In my alternate reality, Earth is not a planet. It is a moon and orbits a gas giant (however it has all of Earth's characteristics, it is also full of humans and life as we know it). This is the only moon the gas giant has. This alternate Earth is tidally locked. This means that the people who live on the "outer" side of the moon have never seen the planet they orbit. And here comes the question: Assuming Astronomy develops as it did on our Earth. When will they be able to discover they don´t rotate around the sun alone?

When I say "when" I am asking which stage of astronomical development. Could Galileo and Copernicus have noticed that? Ptolomeo, perhaps? Or maybe the Greek astronomer Aristarco de Samos (310-230 BC) could have noticed that with his observations of the sky? (No, these are not multiple questions. I am just explaining the type of answer I am looking for).

Of course, as I said before, I am assuming all these inhabitants of the continent on the "outer" side of the Earthly moon have never navigated to the other side of their moon, so they have never seen the big gas giant in the sky.

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    $\begingroup$ It's pretty likely they will circumnavigate their moon, and have a really big HOLY COW moment, before they will figure out the planetary law of motion. In addition, if there are people who live on the other side, their legends / primitive science / etc will probably be passed mouth-to-mouth over their trade routes before anyone circumnavigates. $\endgroup$ – tbrookside Sep 20 '18 at 18:12
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    $\begingroup$ Does anyone know if, at a distance from the sun that's in the habitable zone, the planet would cast enough light on the tidally locked side to effect either weather on the moon OR to have effects in the night sky on the opposite side, by diffusion of that light across the terminator? $\endgroup$ – tbrookside Sep 20 '18 at 19:51
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    $\begingroup$ @tbrookside - Exactly so. The gas giant theory would spread to become common knowledge long before any detailed scientific observation takes place. $\endgroup$ – Richard Sep 20 '18 at 20:07
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    $\begingroup$ @tbrookside, perhaps this answer may help you: worldbuilding.stackexchange.com/questions/11505/… $\endgroup$ – Carlos Zamora Sep 20 '18 at 22:07
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    $\begingroup$ Based on Carlos' link, if there are any people at all on the planet side, the planet is likely to be the central point of a huge proportion of their local mythologies and customs. More than our sun; more than our moon. It's by far the most important thing in the sky, every day. Frankly a story about people on the far side trying to interpret what they're hearing, at a distance and without ever seeing the planet, sounds like an excellent story opportunity all on its own. $\endgroup$ – tbrookside Sep 21 '18 at 15:14

11 Answers 11


I think astronomy should advance to the level of Johannes Kepler (early XVII century) to correctly theorize the presence of a host planet.

To the eyes of early astronomers (like Ptolemy), the world would be still Earth-centric. The only odd thing would be a minor parallax caused by orbital movement. Without any scientifically sound theory of planetary movement, this parallax would be likely explained as a feature of celestial movement.

Copernicus would have every reason to put the sun to the center of the universe and even propose a correct explanation that the parallax is caused by Earth's own movements - but he would have no mechanism to explain these movements. He may theorize the host planet, but this theory would have no way of being proved.

It would take a telescope and accurate observation of another planets to suggest that the most plausible explanation of planet's own movement is the presence of a massive host planet.

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    $\begingroup$ I agree with this answer - but Magellan comes before Kepler, so sea navigation will reveal the secret before astronomy does. $\endgroup$ – tbrookside Sep 20 '18 at 18:13
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    $\begingroup$ @tbrookside - I agree here. They may not even need Magellan, Columbus may be sufficient. $\endgroup$ – Alexander Sep 20 '18 at 18:23
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    $\begingroup$ This is not right. The parallax change is not minor. Do the calculation. And what about the luminosity change? $\endgroup$ – kingledion Sep 20 '18 at 18:43
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    $\begingroup$ No. The size relation between their moon and the planet will be obvious to the earliest astronomers, and then the "earth"centric scheme looks very shifty. $\endgroup$ – Karl Sep 20 '18 at 22:26
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    $\begingroup$ @Alexander The orbit of the earth-moon is dependent on it's day length. If it has a 24 hours day it's not in a wide orbit. Assuming the mass of the gas giant is the same as Jupiter a 24 hours orbit has a radius of about 300000 km $\endgroup$ – Taemyr Sep 21 '18 at 8:45

Unless all the land mass on the tidally locked moon is on the side facing away from the gas giant, the humans will discover they're orbiting a gas giant during the stone age:

Human migration

If the land mass is indeed concentrated on the far side of the moon only, they will discover it a bit later, maybe as early as prehistory but certainly no later than the invention of the lateen sail during Roman times...

