A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Universe of the ancient Greeks.
From: Lu Abel
Date: 2006 Mar 12, 21:44 -0800
From: Lu Abel
Date: 2006 Mar 12, 21:44 -0800
While I hesitate to disagree with such giants of this list as George Huxtable and Frank Reed, I have to ask: what about the motion of the planets? Since ancient times, astronomers have understood the path described by the planets in the Earth's sky to be epicycloidal (the loop-the-loop motion of a point on a wheel as it rolls along the ground), leading to the planets to appear to wander through the sky (in fact, the word planet means "wanderer") and at times even reverse (or "retrograde") their motion through the sky. To me, the great leap of imagination by Copernicus and Galileo was to realize that a heliocentric universe could explain the motion of the planets quite simply. If earth and the planets all were in circular orbits around the Sun, the apparent motion of the planets was easily explained by their relative motion with respect to the earth. Given the level of scientific knowledge, inquiry, and downright genius among the ancient Greeks, I'm rather surprised that the idea of a heliocentric universe didn't occur to one of them. Lu Abel George Huxtable wrote: > Frank Reed wrote- > > | George H wrote: > | "Ptolemy, in the early pages of his "Almagest", in about 200 AD, considers > | the arguments for and against motion of the Earth, and decides that indeed the > | Earth is stationary at the centre of the Universe. I doubt if many of us > | would have argued otherwise, if we had found ourselves in the same situation, in > | the same state of knowledge." > | > | I agree with that completely, and I would even go a little farther. If we > | were placed back in time 1800 years with our modern sense of scientific > | methodology and principles intact, we would still reach the same conclusion. The > | most damning evidence against the motion of the Earth is the apparent lack of > | stellar parallax. The reply from advocates of a moving Earth --"well... maybe > | the stars are really far away..."-- would immediately strike us an ad hoc > | assumption designed to avoid facing up to observational evidence that clearly > | rules out the theoretical model. That the ad hoc assumption turns out to be > | entirely correct is one of those things that makes the history of science > | interesting... > > ============== > > Indeed, the lack of stellar parallax was an important, even a clinching argument agains the > implausible notion that the Earth might be making a circuit around the Sun. It's nice to agree so > well with Frank. > > Perhaps it might be of interest to set out the steps in that argument, as it appeared to the Greeks. > By "the Greeks", I'm really referring to that late, great authority on these matters, Ptolemy, in > about 200 AD, but even in the time of Hipparchus, 3 centuries earlier, such matters were rather well > understood. > > First, there was no doubt that the Earth was a sphere, and its radius was quite precisely known, by > the differing altitudes of the noon Sun from different latitudes, a known distance apart. > > The distance of the Moon was also reasonably well known. There were a number of ways of getting a > handle on that. One was the shape of the Earth's shadow, cast on the Moon at the time of a lunar > eclipse, which gave an idea of the relative sizes of Earth and Moon. Another was the way that the > totality of a solar eclipse varied with position of the observer on the Earth's surface. The Moon's > distance is actually about 60 Earth radii, and the Greeks had that nearly right. > > It was clear that the Sun was many times further away from the Earth than was the Moon, from > measurements made when the Moon's disc was exactly half-illuminated by the Sun. Then the angle > between Moon and Sun should give their relative distances by triangulation, but no difference from > an angle of 90 degrees could be distinguished. So all that could be deduced was that the Sun was at > least many times (more than 10, say) further than the Moon, whereas in fact it is nearly 400 x the > Moon's distance. > > Even so, that meant that if the Earth was going round the Sun, the radius of its journey would be at > least something like 1000 Earth radii (in fact, it was 24 x greater still). In that case, they would > argue, you would expect to see some apparent motion of stars in the sky, as the viewpoint from the > Earth changed by such an immense amount over the year. But no such relative motion (parallax) could > be detected, between stars in one direction and stars in another. If the Earth really was moving > round the Sun, the stars would have to be inconceivably distant, for there to be no discernable > parallax. We now know, of course, that that was indeed the case. To the Greeks, however, it was more > logical to presume that the Earth stayed put at the centre of the universe. Can we blame them for > rejecting the other possibility? I think not. > > George. > > contact George Huxtable at firstname.lastname@example.org > or at +44 1865 820222 (from UK, 01865 820222) > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > >