<h2>Part One: Moons of other planets.</h2>
<p>The discovery of the Galilean moons of Jupiter in 1609-1610 proved that not everything had to orbit directly around one single object.</p>
<p>The discovery of the Galilean moons showed that objects could revolve around an object which revolved around another object.  There was no way for all known objects to revolve directly around each other.</p>
<p>Either:</p>
<ol>
<li>The Sun, the Moon, and the planets revolved around Earth, and the 4 Galilean moons revolved around Jupiter, thus only indrectly orbiting the Earth.</li>
</ol>
<p>or:</p>
<ol start="2">
<li>Earth, the Moon, and the planets orbited around the Sun, and the 4 Galilean moons revolved around Jupiter, thus only indirectly orbiting the Sun.</li>
</ol>
<p>or:</p>
<ol start="3">
<li>The Sun, the Earth, the other planets, and the 4 Galilean moons, all revolved around Jupiter, and the Moon revolved around Earth, thus only indirectly orbiting Jupiter.</li>
</ol>
<p>Decades later, Titan was discovered in 1654, and four other large moons of Saturn were discovered in 1671 to 1684.</p>
<p>So now there were three objects in the solar system which certainly had objects orbiting them, Earth, Jupiter, and Saturn, and there was a big astronomical question whether everything ultimately orbited the Earth or the Sun.</p>
<h2>Part Two: Phases of the Planets.</h2>
<p>When telescopes were first used to observe the planets starting in 1609-1610, the phases of the planets began to be observed, phases caused by the changing angles between the Sun, the planets, and Earth.</p>
<p>It became noticed that the phases of Mercury and Venus went all the way from thin crescents to 100 percent full, and that their angular diameters were larger when they were thin crescents and smaller when they were full. Half the times they appeared close to the Sun they appeared small and full, and half the times they appeared close to the son they appeared large and crescent shaped. That was consistent with them orbiting around the Sun at a distance less than the distance between Earth and the Sun.</p>
<p>But the phases of Jupiter and Saturn were different.  They ranged from slightly more than half full to totally full.   Jupiter and Saturn always appeared smaller and less full the closer they were to the direction of the Sun but were never less than half full.  They never appeared larger and less than half full when they were close to the direction of the Sun.  So they must have orbited around either the Earth or the Sun at distances greater than the distance between the Earth and the Sun.</p>
<h2>Part Three: Parallax.</h2>
<p><strong>Diurnal parallax</strong></p>
<blockquote>
<p>Diurnal parallax is a parallax that varies with rotation of the Earth or with difference of location on the Earth. The Moon and to a smaller extent the terrestrial planets or asteroids seen from different viewing positions on the Earth (at one given moment) can appear differently placed against the background of fixed stars.[12][13]</p>
<p>The diurnal parallax has been used by John Flamsteed to measure the distance to Mars at its opposition and through that to estimate the astronomical unit and the size of the solar system.[14]</p>
<p><sub>https://en.wikipedia.org/wiki/Parallax#Distance_measurement</sub></p>
</blockquote>
<p>john Flamsteed lived from 1646 to 1719.</p>
<p><strong>Solar parallax</strong></p>
<blockquote>
<p>After Copernicus proposed his heliocentric system, with the Earth in revolution around the Sun, it was possible to build a model of the whole Solar System without scale. To ascertain the scale, it is necessary only to measure one distance within the Solar System, e.g., the mean distance from the Earth to the Sun (now called an astronomical unit, or AU). When found by triangulation, this is referred to as the solar parallax, the difference in position of the Sun as seen from the Earth's centre and a point one Earth radius away, i. e., the angle subtended at the Sun by the Earth's mean radius. Knowing the solar parallax and the mean Earth radius allows one to calculate the AU, the first, small step on the long road of establishing the size and expansion age[20] of the visible Universe.</p>