Source for map

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    $\begingroup$ Which is interesting, because it also means they will be much less likely to believe the world is flat. $\endgroup$ – Wildcard Sep 20 '18 at 23:34
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    $\begingroup$ Yup, no Flat-Earthers there! :-) @Wildcard $\endgroup$ – Fabby Sep 21 '18 at 0:03
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    $\begingroup$ There being a flat Earth has never been a common belief; even in the middle ages when it was first suggested. The fact that a small percentage of the population believe that it is doesn't make it the popular or accepted believe. $\endgroup$ – UKMonkey Sep 21 '18 at 10:25
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    $\begingroup$ @UKMonkey We were taking pot shot at the 21st century Flat Earthers... $\endgroup$ – Fabby Sep 21 '18 at 10:30
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    $\begingroup$ Simply seeing the gas giant constantly floating in the sky doesn't mean that people will realise they are orbiting it. They have the gas giant god constantly watching over them, and the sun which travels across the sky, sometimes going behind the gas giant. It takes Keplar levels of astronomy to work out that objects in the sky don't orbit the earth. $\endgroup$ – UEFI Sep 21 '18 at 16:02

I think @Alexander 's answer is good but not entirely correct.

Consider the gas giant's axial tilt. Gas giants tend to rotate rapidly due to conservation of angular momentum and the collapse of an enormous volume into a comparatively tiny size of the gas giant planet. That rotation, which would draw the Moon over the gas giant's equator makes the Axial tilt important. Jupiter has a 3 degree Axial Tilt, Saturn has 25 degrees tilt, and Uranus has 98 degrees tilt - basically flipped on it's side.

Due to the gas giant's rotation and equatorial bulge, it's likely that the planet-moon would orbit around the gas giant's equator. That means that the Axial tilt of the gas giant would affect the movement of the moon above and below the gas giant's ecliptic. For the stars, this wouldn't make much difference, but the other planets and Sun would observably move up and down in a sine wave pattern. Against the fixed stars, this would be noticeable against the fixed stars (hmm, Mars was in a different place relative to that star last time around), but probably explained by additional epicycles early-on, similar to the Ptolemaic models that were the standard for over 1,500 years.

It's worth noting that Aristarchus and his early heliocentric model was based on observing Earth's shadow on the Moon and that would no longer be an option, so a version of the Ptolemaic model is highly likely for your scenario.

So, it matters how much your planet-moon moves above and below the ecliptic because that would be observable relative to the other planets and the position of the Sun at Sunrise and Sunset. Even if the movement was less than 1 degree, it would be measurable with equipment and buildings like the Mayans had.

If the gas giant's axial tilt is zero, then the planet-moon simply moves closer and further from the Sun as it orbits the gas giant. That would cause the observed other planet's motions to not move at Keplerian predicted velocity but wouldn't move them up and down.

Up and Down movement would be probably be tied to epicycles up and down from the ecliptic, but it might not be too long before somebody says, "wait, instead of all these circles, if the Earth moves, that would explain everything", and then they get persecuted by the church and all that good stuff.

If the gas giant has close to zero eccentricity, then everything becomes much harder. Kepler was only able to do what he did, using both very careful observations over many years and the best astronomical observation equipment ever made, because the Earth returns to the same spot relative to the Sun every year. Having a fixed observation point makes triangulation possible and Kepler relied on triangulation to work out his formulas.

If you have the planet-moon moving around a gas giant in a tidally locked orbit, you lose that "fixed" position at the same date every year unless the Planet-Moon's orbit around the gas giant and the gas giant's orbit around the Sun are neatly divisible, which is unlikely. Now if the planet is very close to the Moon, it's movement becomes smaller and maybe this problem goes away. If it's a significant variation in distance, then that would makes Kepler's work more difficult.

If you lose the fixed position you can't perform triangulation, or, you need to wait several years for a relatively equal position and you'd need to know how many years to wait. That makes Kepler's calculations much harder and I'm not sure he pulls it off.

And without Kepler, Newton might still work out Calculus, but it's unclear if he works out orbits, which wouldn't make sense based on observation.

Worst case, I would guess that they'd need advanced telescopes to begin to observe objects passing into the shadow of the gas giant (that's funny, I was tracking that object and it vanished), and circumnavigation would almost certainly precede that. It might take a mathematician of the skill of Laplace to work out the specifics from observations of the night sky because your scenario could be quite a bit more complicated. (IMHO).

