Question about ancient view of space distances?

[robnissen]

Does anyone know when humans first became aware of the size of our part of Milky Way, i.e., had some rough idea of the distances to the closest stars, like with in a factor of 2. Is this 20th century knowledge or was it known in the time of Halley, or did the ancients have some idea? Note, I am not asking about other galaxies, that was in the 1920s, IIRC.

 

[SpeedFreek]

Well the first clues to the vast distances involved seem to have been considered by Tycho Brahe in the 16th century. But let's go back a little further...<br /><br />Around 4000 years ago the Babylonians could predict the motions of the Moon and Sun and thus predicted eclipses. But it was the ancient Greeks who we know as the first cosmologists. A little over 2000 years ago they developed the idea that the stars were fixed on a celestial sphere which rotated about the spherical Earth every 24 hours, and the planets, the Sun and the Moon, moved in the ether between the Earth and the stars.<br /><br />A few centuries later in the second century AD, Ptolemy developed the idea, working out that perfect motion should be in circles, so the stars and planets, being heavenly objects, moved in circles. However, to account for the complicated motion of the planets, which appear to periodically loop back upon themselves, epicycles had to be introduced so that the planets moved in circles upon circles about the fixed Earth.<br /><br />This geocentric (and vastly complicated!) view endured right up to the 16th century, when Copernicus proposed a heliocentric model. But his model couldn't actually match the accuracy of Ptolemy's geocentric one! The problem was that the stars were still considered to be in a celestial sphere.<br /><br />Tycho Brahe stuck to the geocentric model because he realised that if the Earth was moving about the Sun, then the relative positions of the stars should change as viewed from different parts of the Earth's orbit. But there was no evidence of this shift, called parallax. Either the Earth was fixed, or else the stars would have to be what he thought was impossibly far away. He got it wrong, but for the right reason! <img src="/images/icons/wink.gif" /><br /><br />In the 17th century Galileo built his telescope and proved that the Sun was the centre of the solar system. It took a while for his ideas to be accepted but once they were, then it was known that the s <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[spacester]

Good question, good answer. This 4.1 MB pdf adds a bit of info: at the time of Copernicus, no one had been able to measure any parallax, and the limit of observation was about 1 arcsec. That leads to a conclusion that the closest stars were no closer than 3400 AU. But they didn't know what an AU was! <div class="Discussion_UserSignature"> </div>

 

[robnissen]

Great Answer. <img src="/images/icons/smile.gif" /> Its posts like this that keep me coming back to this board.<br /><br />Thx.