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Jason Thompson
2003-Jun-25, 01:18 PM
Just an interesting hypothetical question here (well, I think it's interesting, anyway!).

How much younger do we think the heliocentric model of the solar system would be without the other planets around?

As I understand it, it was the odd motion of the planets in the sky that led people to start thinking that they couldn't just be orbiting the Earth. The ancient Greeks used epicylces to explain it (a planet orbits a point that orbits a point that orbits another point that orbits the Earth; by the time they came to explain some of the outer planets they were using about twenty to thirty epicycles to fit the motion with the model!), then centuries later Tycho Brahe came up with a rather elegant solution in which the planets orbited the sun, which in turn orbited the Earth, and then Copernicus came up with the heliocentric model (and was terrified of what would happen to him if he published his finding that the Earth was not the centre of the universe after all).

Thing is, all this seems to have been based upon the observation of the other planets in the system. Without that reference (i.e. if the system had comprised only sun, Earth and moon against a background of stars), how long would it have taken people to realise that it was more likely that Earth went round the sun?

Glom
2003-Jun-25, 01:50 PM
Good question.

I can cop out of it by pointing out that without Jupiter, it's unlikely that a developed enough civilisation would have developed because of all the debris running around that would smash into Earth.

Crimson
2003-Jun-25, 03:10 PM
Actually, long before Copernicus, Greek astronomer Aristarchus had concluded that the Earth circled the Sun. He didn't need the planets. Instead, crude estimates of the Sun's distance showed the Sun to be larger than the Earth, so Aristarchus deduced that the Sun was the more logical center.

Trouble is, almost no one believed him. Artistotle argued that if the Earth really circled the Sun, stars should show parallax--which they didn't.

Of course, at that time, stellar parallax was far beyond the threshold of detection. But after Copernicus's theory became established, astronomers began to search for parallax. In 1728, as a result of this search, they discovered stellar aberration, an effect that arises from the Earth's motion around the Sun, and in 1838 genuine stellar parallax was detected. So either of those years might have been the date that heliocentrism was established.

However, without Copernicus, would astronomers have tried to detect parallax?

ToSeek
2003-Jun-25, 03:57 PM
I would go along with Crimson: geocentricism would probably hold until either:

- parallax was observed (and in an environment where it would probably not be looked for)
- it was determined that the Sun was much larger than the Earth

tracer
2003-Jun-25, 06:51 PM
I can cop out of it by pointing out that without Jupiter, it's unlikely that a developed enough civilisation would have developed because of all the debris running around that would smash into Earth.
Nonsense! Sure, the Earth would've gotten bombarded by more planetessimals over a longer period, but eventually things would've calmed down. So what if the Earth's surface remained molten for another billion years, we'd still have plenty of time!

And besides, without Jupiter, the material in the asteroid belt would probably have coalesced to form its own planet, which would take some of the heat off of us, so to speak.

dgruss23
2003-Jun-25, 07:18 PM
It is important to clarify that the heliocentric reference frame is the preferred reference frame in Newtonian gravity. In General Relativity, any reference frame is equally valid - as we've discussed here. (http://www.badastronomy.com/phpBB/viewtopic.php?t=5170&start=0)



Tracer wrote: Nonsense! Sure, the Earth would've gotten bombarded by more planetessimals over a longer period, but eventually things would've calmed down.

I wouldn't go so far as to say that its definitely established, but some research suggested that if it were not for the shielding of Jupiter and Saturn the Earth would STILL be getting slammed every 100,000 years by a K-T type event instead of the ~ 100 million years that is typically takes between such events. I think that is probably what Glom was referring to. I don't want to divert the discussion, but that is one of the "Rare Earth" arguments.


crimson wrote: However, without Copernicus, would astronomers have tried to detect parallax?

I suspect that they would have because they were constantly trying to make parallax measurements within the solar system. It would seem natural that they would want to try it on stars to establish how far away they are - even if they clung to a geocentric point of view.

TriangleMan
2003-Jun-25, 08:29 PM
Actually, long before Copernicus, Greek astronomer Aristarchus had concluded that the Earth circled the Sun. He didn't need the planets. Instead, crude estimates of the Sun's distance showed the Sun to be larger than the Earth, so Aristarchus deduced that the Sun was the more logical center.