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    $\begingroup$ "...observing Earth's shadow on the Moon and that would no longer be an option" - Why not? The gas giant would serve as a huge back-drop for casting the inhabited moon's shadow onto. That shadow would be visible half the time, and it would be easy to figure out the shadow's relation with the position of the sun. $\endgroup$ – void_ptr Sep 21 '18 at 16:12
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    $\begingroup$ @void_ptr if you are already on the side of the moon facing the giant, you already know the gas giant is there. tracking the shadow would help determine you are orbiting that thing in the sky, but they will know about their constant companion long before they get around to looking at their own shadow. $\endgroup$ – Mr.Mindor Sep 21 '18 at 16:37
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    $\begingroup$ @Mr.Mindor Sure. But the question asks about "discovering they live on a moon". And observing the shadow can make a difference between "we live on a pretty huge ball orbiting another enormously huge one" and "we live on a ball in the center of the universe, and half of our sky firmament sphere is covered in a fancy colorful swirly thingy". $\endgroup$ – void_ptr Sep 21 '18 at 17:58
  • $\begingroup$ @void_ptr Shadow's are pretty straight forward, and measuring the size of the Earth or theoretical planet would probably precede much of this. Aristophanes did it 2000 yeas ago. Some good telescopes that can track nearby asteroids could notice that something was amiss because the shadow would be too big. $\endgroup$ – userLTK Sep 21 '18 at 22:30
  • $\begingroup$ @void_ptr Earth's shadow on its parent is only visible for significant amounts of the orbit if that orbit is not significantly inclined to the plane of the parent's orbit around its sun. But in that case, this version of Earth would have no seasons (see my comments on the question, above). $\endgroup$ – Robin Saunders Sep 22 '18 at 20:56

There's a factor you haven't considered: what size is the primary, and how far from the primary does pseudo-Earth orbit?

For why this matters, consider a primary the size of Jupiter. Let's say pseudo-Earth orbits at the distance of Ganymede, at 1 million km. At that distance, the Jupiter-sized primary would span roughly 7.5 degrees of the sky (the Moon spans about 0.5 degrees). This means that the primary would be visible for more than half the planet; you wouldn't see it all, but you'd see something freaking big in the sky on the horizon. At a rough approximation, ignoring diffraction and such, you'd see at least part of it from -97 to +97 degrees of longitude (where 0 is directly "below" the primary), and a similar amount past the poles.

If it has all of Earth's characteristics, including the geography and orientation, and the primary is in the best possible position on the equator to get maximum distance from the major landmasses, in the central Pacific, (at roughly 145 W on our Earth), then it would be visible anywhere from about 48 West in the Americas to 118 East in Asia.

That means it's visible in Central America--in fact, they'd see all of it, and Mayans knew astronomy. It would be seen from eastern China, Korea, and Japan...the Chinese also having pretty good astronomers. During the Yuan Dynasty (ie, the Mongol Empire), Chinese astronomers collaborated with Islamic astronomers, which means Chinese observations of that big thing in the sky would have been available to people in Europe by the 13th century, and the space accessible to the Chinese (from Eastern China to Japan) would have clearly shown that it was a disk-like or spherical object as they'd be able to see it rising as they moved eastward.

They might, in fact, have been inspired to send expeditions out even further to get more observations.

Obviously this changes depending on size of the primary and distance from it, but direct observation might not have been as difficult as you assume.

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    $\begingroup$ To add to this a bit: planetary eclipses would be both lengthy and frequent on that particular side of the planet with such a huge swath of the sky occupied. Observers would likely detect the pattern quickly and begin theorizing. $\endgroup$ – Michael W. Sep 21 '18 at 18:46
  • $\begingroup$ Worth also adding that if the planet has rings, then they will be visible even further round. Also, the planet is likely to have additional moons, which will also be visible as easily observable disks from pretty much anywhere. Their motions will definitely help any observers trying to work it out. $\endgroup$ – Spudley Sep 21 '18 at 21:19

Not very long at all. Here is a picture of what Jupiter would look like if it was just over 385,000km away. Reference:https://twistedsifter.com/2012/07/picture-of-the-day-if-jupiter-was-the-same-distance-as-the-moon/

Here is the list of moons of Jupiter and their distances: https://web.pa.msu.edu/people/horvatin/Astronomy_Facts/planet_pages/Jupiters_moons.htm

You will notice that the closest moon is the same distance roughly as our moon; and the next furthest is only double the distance. This means that depending on the orbit, it would be really, really hard to not notice the other body. You'd have to do the same research into the orbital mechanics; but given this sort of work was done in the BC period to a reasonable degree of accuracy; and the size of this thing in the sky clearly dwarfs the sun; I suspect that the research into the planets would start with the massive thing that blocks the sun half the time rather than the sun.