<p>A primitive way to determine the distance to the Sun in terms of the distance to the Moon was already proposed by Aristarchus of Samos in his book On the Sizes and Distances of the Sun and Moon. He noted that the Sun, Moon, and Earth form a right triangle (with the right angle at the Moon) at the moment of first or last quarter moon. He then estimated that the Moon–Earth–Sun angle was 87°. Using correct geometry but inaccurate observational data, Aristarchus concluded that the Sun was slightly less than 20 times farther away than the Moon. The true value of this angle is close to 89° 50', and the Sun is actually about 390 times farther away.[18] He pointed out that the Moon and Sun have nearly equal apparent angular sizes and therefore their diameters must be in proportion to their distances from Earth. He thus concluded that the Sun was around 20 times larger than the Moon; this conclusion, although incorrect, follows logically from his incorrect data. It does suggest that the Sun is clearly larger than the Earth, which could be taken to support the heliocentric model.[21]</p>
<p>[...]</p>
<p>Although Aristarchus' results were incorrect due to observational errors, they were based on correct geometric principles of parallax, and became the basis for estimates of the size of the Solar System for almost 2000 years, until the transit of Venus was correctly observed in 1761 and 1769.[18] This method was proposed by Edmond Halley in 1716, although he did not live to see the results. The use of Venus transits was less successful than had been hoped due to the black drop effect, but the resulting estimate, 153 million kilometers, is just 2% above the currently accepted value, 149.6 million kilometers.</p>
<p>Much later, the Solar System was &quot;scaled&quot; using the parallax of asteroids, some of which, such as Eros, pass much closer to Earth than Venus. In a favourable opposition, Eros can approach the Earth to within 22 million kilometres.[22] Both the opposition of 1901 and that of 1930/1931 were used for this purpose, the calculations of the latter determination being completed by Astronomer Royal Sir Harold Spencer Jones.[23]</p>
<p>Also radar reflections, both off Venus (1958) and off asteroids, like Icarus, have been used for solar parallax determination. Today, use of spacecraft telemetry links has solved this old problem. The currently accepted value of solar parallax is 8&quot;.794 143.[24]</p>
<p><sub>https://en.wikipedia.org/wiki/Parallax#Solar_parallax</sub></p>
</blockquote>
<p><strong>Part Four: Stellar Parallax.</strong></p>
<p>The heliocentric theory implies that as the Earth orbits the Sun, the angles to stars should change. Because of the vast distances to the stars, astronomers failed to detect the parallaxes and distances to the stars for centuries.</p>
<p>Advances in instruments and techniques led to the first successfull measurements of stellar parallax in the 1830s, when the parallaxs and distances of Alpha centauri, 61 Cygni, and Vega were first measured.  Alpha Centauri is the closest star system to the Sun, 61 Cygni is the 15th, and there should be a few hundred star systems closer - and easier to measure their parallaxes - than Vega.</p>
<p><strong>Part Five: Conclusion.</strong></p>
<p>Any amateur astronomer who has telescopes, instruments, and observing techniques as good as those of professional astronomers of 17th and 18th centuries, or the 19th century up to the 1830s, can duplicate those observations which give stronger and stronger evidence that the Earth orbits the Sun instead of the Sun orbiting the Earth.</p>

