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How long will it take to discover they live in a moon and not in a planet?
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In my alternate reality, Earth is not a planet. Is a moon and orbits a gas giant (however it has all the Earth 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. It means: the people who live in the outer side of the moon has never seen the planet they orbit. And here comes the question:
Assuming an evolution of the Astronomy Science like our Earth. When will they be able to discover they don´t rotate around the sun alone?
When I say "when" I am talking about at which stage of their astronomy evolution. ¿Could Galileo and Copernico 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 in 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.
science-based astronomy alternate-reality
asked 1 hour ago
Carlos Zamora
1,315216
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up vote
5
down vote
favorite
In my alternate reality, Earth is not a planet. Is a moon and orbits a gas giant (however it has all the Earth 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. It means: the people who live in the outer side of the moon has never seen the planet they orbit. And here comes the question:
Assuming an evolution of the Astronomy Science like our Earth. When will they be able to discover they don´t rotate around the sun alone?
When I say "when" I am talking about at which stage of their astronomy evolution. ¿Could Galileo and Copernico 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 in 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.
science-based astronomy alternate-reality
asked 1 hour ago
Carlos Zamora
1,315216
2
Does your gas giant have other moons? That will make a big difference.
– Mike Scott
1 hour ago
You are right Mike. I have edited the question. It is the only moon the gas giant has.
– Carlos Zamora
1 hour ago
1
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.
– tbrookside
41 mins ago
add a comment |Â
up vote
5
down vote
favorite
up vote
5
down vote
favorite
In my alternate reality, Earth is not a planet. Is a moon and orbits a gas giant (however it has all the Earth 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. It means: the people who live in the outer side of the moon has never seen the planet they orbit. And here comes the question:
Assuming an evolution of the Astronomy Science like our Earth. When will they be able to discover they don´t rotate around the sun alone?
When I say "when" I am talking about at which stage of their astronomy evolution. ¿Could Galileo and Copernico 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 in 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.
science-based astronomy alternate-reality
asked 1 hour ago
Carlos Zamora
1,315216
In my alternate reality, Earth is not a planet. Is a moon and orbits a gas giant (however it has all the Earth 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. It means: the people who live in the outer side of the moon has never seen the planet they orbit. And here comes the question:
Assuming an evolution of the Astronomy Science like our Earth. When will they be able to discover they don´t rotate around the sun alone?
When I say "when" I am talking about at which stage of their astronomy evolution. ¿Could Galileo and Copernico 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 in 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.
science-based astronomy alternate-reality
science-based astronomy alternate-reality
asked 1 hour ago
Carlos Zamora
1,315216
asked 1 hour ago
Carlos Zamora
1,315216
edited 1 hour ago
asked 1 hour ago
Carlos Zamora
1,315216
asked 1 hour ago
Carlos Zamora
1,315216
asked 1 hour ago
Carlos Zamora
1,315216
1,315216
2
Does your gas giant have other moons? That will make a big difference.
– Mike Scott
1 hour ago
You are right Mike. I have edited the question. It is the only moon the gas giant has.
– Carlos Zamora
1 hour ago
1
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.
– tbrookside
41 mins ago
add a comment |Â
2
Does your gas giant have other moons? That will make a big difference.
– Mike Scott
1 hour ago
You are right Mike. I have edited the question. It is the only moon the gas giant has.
– Carlos Zamora
1 hour ago
1
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.
– tbrookside
41 mins ago
2
2
Does your gas giant have other moons? That will make a big difference.
– Mike Scott
1 hour ago
Does your gas giant have other moons? That will make a big difference.
– Mike Scott
1 hour ago
You are right Mike. I have edited the question. It is the only moon the gas giant has.
– Carlos Zamora
1 hour ago
You are right Mike. I have edited the question. It is the only moon the gas giant has.
– Carlos Zamora
1 hour ago
1
1
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.
– tbrookside
41 mins ago
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.
– tbrookside
41 mins ago
add a comment |Â
3 Answers
3
active
oldest
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up vote
7
down vote
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.
answered 42 mins ago
Alexander
15.9k42663
2
I agree with this answer - but Magellan comes before Kepler, so sea navigation will reveal the secret before astronomy does.
– tbrookside
40 mins ago
@tbrookside - I agree here. They may not even need Magellan, Columbus may be sufficient.
– Alexander
29 mins ago
This is not right. The parallax change is not minor. Do the calculation. And what about the luminosity change?
– kingledion
9 mins ago
add a comment |Â
up vote
1
down vote
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.
Conclusion
- 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.
answered 10 mins ago
kingledion
66.2k22217378
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.
– Alexander
5 mins ago
@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.
– kingledion
1 min ago
add a comment |Â
up vote
0
down vote
Consider two different scenarios:
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.
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.
answered 20 mins ago
o.m.
54.7k677182
add a comment |Â
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
7
down vote
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.
answered 42 mins ago
Alexander
15.9k42663
2
I agree with this answer - but Magellan comes before Kepler, so sea navigation will reveal the secret before astronomy does.
