Phigla Posted May 27, 2022 Share Posted May 27, 2022 Good morning, I just registered on this website. As said in my presentation since recently I'm passionate about the history of the Astronomy...It's my first question I read that the first parallax measurement was made by , Friedrich Bessel in In 1838 using a Fraunhofer heliometer. Looking on the internet this topic the only thing I read is that is a refracting telescope with the lens cut in two, one half is fixed, the other half mobile with a micrometer. At the origin it was made for sun diameter measurement and Bessel used it for parallax measurement If a member can tell me more about the principle and how it's work I will be very happy, because I do not find any info on the net. Thanks in advance 1 Link to comment Share on other sites More sharing options...
Gfamily Posted May 29, 2022 Share Posted May 29, 2022 (edited) The principle is that the split objective will produce two images, which move relative to each other as the micrometer screw is turned. When used as a heliometer, the two images of the sun start aligned and the micrometer moves them until the top edge of one image of the Sun lines up with the bottom edge of the other image. This allows the angular diameter of the Sun to be measured at different points in the Earth's orbit For Bessel's work, he used the heliometer to measure the angular difference between the fast moving 61Cygni and two reference stars several times over a period of a year or more. The telescope is turned so that the split images move along the line between the stars, so that one image of 61Cyg lines up with the other image of the reference star. The micrometer movement gives the angular distance between them. The two sets of distances produce a trace showing the average proper motion across the sky with a superimposed sinusoidal curve. The size of the curve gives a measure of the parallax. Here's a mini gif of some slides from a talk I gave to our Astro Society about Bessel's work Edited May 29, 2022 by Gfamily add gif animation 3 1 Link to comment Share on other sites More sharing options...
Phigla Posted May 30, 2022 Author Share Posted May 30, 2022 Dear Gfamily, Thanks for your reply, it clarify a little bit the Heliometer principle for parallax measurement made by Bessel. But is sill remaining a bit nebulous Have a nice week Link to comment Share on other sites More sharing options...
Sunshine Posted May 30, 2022 Share Posted May 30, 2022 1 hour ago, Phigla said: But is sill remaining a bit nebulous "nebulous" brilliant, haven't heard that word used that way but its a sure be I will be using it when referring to my lack of clarity with something, love it. 3 Link to comment Share on other sites More sharing options...
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