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    $\begingroup$ "Of course, as I said before, I am assuming all these inhabitants of the continent on the "outer" side of the Earthly moon have never navigated to the other side of their moon, so they have never seen the big gas giant in the sky." $\endgroup$ – Micah Sep 21 '18 at 20:09
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    $\begingroup$ @Micah, they wouldn't have to migrate to "the other side". All they have to do is go less than a quarter of the way around and they'd start seeing it. $\endgroup$ – Keith Morrison Sep 22 '18 at 6:00

There have been other questions about habitable moons, and you should read them for other information about hypothetical habitable moons.

This one, for example:

Characteristics of a habitable satellite planet1

I have answered enough such questions that I have got in the habit of referring to my previous answers.

My answer to this question:

What should the size of my moons be? 2

points out that its is considered impossible for a moon to have a month that is the same length as the year of the planet it orbits. A moon's orbit wouldn't be stable unless it orbited the planet at least 9 times during one orbit of the planet around their sun.

If a moon orbits a planet, it should have either a normal rotation period or else have a locked rotation period, so that one side of the moon would always face the planet and one side would always face away from the planet.

If the moon has a normal rotation period, the planet it orbits will be visible for half of every day from almost every part of the moon.

If the moon is tidally locked, the natives of the outer or far side of the moon will never see the planet in their skies and will never directly observe it from their side of the moon. They can only hear about it from natives of the near side or see it themselves when they explore the near side of their moon.

This reminds me of the James Blish story "Get Out of My Sky" (1960) and the Poul Anderson story "The Longest Voyage" (1960).

http://www.isfdb.org/cgi-bin/title.cgi?97951 3


Because of atmospheric refraction of light and libration effects the planet will be visible from some parts of the far side that are close to the line between the far and near sides.

So your far side natives should be limited to only part of the far side of their moon so they will never see their primary planet.

One suggestion would be that there should be a giant impact feature on the far side of the moon from the planet with alternating rings of flat plains and high ring wall mountains. There can be a central mountain range surrounded by higher and dry plains surrounded by lower plains covered by water in a ring shaped ocean surrounded by a circle of land with high mountains in the spine of the circle of land, surrounded by a ring shaped ocean surrounded by a ring of land, and so on.

The central body of land should be like a small continent in size, large enough for a large civilization to arise on the central continent and on the flatter parts of the ring shaped continent of land beyond the ring shaped ocean around the central continent.

The immediate ring shaped continent beyond the ring shaped ocean should have a ring shaped spine of central mountains running all the way around, and they should be high enough for most of the passes to be covered in glaciers all year round. So almost nobody has ever crossed those mountains from one side of the mountains to the other side. As far as people on the inner side of the ring shaped continent - and those on the innermost continent - know the ring shaped glaciated mountain chain could be at the edge of a presumably flat, disc shaped world. They might believe the gods built the ring mountains to keep air and water from falling off the edge of the world.

And possibly any farther out ring shaped continents may also have mountain rings tall enough to be glaciated all year and impassable.

And so the civilizations at the center of the giant impact feature might never have heard anything about the giant planet that is always visible on the other side of their moon.

As said above there should be at least nine month/days when the moon orbits the planet for each year of the planet as it orbits around the sun and possibly tens or hundreds of month/days per year.

The gas giant planet the habitable moon orbits should have an axial tilt, and tidal forces will have regularized the habitable moon's orbit so that the moon's axial tilt will be almost exactly the same as the planet's and so that the moon's orbital plane will be almost exactly in the equatorial plane of the planet.

So the moon will share the axial tilt of the planet, and the lower the axial tilt is the less noticeable the seasons will be on the habitable moon, and the higher the axial tilt the more noticeable the seasons will be on the habitable moon.

So the natives of the habitable moon should notice seasons which are better or worse to some degree for hunting, fishing, food gathering, planting, and harvesting cops. And thus they will keep track of time and develop calendars and observational astronomy to keep track of and predict the passage of time and the seasons.

And the natives will also keep track of their days and nights, which of course will be each be about one half of an orbital period around the primary planet.