CC BY-SA 4.0
2021-04-23T18:04:46.617Z
2022-12-23T11:45:45.087Z
2022-12-23T11:45:45.087Z
M. A. Golding
<p>Ancient Greeks already had measured the size of the Earth, the size and distance of the Moon, and the size and distance of the Sun. They got the wrong values, but their reasoning was sound—it’s only because their measurements were not that great that they were off in their results. But eventually, <em>someone</em> would have gotten the right measurements, and realized how <strong>huge</strong> the Sun is. Does it make sense that the huge Sun revolves around the minuscule Earth? Of course not! It makes much more sense that the minuscule Earth revolves around the huge Sun.</p>
<p>Some Ancient Greeks had already thought of that and did preach for an heliocentric system. To put it <em><strong>very simply</strong></em>, it’s only because Aristotle had more followers that his ideas prevailed and eventually became The Truth that nobody was allowed to contest.</p>

CC BY-SA 4.0
2021-04-23T04:40:19.827Z
I don't understand How it makes more sense for the earth to move around the sun unless you know about the gravitational force. Is it possible to use lunar phases to prove this?
Himanshu
CC BY-SA 4.0
2021-04-23T18:43:45.903Z
@YoungKindaichi A proof depends on the axioms of your logical system.  When you're arguing with someone whose axioms includes "there is an invisible giant snake floating through the sky", you're gonna have a harder time using phases and eclipses to prove things like heliocentricism than you would proving "[the snake eats the moon](https://en.wikipedia.org/wiki/Bakunawa)".
zibadawa timmy
CC BY-SA 4.0
2021-04-23T00:45:41.793Z
2021-04-23T00:45:41.793Z
Pierre Paquette
<p>I always think the strongest demonstration that the earth moves is using a Foucault Pendulum at different latitudes. If you combine this with Eratosthenes measurement of the circumference of the earth, you at least have a solid argument that the earth is round and spinning. It's not all the way to Heliocentric, but starts getting things close and doesn't require a telescope.</p>

CC BY-SA 4.0
2021-04-23T01:25:40.853Z
'Eppur si muove', it kills one dogma, but doesn't destroy the geocentric idea.
AtmosphericPrisonEscape
CC BY-SA 4.0
2021-04-22T23:06:02.607Z
2021-04-22T23:06:02.607Z
I.P. Freeley
<p>I'm new to Astrophysics, So pardon me if you find it silly to ask. I was just started reading about the basics of astrophysics. I read about the Greek Tradition, the concept of a geocentric universe. If I were asked in my childhood, to look at the sky and tell what do you think about the clockwork of the planet, I would have said that the Earth is at the center and everything moves around it as said by the greeks.</p>
<p>I read, How different peoples found difficulty explaining the motion through this Hypothesis. Though they were able to make some complicated models understand the wandering stars etc. And then Copernicus suggested a heliocentric model of planetary motion which is as believed, to make things simpler to make calculation simple. And So on...</p>
<p>It seems the things were based on more or less explaining the data by making an appropriate model and that's How science more or less works.</p>
<p>I want to know if there is some more basic argument that could be given to reject the Geocentric universe.</p>
<p>If not, Could there be some basic experiment that can be done to approve the Heliocentric Hypothesis and reject the other?</p>
<hr />
<p>Edit: One more thing to add, Apart from the minimal argument, it would be great if the argument would remain valid at that time that is it should not contain any new information that wasn't known at that time.</p>

CC BY-SA 4.0
2021-04-22T15:59:32.303Z
You can ask why the Sun and Moon don't fall down to the Earth?  What holds them up?
eshaya
CC BY-SA 4.0
2021-04-22T16:22:31.477Z
But At that time people do not know about gravitational force. I don't think they would have any idea about the mass of the sun or other planets.
Himanshu
CC BY-SA 4.0
2021-04-22T18:11:00.147Z
I don't think there is another answer if you restrict it to just what humans knew in ancient Greece.  If you can't use arguments stemming from concepts like gravity or conservation of angular momentum, physics arguments are unconvincing.  If you don't allow for building a telescope to see Jupiter's Moons (to show not all bodies orbit the Earth) or phases of Venus, observational arguments are unconvincing.  Perhaps the only argument you have left is that the Heliocentric model is simpler than the Geocentric.
Connor Garcia
CC BY-SA 4.0
2021-04-22T18:13:39.587Z
Can I being at my home, take some sort of data to prove the sun is in the middle, and this time I can use physics?
Himanshu
CC BY-SA 4.0
2021-04-22T19:05:59.100Z
Annual stellar parallax is a relatively simple observation the ancient Greeks could have understood. Unfortunately, the stars turn out to be so far away that the parallax shifts are *really small*, which is why they weren't seen until the 19th Century; you need high-precision telescopes to observe it.
Peter Erwin
CC BY-SA 4.0
2021-04-22T19:41:37.443Z
@YoungKindaichi Planetary angular momentum is conserved in a Sun-centered model (allowing for the barycenter) and it isn't conserved in an Earth-centered model (where the planets will slow down and stop and then accelerate in the opposite direction without any changes to the force applied to them).
Connor Garcia
CC BY-SA 4.0
2021-04-23T19:03:58.270Z
This question and the answers are just all over the place.  The question is about what people knew and didn't know at some time in the past, but does not specify what time.  Is this about ancient Greek times, Roman times, pre-Copernicus, pre-Galileo, after Kepler, after Newton, or the present.  The use of present tense verbs made me think this is about today.
eshaya
<the-sun><solar-system><astrophysics>
What's the minimal argument I can give to reject The Geocentric Universe?
Himanshu
# What's the minimal argument I can give to reject The Geocentric Universe?

