Talk:1639 transit of Venus

How Horrocks (accidentally...) arrived at the distance to the sun
I added a few lines at the beginning of the "Results" section. His observation didn't really allow him to determine the distance (AU=astronomical unit). (It takes careful measurements, including timing, at two places far removed from each other, as was done in later observation campaigns.) It seems Horrocks was mainly interested in measuring the apparent size of Venus (as was his colleague Crabtree). He then used an assumption which it seems was more or less shared by some other contemporaries, that the sizes (diameters) of the planets were proportional to their distance from the sun. This then would of course allows to determine the actual distance of Venus (expressed in earth radii) and thus the AU. Thus this was more of a lucky guess, as Venus is only some rough 30% larger than what would be expected from the assumed "law". For the moment I don't have a better reference than: http://adsabs.harvard.edu/full/2005A%26G....46a..14H

Greetings,79.3.206.62 (talk) 17:05, 3 June 2012 (UTC)


 * Yes, that sounds about right. I knew about the false assumption he made about the size of the planets but I'm no astronomer or mathematician and I still haven't had time to get my head round the concept properly. Too busy trying to get this and the Transit of Venus article ready for the main page on the 5th of June. Richerman ''   (talk) 18:18, 3 June 2012 (UTC)


 * Thanks for clearing that up as I have been wondering about it. Someone needs to add the above link as a reference (I'll get around to it, but no time now, particularly since I'll have to examine the document first). Johnuniq (talk) 02:35, 4 June 2012 (UTC)


 * I've added the reference but this needs a bit more work. I tried to rewrite it but I'm not sure if I got it completely right. My proposed text is below:

 Kepler had found that the distance between the planets increased in proportion to their distance from the Sun and his mystical mind led him to assume that the universe was created with a divine harmony and the size of the planets would increase in the same way. He had written in 1618 "Nothing is more in concord with nature than that the order of magnitude should be the the same as the order of the spheres". When Horrocks found that this was true of Venus and Mercury he tentatively proposed a law that said "All planets, (with the exception of Mars) are the same angular size, this being 28 arc seconds" This meant that the assumption his hero Kepler had made about the sizes of the planets held true and led to the erroneous conclusion that the distance between each planet and the Sun was about 15,000 times its radius. Thus he estimated the distance from the Earth to the Sun to be approximately 60 million miles (97 million km) – suggesting the solar system was ten times larger than traditionally believed. 

Johnuniq (talk) 11:26, 4 June 2012 (UTC)
 * From my reading of Aughton's book there was more evidence from his observations of the other planets that led him to propose the law but I've not digested it all yet. Does that summary look about right? Richerman ''   (talk) 10:58, 4 June 2012 (UTC)
 * I believe that's reasonable, although there should be an explanation that the 28 arcseconds is the angular diameter of a planet, as seen from the Sun. I think some rewording is needed as it may suggest that Horrocks observed Mercury, whereas he did calculations from Piere Gassendi's observations of the 1631 transit of Mercury, and used Kepler's (incorrect) values for the Mercury–Earth/Mercury–Sun distances; those calculations (according to the article) concluded that Mercury had an angular diameter of 28″ seen from the Sun. Also, some improvement on "hero" is needed. I have to go now, so can't do anything useful, but here is an extract from some notes I took a couple of hours ago, while reading the harvard.edu link above (I'm not proposing this for the article, but it is a summary of a signficant point that I think was made by the article, and "veneration" is more suitable than "hero"):"Perhaps Horrocks thought his unfounded law was reasonable given a veneration of Kepler, and the mystery of Kepler's third law (for which they had no explanation). If the cube of the planet–Sun distance was proportional to the square of the period of the planet's orbit, why should there not be a proportionality between the planet–Sun distance and the planet radius?"
 * Thanks John, I've changed the text now. Richerman ''   (talk) 01:16, 5 June 2012 (UTC)