:o I had to look that up - I had never heard of Aristarchus. Here is a short biography of Aristarchus (http://www-groups.dcs.st-andrews.ac.uk/~history/Mathematicians/Aristarchus.html)

russ_watters
2003-Jun-25, 08:44 PM
Parallax is good, but what about Newton's gravity itself? With it you can predict quite accurately where the center of an earth-sun solar system would be.

ToSeek
2003-Jun-26, 12:12 AM
Parallax is good, but what about Newton's gravity itself? With it you can predict quite accurately where the center of an earth-sun solar system would be.

Only if you know how massive the Sun is.

Celestial Mechanic
2003-Jun-26, 04:14 AM
[Snip!]The ancient Greeks used epicylces to explain it (a planet orbits a point that orbits a point that orbits another point that orbits the Earth; by the time they came to explain some of the outer planets they were using about twenty to thirty epicycles to fit the motion with the model!),[Snip!]
Actually, Ptolemy did not pile epicycle upon epicycle, and the Arab astronomers who followed him added maybe one to each planet. Ptolemy had the center of the planet's orbit displaced from the Earth and the planet described its orbit at a uniform rate about that point. This is actually good to the first order in the eccentricity but not the second. Since the eccentricities of Earth, Venus, Jupiter and Saturn are less than 0.05, it worked reasonably well for those planets. Ptolemy's model of Mercury was so bizarre and was off by as much as 10 degrees, but lack of observations made this unnoticeable. Mars, however, has an eccentricity of 0.093 and it could be off by as much as two degrees, which was noticed by Copernicus. Of course it was the motion of Mars that led Kepler to ultimately use an ellipse instead of off-center circles.

Owen Gingerich's book The Eye of Heaven (short title) has much to say about how we have come to believe in multiple epicycles (he traces it to an Encyclopedia Britanica article in 1969!).

Geo3gh
2003-Jun-26, 05:27 AM
[snip]
Owen Gingerich's book The Eye of Heaven (short title) has much to say about how we have come to believe in multiple epicycles (he traces it to an Encyclopedia Britanica article in 1969!).


It reminds me of how everyone knows that Columbus was trying to prove that the Earth was round. That wasn't really in contention, at least not amongst the educated. He disagreed with the commonly accepted circumfrence of the Earth, I believe, not with the accepted shape of the Earth.

I recall that the thought that Europeans thought the Earth was flat in Columbus' day was introduced by Washington Irving in one of his essays. I don't remember which one. Anyhow, that got passed around until it was something everyone knew. No one bothered to check up on it, we just knew.

I do get amused at how often we exagerate how far off the ancients were, and how much better the theories that supplanted them were. I commonly hear from people talking about Copernicus that his new heliocentric model did away with epicycles. Wrong. He had them as well. He was still using circular orbits, and so needed epicycles to fit the model to the observed data. And those same people's jaws drop when I tell them this.

Ah well. I'm amused by simple things.

kilopi
2003-Jun-26, 06:14 AM
I do get amused at how often we exagerate how far off the ancients were, and how much better the theories that supplanted them were. I commonly hear from people talking about Copernicus that his new heliocentric model did away with epicycles. Wrong. He had them as well. He was still using circular orbits, and so needed epicycles to fit the model to the observed data. And those same people's jaws drop when I tell them this.
Arthur Koestler, in his book The Sleepwalkers, points out that Copernicus's theory used more epicycles than Ptolemy's theory.

ToSeek
2003-Jun-26, 03:01 PM
I do get amused at how often we exagerate how far off the ancients were, and how much better the theories that supplanted them were. I commonly hear from people talking about Copernicus that his new heliocentric model did away with epicycles. Wrong. He had them as well. He was still using circular orbits, and so needed epicycles to fit the model to the observed data. And those same people's jaws drop when I tell them this.
Arthur Koestler, in his book The Sleepwalkers, points out that Copernicus's theory used more epicycles than Ptolemy's theory.

I think also according to Copernicus that the Moon's apparent size would change by a factor of 2 during the course of a cycle, due to the size of the epicycles he used.

kilopi
2003-Jun-26, 03:21 PM
I think also according to Copernicus that the Moon's apparent size would change by a factor of 2 during the course of a cycle, due to the size of the epicycles he used.
No way! Really? Why? I read Sleepwalkers before, but I don't remember this. Is it in there, or some other reference?

I googled but didn't find anything. This article (http://csep10.phys.utk.edu/astr161/lect/retrograde/copernican.html) says Copernicus used few epicycles than Ptolemy, but I'm pretty sure Koestler said that Copernicus actually used more--that there was a miscount somewhere along the way.