(Asked by: username_0 on 2021-04-22)

I'm new to Astrophysics, So pardon me if you find it silly to ask. I was just started reading about the basics of astrophysics. I read about the Greek Tradition, the concept of a geocentric universe. If I were asked in my childhood, to look at the sky and tell what do you think about the clockwork of the planet, I would have said that the Earth is at the center and everything moves around it as said by the greeks.

I read, How different peoples found difficulty explaining the motion through this Hypothesis. Though they were able to make some complicated models understand the wandering stars etc. And then Copernicus suggested a heliocentric model of planetary motion which is as believed, to make things simpler to make calculation simple. And So on...

It seems the things were based on more or less explaining the data by making an appropriate model and that's How science more or less works.

I want to know if there is some more basic argument that could be given to reject the Geocentric universe.

If not, Could there be some basic experiment that can be done to approve the Heliocentric Hypothesis and reject the other?

---

Edit: One more thing to add, Apart from the minimal argument, it would be great if the argument would remain valid at that time that is it should not contain any new information that wasn't known at that time.

**Tags:** the-sun, solar-system, astrophysics

> **Comments:**
> **username_1**: You can ask why the Sun and Moon don't fall down to the Earth?  What holds them up?  
> **username_0**: But At that time people do not know about gravitational force. I don't think they would have any idea about the mass of the sun or other planets.  
> **username_2**: I don't think there is another answer if you restrict it to just what humans knew in ancient Greece.  If you can't use arguments stemming from concepts like gravity or conservation of angular momentum, physics arguments are unconvincing.  If you don't allow for building a telescope to see Jupiter's Moons (to show not all bodies orbit the Earth) or phases of Venus, observational arguments are unconvincing.  Perhaps the only argument you have left is that the Heliocentric model is simpler than the Geocentric.  
> **username_0**: Can I being at my home, take some sort of data to prove the sun is in the middle, and this time I can use physics?  
> **username_3**: Annual stellar parallax is a relatively simple observation the ancient Greeks could have understood. Unfortunately, the stars turn out to be so far away that the parallax shifts are *really small*, which is why they weren't seen until the 19th Century; you need high-precision telescopes to observe it.  
> **username_2**: @YoungKindaichi Planetary angular momentum is conserved in a Sun-centered model (allowing for the barycenter) and it isn't conserved in an Earth-centered model (where the planets will slow down and stop and then accelerate in the opposite direction without any changes to the force applied to them).  
> **username_1**: This question and the answers are just all over the place.  The question is about what people knew and didn't know at some time in the past, but does not specify what time.  Is this about ancient Greek times, Roman times, pre-Copernicus, pre-Galileo, after Kepler, after Newton, or the present.  The use of present tense verbs made me think this is about today.  

---

## 3 answers

### Answer by username_4 on 2021-04-23. Score: 6

## Part One: Moons of other planets.

The discovery of the Galilean moons of Jupiter in 1609-1610 proved that not everything had to orbit directly around one single object.