The natives on the near side will see the gas giant planet and may assume for many thousands of years that it - along with their sun and other planets - orbits their moon before they eventually become advanced enough to realize that all the planets orbit their sun and that they are on a moon orbiting the gas giant planet.

And the natives of the far side of the moon wouldn't see the planet or know it was there, but will eventually be able to discover that the planets in their solar system orbit around their sun, and that their moon seems to be one of those planets. And then they might discover that there are so many problems with the orbit of their "planet" that making it a moon orbiting a planet that can't be seen from their side is the simplest explanation.

As we all know, every night at midnight the stars that are on a line across the sky from north to south are at the opposite side of Earth - and/or of the hypothetical celestial sphere that for thousands of years they were supposed to be attached to - from the Sun.

On Earth a sidereal year is the time it takes the Earth to make a complete orbit of the Sun as measured against the stars. It is 365.256 days. In an average sidereal year the Earth travels about 1.0146 degrees along its orbit around the Sun. A stellar day is the time it takes Earth to rotate 360 degrees with respect to the stars.

So each day at midnight the stars appear to have moved about 1.0146 degrees from their positions the previous midnight, and over the course of a year the midnight line will appear to move 360 degrees around the celestial sphere to its original position.

But the natives will originally discover what are called tropical years and solar days. Because the Earth moves along its orbit during a sidereal day, at the end of a sidereal day the direction that used to point to the Sun is now pointing about 1.0146 degrees away from the Sun. A stellar day is the time period when the Earth turns 360 degrees relative to the Sun.

And a tropical year is the time period for a complete cycle of the seasons, and is about 365.242 days long.

Suppose that it took exactly 450 month/days of the habitable moon for the planet to orbit its sun. Each month/day the midnight line would point 0.8 degrees off of where it pointed the previous midnight.

Suppose that it took exactly 90 month/days of the habitable moon for the planet to orbit its sun. Each month/day the midnight line would point 4 degrees off of where it pointed the previous midnight.

Suppose that it took exactly 9 month/days of the habitable moon for the planet to orbit its sun. That is about the least possible number of month/days for the planet to orbit its sun. Each month/day the midnight line would point 40 degrees off of where it pointed the previous midnight.

Of course the year of the planet wouldn't be evenly divisible by the month/day of the habitable moon.

Obviously the fewer month/days of the habitable moon there are in a year of the giant planet, the more noticeable will be the differences between sidereal and tropical years, and between stellar days and solar days. And the more month/days of the habitable moon that there are in a year of the giant planet, the less noticeable will those differences be.

In our solar system, and in any solar system like ours, the distances between the planetary orbits will be so great that every planet will look like a dot of light when seen with the naked eye from another planet even at their closest approaches. But when telescopes are invented (first used for astronomical observations from Earth in 1609) and used for astronomical observations some of the planets should show discs in the telescopic views, and thus their phases should be observable.

The differences between the phase cycles of inner and outer planets should provide strong evidence in favor of a theory that the planets orbit around their stars.

All four of the Galilean moons of Jupiter are bright enough to theoretically be seen with the naked eye from Earth. When Jupiter and Earth are closest, their apparent magnitudes range from 4.6 to 5.6. But their angular separation from Jupiter never gets to be more than about the absolute minimum angle that the human eye can see, so they appear as part of the same dot of light as Jupiter.

Even cheap binoculars of the present time are superior to the early telescopes which discovered the Galilean moons of Jupiter that were discovered in December, 1609 or January, 1610. The discovery of the Galilean satellites clearly seen to orbit Jupiter showed that astronomical objects could orbit around other astronomical objects that were not the earth, and was a strong argument in favor of the heliocentric theory.

Since your fictional star system is different from our solar system in some ways - since it has a gas giant planet with a giant habitable moon orbiting in its habitable zone - it could be different from our solar system in other ways, including the relative and absolute spacing of the planets.

Many exoplanets and systems of exoplanets have been discovered, so it is known that the majority of star systems are greatly different from ours in various ways.

For example, CVSO 30 has the widest spacing, in both absolute and relative terms, between two consecutive (known) planets of a star. CVSO 30 c is about 78,998 times as far from their star CVSO 30 as CVSO 30 b is, or about 662 Astronomical Units (or AU) - an AU is the distance from Earth to the Sun.


If the nearest planet to your habitable moon and its giant planet orbits hundreds of AU farther away from their star, it may appear like a mere dot of light in early telescopes, and later and better telescopes, and still later and better telescopes, and so on. It might not be seen as a disc with phases until 20th century telescopes or 21st century telescopes are invented.