– tbrookside
40 mins ago
@tbrookside - I agree here. They may not even need Magellan, Columbus may be sufficient.
– Alexander
29 mins ago
This is not right. The parallax change is not minor. Do the calculation. And what about the luminosity change?
– kingledion
9 mins ago
add a comment |Â
up vote
7
down vote
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.
answered 42 mins ago
Alexander
15.9k42663
2
I agree with this answer - but Magellan comes before Kepler, so sea navigation will reveal the secret before astronomy does.
– tbrookside
40 mins ago
@tbrookside - I agree here. They may not even need Magellan, Columbus may be sufficient.
– Alexander
29 mins ago
This is not right. The parallax change is not minor. Do the calculation. And what about the luminosity change?
– kingledion
9 mins ago
add a comment |Â
up vote
7
down vote
up vote
7
down vote
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.
answered 42 mins ago
Alexander
15.9k42663
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.
answered 42 mins ago
Alexander
15.9k42663
answered 42 mins ago
Alexander
15.9k42663
answered 42 mins ago
Alexander
15.9k42663
answered 42 mins ago
Alexander
15.9k42663
15.9k42663
2
I agree with this answer - but Magellan comes before Kepler, so sea navigation will reveal the secret before astronomy does.
– tbrookside
40 mins ago
@tbrookside - I agree here. They may not even need Magellan, Columbus may be sufficient.
– Alexander
29 mins ago
This is not right. The parallax change is not minor. Do the calculation. And what about the luminosity change?
– kingledion
9 mins ago
add a comment |Â
2
I agree with this answer - but Magellan comes before Kepler, so sea navigation will reveal the secret before astronomy does.
– tbrookside
40 mins ago
@tbrookside - I agree here. They may not even need Magellan, Columbus may be sufficient.
– Alexander
29 mins ago
This is not right. The parallax change is not minor. Do the calculation. And what about the luminosity change?
– kingledion
9 mins ago
2
2
I agree with this answer - but Magellan comes before Kepler, so sea navigation will reveal the secret before astronomy does.
– tbrookside
40 mins ago
I agree with this answer - but Magellan comes before Kepler, so sea navigation will reveal the secret before astronomy does.
– tbrookside
40 mins ago
@tbrookside - I agree here. They may not even need Magellan, Columbus may be sufficient.
– Alexander
29 mins ago
@tbrookside - I agree here. They may not even need Magellan, Columbus may be sufficient.
– Alexander
29 mins ago
This is not right. The parallax change is not minor. Do the calculation. And what about the luminosity change?
– kingledion
9 mins ago
This is not right. The parallax change is not minor. Do the calculation. And what about the luminosity change?
– kingledion
9 mins ago
add a comment |Â
up vote
1
down vote
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.
Conclusion
- 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.
answered 10 mins ago
kingledion
66.2k22217378
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.
– Alexander
5 mins ago
@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.
– kingledion
1 min ago
add a comment |Â
up vote
1
down vote
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.
Conclusion
- 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.
answered 10 mins ago
kingledion
66.2k22217378
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.
– Alexander
5 mins ago
@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.
– kingledion
1 min ago
add a comment |Â
up vote
1
down vote
up vote
1
down vote
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.
Conclusion
- 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.
answered 10 mins ago
kingledion
66.2k22217378
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.
Conclusion
- 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.
answered 10 mins ago
kingledion
66.2k22217378
answered 10 mins ago
kingledion
66.2k22217378
answered 10 mins ago
kingledion
66.2k22217378
answered 10 mins ago
kingledion
66.2k22217378
66.2k22217378
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.
– Alexander
5 mins ago
@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.
– kingledion
1 min ago
add a comment |Â
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.
– Alexander
5 mins ago
@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.
– kingledion
1 min ago
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.
– Alexander
5 mins ago
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.
– Alexander
5 mins ago
@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.
– kingledion
1 min ago
@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.
– kingledion
1 min ago
add a comment |Â
up vote
0
down vote
Consider two different scenarios:
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.
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.
answered 20 mins ago
o.m.
54.7k677182
add a comment |Â
up vote
0
down vote
Consider two different scenarios:
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.
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.
answered 20 mins ago
o.m.
54.7k677182
add a comment |Â
up vote
0
down vote
up vote
0
down vote
Consider two different scenarios:
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.
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.
answered 20 mins ago
o.m.
54.7k677182
Consider two different scenarios:
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.
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.
answered 20 mins ago
o.m.
54.7k677182
answered 20 mins ago
o.m.
54.7k677182
answered 20 mins ago
o.m.
54.7k677182
answered 20 mins ago
o.m.
54.7k677182
54.7k677182
add a comment |Â
add a comment |Â
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2
Does your gas giant have other moons? That will make a big difference.
– Mike Scott
1 hour ago
You are right Mike. I have edited the question. It is the only moon the gas giant has.
– Carlos Zamora
1 hour ago
1
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.
– tbrookside
41 mins ago