The discovery of the Galilean moons showed that objects could revolve around an object which revolved around another object. There was no way for all known objects to revolve directly around each other.

Either:

1. The Sun, the Moon, and the planets revolved around Earth, and the 4 Galilean moons revolved around Jupiter, thus only indrectly orbiting the Earth.

or:

2. Earth, the Moon, and the planets orbited around the Sun, and the 4 Galilean moons revolved around Jupiter, thus only indirectly orbiting the Sun.

or:

3. The Sun, the Earth, the other planets, and the 4 Galilean moons, all revolved around Jupiter, and the Moon revolved around Earth, thus only indirectly orbiting Jupiter.

Decades later, Titan was discovered in 1654, and four other large moons of Saturn were discovered in 1671 to 1684.

So now there were three objects in the solar system which certainly had objects orbiting them, Earth, Jupiter, and Saturn, and there was a big astronomical question whether everything ultimately orbited the Earth or the Sun.

## Part Two: Phases of the Planets.

When telescopes were first used to observe the planets starting in 1609-1610, the phases of the planets began to be observed, phases caused by the changing angles between the Sun, the planets, and Earth.

It became noticed that the phases of Mercury and Venus went all the way from thin crescents to 100 percent full, and that their angular diameters were larger when they were thin crescents and smaller when they were full. Half the times they appeared close to the Sun they appeared small and full, and half the times they appeared close to the son they appeared large and crescent shaped. That was consistent with them orbiting around the Sun at a distance less than the distance between Earth and the Sun.

But the phases of Jupiter and Saturn were different. They ranged from slightly more than half full to totally full. Jupiter and Saturn always appeared smaller and less full the closer they were to the direction of the Sun but were never less than half full. They never appeared larger and less than half full when they were close to the direction of the Sun. So they must have orbited around either the Earth or the Sun at distances greater than the distance between the Earth and the Sun.

## Part Three: Parallax.

**Diurnal parallax**

> Diurnal parallax is a parallax that varies with rotation of the Earth or with difference of location on the Earth. The Moon and to a smaller extent the terrestrial planets or asteroids seen from different viewing positions on the Earth (at one given moment) can appear differently placed against the background of fixed stars.[12][13]
>
> The diurnal parallax has been used by John Flamsteed to measure the distance to Mars at its opposition and through that to estimate the astronomical unit and the size of the solar system.[14]
>
> https://en.wikipedia.org/wiki/Parallax#Distance\_measurement

john Flamsteed lived from 1646 to 1719.

**Solar parallax**

> After Copernicus proposed his heliocentric system, with the Earth in revolution around the Sun, it was possible to build a model of the whole Solar System without scale. To ascertain the scale, it is necessary only to measure one distance within the Solar System, e.g., the mean distance from the Earth to the Sun (now called an astronomical unit, or AU). When found by triangulation, this is referred to as the solar parallax, the difference in position of the Sun as seen from the Earth's centre and a point one Earth radius away, i. e., the angle subtended at the Sun by the Earth's mean radius. Knowing the solar parallax and the mean Earth radius allows one to calculate the AU, the first, small step on the long road of establishing the size and expansion age[20] of the visible Universe.
>
> A primitive way to determine the distance to the Sun in terms of the distance to the Moon was already proposed by Aristarchus of Samos in his book On the Sizes and Distances of the Sun and Moon. He noted that the Sun, Moon, and Earth form a right triangle (with the right angle at the Moon) at the moment of first or last quarter moon. He then estimated that the Moon–Earth–Sun angle was 87°. Using correct geometry but inaccurate observational data, Aristarchus concluded that the Sun was slightly less than 20 times farther away than the Moon. The true value of this angle is close to 89° 50', and the Sun is actually about 390 times farther away.[18] He pointed out that the Moon and Sun have nearly equal apparent angular sizes and therefore their diameters must be in proportion to their distances from Earth. He thus concluded that the Sun was around 20 times larger than the Moon; this conclusion, although incorrect, follows logically from his incorrect data. It does suggest that the Sun is clearly larger than the Earth, which could be taken to support the heliocentric model.[21]
>
> [...]
>
> Although Aristarchus' results were incorrect due to observational errors, they were based on correct geometric principles of parallax, and became the basis for estimates of the size of the Solar System for almost 2000 years, until the transit of Venus was correctly observed in 1761 and 1769.[18] This method was proposed by Edmond Halley in 1716, although he did not live to see the results. The use of Venus transits was less successful than had been hoped due to the black drop effect, but the resulting estimate, 153 million kilometers, is just 2% above the currently accepted value, 149.6 million kilometers.
>
> Much later, the Solar System was "scaled" using the parallax of asteroids, some of which, such as Eros, pass much closer to Earth than Venus. In a favourable opposition, Eros can approach the Earth to within 22 million kilometres.[22] Both the opposition of 1901 and that of 1930/1931 were used for this purpose, the calculations of the latter determination being completed by Astronomer Royal Sir Harold Spencer Jones.[23]
>
> Also radar reflections, both off Venus (1958) and off asteroids, like Icarus, have been used for solar parallax determination. Today, use of spacecraft telemetry links has solved this old problem. The currently accepted value of solar parallax is 8".794 143.[24]
>
> https://en.wikipedia.org/wiki/Parallax#Solar\_parallax