And similarly it may not be possible to see moons orbiting such a distant planet, supporting a theory that the planets orbit around their star, until 20th century or 21st century telescopes are invented.

And on the other hand, in some star systems exoplanets orbit many times closer together than any planets in our solar system.

The smallest absolute difference between the orbits of two consecutive planets is between Kepler-70b and Kepler-70c. It is 0.0016 AU or about 240,000 kilometers.

During closest approach, Kepler-70c would appear 5 times the size of the Moon in Kepler-70b's sky.



Note that there are reports of an unconfirmed planet orbiting between the orbits of Kepler-70b and Kepler-70c!

The Kepler-36 system has the smallest known relative difference between the orbits of two consecutive planets. Kepler-36c is believed to have an orbit only 11 percent wider than that of Kepler-36b.

Kepler-36b and c have semi-major axes of 0.1153 AU and 0.1283 AU respectively, c is 11% further from star than b.



The potentially habitable planets in the habitable zone of TRAPPIST-1 also orbit quite close to each other.

The system is very flat and compact. All seven of TRAPPIST-1's planets orbit much closer than Mercury orbits the Sun. Except for TRAPPIST-1b, they orbit farther than the Galilean satellites do around Jupiter,[42] but closer than most of the other moons of Jupiter. The distance between the orbits of TRAPPIST-1b and TRAPPIST-1c is only 1.6 times the distance between the Earth and the Moon. The planets should appear prominently in each other's skies, in some cases appearing several times larger than the Moon appears from Earth.[41] A year on the closest planet passes in only 1.5 Earth days, while the seventh planet's year passes in only 18.8 days.[38][34]


Thus it is possible for some solar systems to have planets so close that they sometimes or always have visible discs as seen with the naked eye from the surfaces of some or all other planets in that system.

If planets are close enough to show visible discs with the naked eye, their phases can be seen with the naked eye, and the naked eye can tell the difference between the phases of inner planets and outer planets, thus forming strong evidence for a theory that the planets orbit around their star instead of the star and planets orbiting round the habitable moon.

In our solar system, some people allegedly have seen phases of Venus with the naked eye:

The extreme crescent phase of Venus can be seen without a telescope by those with exceptionally acute eyesight, at the limit of human perception. The angular resolution of the naked eye is about 1 minute of arc. The apparent disk of Venus' extreme crescent measures between 60.2 and 66 seconds of arc,4 depending on the distance from Earth. Nevertheless it is possible for observers with extremely acute eyesight to see a crescent Venus under ideal atmospheric circumstances.

There have been numerous reports stating such observations. The phases of Venus are alleged to have been seen in Mesopotamian times by priest-astronomers. Ishtar (Venus) is described in cuneiform text as having horns.1 However, other Mesopotamian deities were depicted with horns, so the phrase could have been simply a symbol of divinity.


So in a solar system where planets were a little closer together than in ours people might be able to see the crescent phase of the next innermost planet with the naked eye. And if the planets were much closer together they might be able to see all the phases of that planet with the naked eye.

And if in some solar systems planets can get close enough to show visible discs and phases to the naked eye, in some solar systems planets might get close enough to see moons orbiting those planets with the naked eye, which would be a strong argument in favor of a theory that the planets orbit their star.

It may be noted that it is possible that some humans have seen one or more of the moons of Jupiter with the naked eye.

http://www.denisdutton.com/jupiter_moons.htm 10

Clearly if Jupiter could get half as far, or a quarter as far, as it actually gets, it might be possible to see the Galilean satellites regularly enough with the naked eye to plot their obits around Jupiter. If Jupiter could get much closer than that it would be even easier to see the orbits of the Galilean moons.

So the structure of your fictional star system will determine how advanced the natives of the far side of your moon have to be in order to discover that their world orbits around a point in space that orbits around their star, and that there should be an astronomical body at that point in space, a point in space and astronomical body that their side of the moon is always turned away from.

