Heliometer

[Phigla]

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

 

[Gfamily]

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

[Phigla]

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

[Sunshine]

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.