**Part Four: Stellar Parallax.**

The heliocentric theory implies that as the Earth orbits the Sun, the angles to stars should change. Because of the vast distances to the stars, astronomers failed to detect the parallaxes and distances to the stars for centuries.

Advances in instruments and techniques led to the first successfull measurements of stellar parallax in the 1830s, when the parallaxs and distances of Alpha centauri, 61 Cygni, and Vega were first measured. Alpha Centauri is the closest star system to the Sun, 61 Cygni is the 15th, and there should be a few hundred star systems closer - and easier to measure their parallaxes - than Vega.

**Part Five: Conclusion.**

Any amateur astronomer who has telescopes, instruments, and observing techniques as good as those of professional astronomers of 17th and 18th centuries, or the 19th century up to the 1830s, can duplicate those observations which give stronger and stronger evidence that the Earth orbits the Sun instead of the Sun orbiting the Earth.


---

### Answer by username_5 on 2021-04-23. Score: 1

Ancient Greeks already had measured the size of the Earth, the size and distance of the Moon, and the size and distance of the Sun. They got the wrong values, but their reasoning was sound—it’s only because their measurements were not that great that they were off in their results. But eventually, *someone* would have gotten the right measurements, and realized how **huge** the Sun is. Does it make sense that the huge Sun revolves around the minuscule Earth? Of course not! It makes much more sense that the minuscule Earth revolves around the huge Sun.

Some Ancient Greeks had already thought of that and did preach for an heliocentric system. To put it ***very simply***, it’s only because Aristotle had more followers that his ideas prevailed and eventually became The Truth that nobody was allowed to contest.

> **Comments:**
> **username_0**: I don't understand How it makes more sense for the earth to move around the sun unless you know about the gravitational force. Is it possible to use lunar phases to prove this?  
> **username_6**: @YoungKindaichi A proof depends on the axioms of your logical system.  When you're arguing with someone whose axioms includes "there is an invisible giant snake floating through the sky", you're gonna have a harder time using phases and eclipses to prove things like heliocentricism than you would proving "[the snake eats the moon](https://en.wikipedia.org/wiki/Bakunawa)".  

---

### Answer by username_7 on 2021-04-22. Score: 1

I always think the strongest demonstration that the earth moves is using a Foucault Pendulum at different latitudes. If you combine this with Eratosthenes measurement of the circumference of the earth, you at least have a solid argument that the earth is round and spinning. It's not all the way to Heliocentric, but starts getting things close and doesn't require a telescope.

> **Comments:**
> **username_8**: 'Eppur si muove', it kills one dogma, but doesn't destroy the geocentric idea.  

---