  • $\begingroup$ While the answer is interesting, most of it does not contribute much to the question on how much the observations of the "natives on the far side" would be different to observations of people on our earth (assuming e.g. never leaving europe). If the day/month length would be similar to ours (i.e. even a bit shorter than Io's) and the year too (i.e. the distance to their sun also similar), what would be different? $\endgroup$ – Paŭlo Ebermann Sep 24 '18 at 12:52
  • $\begingroup$ @PauloEberman There is something called the retrograde apparent motion of outer planets. As Earth orbits the Sun in much less time than the outer planets do, it catches up with them and then passes them periodically, and they appear to move backwards compared to distant stars. Astronomers who believed that the Sun and planets revolved around the Earth had to invent complicated systems to explain retrograde motion, which is explained very simply by the heliocentric theory. continued. $\endgroup$ – M. A. Golding Sep 24 '18 at 20:28
  • $\begingroup$ @PauloEberman And similarly the orbit of the alien moon around its planet would create a similar but much smaller effect every month/day and would seem to make outer planets speed up and slow down every month/day. So if astronomers on your moon believe it orbits their sun they will also notice a smaller and more frequent retrograde effect and eventually deduce that their world orbits a larger planet that can't be seen from their side. $\endgroup$ – M. A. Golding Sep 24 '18 at 20:42

Somewhere around Galileo's time seems most accurate, but in some cases it could take as late as the early 19th century technology.

The first question that would have to be answered is when they decided that the sun doesn't revolve around them. As motion is relative, both "the sun revolves around us" and "we revolve around the sun" are equivalent until you consider the movement of other planets (which move through the sky in complex shapes). So the lower bound for when they realize they are a moon has to be when they can observe the other planets and realize they don't revolve around you.

The parallax caused by rotation around the gas giant is going to be minor, compared to the distance to other planets. It will be much harder to notice. Or will it? Unstated is how far the moon is from the planet. If it's the same distance as the Earth to the Moon (0.3 Mm), the parallax of Mars from one side of the orbit to the other would be about 19 arc-minutes. This was measurable by Galileo, so that's an upper bound. Some of his measurements against mars were detecting movements of 5-6 arc-minutes, so he had the technology.

On the other hand, if the moon is as far as Neso, Neptune's farthest moon, its much more obvious. Neso is a mighty 48,000 Mm away from Neptune, and that will peak at 72,000 Mm at its apocenter. That's a long distance. It's larger than the apihelion of mercury -- meaning at its furthest from Neptune, it's further from Neptune than mercury is from the sun! That'll get noticed much faster!

But the real question is why they have't explored their moon. As we've seen in other answers, humans explored rather quickly. They would find stories of their gas giant dominating the sky rather quickly. If they don't have this, the fun question is why. Why didn't they do the obvious thing? Why did they develop arc-minute accuracy telescopes before they learned to walk around their own planet?

Perhaps the answer is that there are dangerous aliens on the far side of the moon that eat any adventurers who get close to seeing the gas giant. If so, that other species would be a dominating factor in the development of the culture of our race. Everything would be based around dealing with these aliens.

In which case, Galileo might not find it worth his time to stare at the stars. You might develop a pretty spectacular level of technology when facing half a planet dominated by a species that eats you.

In this case, the reality of our skies may not become apparent until the development of modern artillery. WWII artillery could be aimed within 5 arc-minutes, so all it would take is one curious guy pointing his gun-sights at the stars to start collecting the data that shows you are not alone.


Your inhabitants could be able to detect that something's amiss long before they can travel far enough to see the planet. Simple night-time observations would clue them in that there's something behind them, even if they can't tell that it's a planet.

In your setup, the planet is significantly larger than the moon. Large enough, in fact, to be able to completely eclipse the sun. You wouldn't see the eclipse directly since it would only occur at night, but you would be able to detect that you were passing through the eclipse's umbra. Space objects passing nearby (meteors, etc) in the middle of the night would seem to disappear as they passed through the umbra and no longer had any sunlight to reflect off of them. They could be detected when they obscure distant stars, but they would not be directly visible again until they came out the other side of the planet's shadow. An ancient astronomer could observe this phenomenon, make some rough measurements of where the objects disappear and reappear in the sky, and use a crude scale model to show that the "dark spot in the sky" was far too large to be their world's shadow. Something else must be behind them casting a shadow, something that's much larger.


How far is your Earth-moon from the planet?

Let us assume that the Earth-moon is the same distance from the planet that Callisto is from Jupiter. Callisto's semi-major axis is 1.9 million km relative to Jupiter. Assuming that the Sun is still the same, and the Earth-moon is in the same habitable zone, then the distance to the sun is 150 million km.

The diameter of the Earth-moon's orbit around the gas giant is twice the semi-major axis. This forms an isosceles triangle with a vertex angle of 0.025 radians; or 1.4 degrees. Detecting this angle is well within the capabilities of ancient astronomers (as in Babylonian/Chinese/Indian).

