since they won't see it coming in from way up higher. They are rare, and people are going to have only a fraction of a second to see them between them exiting the bottom of the clouds and hitting the ground (which really limits how far away you'll be able to see it from). $\endgroup$
    – Shufflepants
    Oct 13, 2016 at 15:11
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After being able to light a fire, They'll figure out that light has to have a source. They might initially think it's related to the clouds, but after some simple experiments with water, someone would probably guess that the clouds behave just like regular water vapor.

By fast enough transportation (pursuing the sun or climbing ascending a mountain) they can figure out that the light source just moves to a different location and eventually comes back from the other side.

After enough exploration they can find out that their world is spherical. They'll notice that they can get back to the same point, no matter which direction they choose to go.

These discoveries would lead them to the conclusion that:

  • Either they hover around the light source.
  • Or the light source is hovering around them.

Now, if they have a moon, they'll find out that there is another object above the clouds (by noticing that there is another, smaller source for light, and there are water tides) that they hover around or it hovers around them. This would lead them to the conclusion:

  • At least one sky object hovers around their spherical world.

By noticing/discovering gravity, and by some physics calculation, they would reach conclusions like:

  • If you fall/fly fast enough, you can orbit the world.
  • The moon must be a very big object cause it moves water with its own gravitational field.

By some careful measurements and calculation, they can discover that:

  • The light from the moon is related to the position of the sun.
  • The moon probably just reflects the sun's light, because it simply produces less light.

Lunar and Solar eclipses would lead them to the discovery that:

  • The sun is further than the moon.

When asteroids will fall on their world, they'll discover that something, beyond the clouds is made from a similar material, and by more calculation they would discover, with the assumption that there are a lot more asteroids out there:

  • If an object is big enough, it would form a sphere.

  • Moon and sun are probably spherical too.

Eventually, with advanced enough technology that would let them observe electromagnetic waves, they could discover a lot more of what's going on in our universe.

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Short answer: They wouldn't have astronomy

Long answer: If there's no celestial bodies to be observed, no stars or sun to be recognized, there won't be anything called Astronomy at all. The word planet (or any word meaning planet) might also not come to existence, simply because it implies there being others of the same (but I won't diverge too much here..).
We can deduce that the thing we live on must be spherical, as well as its circumference, as long as we can observe shadows - thus the way physics develop is unlikely to change all-too-much besides branches such as orbital mechanics, etc.

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    $\begingroup$ "Planet" means "wanderer". The idea that the Earth was one of these things and not the center of the universe took centuries to develop. You have sort of internalized this idea, but from a pre-Copernican perspective talking about multiple centers of the universe would be very like discussing the many-worlds theorem today, or perhaps even more bizarre and impractical. I might recommend 'The Clockwork Universe' by E. Dolnick which examines the shifts in worldview during the Copernican Revolution. As to the correctness of your answer, there are two observable celestial bodies: the Earth and Sun. $\endgroup$
    – Kaia Leahy
    Oct 12, 2016 at 14:44
  • $\begingroup$ @sethrin as far as I understood, the sun cannot be seen as it's hidden behind the cloud-ceiling $\endgroup$
    – dot_Sp0T
    Oct 12, 2016 at 15:06
  • $\begingroup$ Visible light encompasses only a very thin part of the EM frequency range. Unless the OP states that all kinds of EM radiation is blocked, there is lots of information which can be picked up. $\endgroup$
    – MauganRa
    Oct 12, 2016 at 21:06
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    $\begingroup$ @MauganRa, Naturally they will eventually have the means, yes. But the astronomy as we refer to that special science will not exist in any way that we know it. Without anything visual, all the EM-Waves that can be measured will not make any sense - not until after they sent the first manned objects above the clouds and understood that they're on one of many spheres. Then science will form new branches that are similar to but are not themselves astronomy $\endgroup$
    – dot_Sp0T
    Oct 13, 2016 at 4:58
  • $\begingroup$ The Sun may not be directly visible but one can detect a day-night cycle and, depending on the exact optical qualities of the cloud layer, detect shadows as well. To figure out what exactly their Astronomy would consist of would require some alt-history, but there are potential observables there. $\endgroup$
    – Kaia Leahy
    Oct 14, 2016 at 13:15
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Radio astronomy

This could be a really excellent opportunity to explore what it would be like if astronomy were discovered at a level of technological development comparable to that of the early 1900s. History could be littered with the corpses of the cooky astronomers (think a mathematician/cosmologist who everyone treated like an insane alchemist), people who inferred the existence of the wider universe from the cycle of days and the tides (if there's a moon or moons). The new radio astronomers would probably be ridiculed for outrageous claims about things nobody can see, only to have the evidence slowly mount that there's a lot going on up there.

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