Furthermore, in this case, there is a 2.5% variation in distance from the sun with various orbital positions around the gas giant. This corresponds to a 4.9% drop in luminosity of the sun from nearest to farthest point. This too would be readily observable to the ancients...as far back as the Paleolithic, I would think.

If you don't want the Earth-moon so far from the gas giant, then these numbers are reduced. At the distance of Ganymede, this becomes 0.014 radians and 2.8% luminosity, both still noticeable. At the distance of Io, this becomes 0.005 (only 20 minutes of arc) and 1.1 % luminosity. I'd have to do more research on ancient instruments to see how noticeable this is; but it is at least plausible that both would be noticed. Once first detected, many would devise experiments to calculate more carefully, so I think both differences would be detected, even if the Earth-moon were very close to the gas giant.

Observation of these distances

So the ancients would know from observation that neither "Earth-moon orbits the Sun" or "Sun orbits the Earth-moon" is a true statement.

The odd rotation of the Earth-moon around the sun is what the Greeks called an epicycle. The Hellenistic era Greeks explained the apparent retrograde motion of the planets in the sky by a system of epicycles. If they were able to apply this concept to something that does not exist in reality, then we could assume that by 300 BC, Greek astronomy would know that the motion of the Earth-moon was in orbit around something, and that something was in turn orbiting the sun.

As far as travel to the far side of the planet to see the gas giant first hand, that is more a matter of exploring. But one whisper of such and explanation would quickly establish itself, as the most reasonable explanation for why the Earth-moon is apparently orbiting a random point in space.


  • The ancient astronomers would know that neither the Earth-moon orbits the sun, nor the sun orbits the Earth-moon because of discrepancies in rotation rate and changes in luminosity of the sun.
  • By the time of the ancient Greeks, this apparent observations could be (accurately) explained by the existing theory of epicycles.
  • By the time the first explorer got to the opposite side of the world and saw the gas giant, looming enormous in the sky, the cause of the epicycles would be fully explained.
  • How long this exploration would take is up to you and the orientation of planets. If Eart's Old World lay in the away-facing hemisphere; Irish or Japanese fishermen in the Atlantic or Pacific would have seen the gas giant in antiquity.
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    $\begingroup$ A consideration - the farthest point from the sun will correspond to the night on the "Earth-moon". The closest point will be high noon. Analyzing distance effect on luminosity will be quite tricky. $\endgroup$ – Alexander Sep 20 '18 at 18:48
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    $\begingroup$ @Alexander Ok, acknowledged that that is an issue. But the parallax from orbit will be apparent against the stars in addition to the sun, so that is more measurable. $\endgroup$ – kingledion Sep 20 '18 at 18:52
  • $\begingroup$ If Eurasia was centered on the away-facing hemisphere, to get it the furthest away from seeing the primary as possible, it would be seen from the mainland. The Eurasia/African mainland spans roughly 207 degrees of longitude, so someone can see the primary in the sky. $\endgroup$ – Keith Morrison Sep 22 '18 at 6:18

Consider two different scenarios:

  1. The moon is the center of the universe. The planet orbits around the moon. The sun orbits around the planet. Lots of other moons orbit around the planet. Lots of other planets orbit around the sun, with their own moons.

  2. The sun is the center of the universe. The planet orbits around the sun, just like other planets and some junk. The moon orbits around planet, just like other moons.

One could go quite far with the first option, building an ever more complicated model of the celestial spheres.

What makes the second option "more scientific" is that it needs fewer special cases, and that it groups like with like. All planets orbit around the sun, and so on.

Note that both models are equally wrong, but one can go quite far with the helicentric model.

  • $\begingroup$ "both models are equally wrong" - Only in a very binary sense. $\endgroup$ – Kelly Thomas Sep 21 '18 at 0:22
  • $\begingroup$ @KellyThomas, both geocentrism and heliocentrism rely on a sense of exceptionalism. The big leap is accepting many suns with their own planetary systems, with Earth "far out in the uncharted backwaters," as Douglas Adams put it. $\endgroup$ – o.m. Sep 21 '18 at 4:44

They might never find out, they might never even see the sky.

If the planet has a thick, permanent cloud cover, the sky may well be something the inhabitants of the moon never experience through the murky gloom in which they live.

If the atmosphere is violent enough they may never manage to build a craft that can lift them above that cloud cover.

There'd be no need for weird geographical layouts preventing them from traveling to the side of the moon facing the planet.

And if they can't see the sky, they probably won't ever even consider there's anything beyond the clouds, thus never consider building rockets to pierce them and see what lies beyond.


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