A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Date: 2014 Oct 25, 05:35 +0100
Thanks Don and Bill,
That is really interesting. Does anyone know of modern equivalents for amateur astronomers to measure small angular differences (eg double stars) via a telescope?
It seems that Cook did in fact use such a device on his voyage to observe the transit of Venus. Called a divided object glass micrometer, it was fitted to his Dolland reflecting telescope. It was also called a heliometer, since it was initially developed to accurately measure the sun's disk.
I suppose that Cook used it to track the path of Venus across the sun.
On Oct 24, 2014, at 5:56 PM, Don Seltzer <NoReply_Seltzer@fer3.com> wrote:
On Fri, Oct 24, 2014 at 1:01 PM, Francis Upchurch <NoReply_Upchurch@fer3.com> wrote:
How do you think the old guys (eg Cook et al) did it in the 1770s without photography and computers? Did they measure the thing real time with lens objective graticles or whatever they are called? Any information on those? I’ll probably try to do it the old way if I could find out how.
I don't know if Cook used it, but in the mid-1700's there were several versions of a double image micrometer telescope available to astronomers for measuring small angles. The basic principle was to modify a refracting telescope by splitting one of the lenses in half, and controlling the separation of the two halves by a micrometer mechanism. With the the two halves together, a single image was formed. Turning the micrometer to separate the two resulted in a double image, with the angular separation proportional to the number of turns of the micrometer. To measure the diameter of the sun, the micrometer would be adjusted so that the two images of the sun were just touching, limb to limb. Astronomers also used this instrument to measure the separation of double stars.
Naval officers of the period adapted the telescope, using it as a 'coming up' glass. Observing a another ship in the distance, they would adjust the glass to have the double images just touching. Some time later, they would again observe the other vessel. If the double images were now overlapping, the ship was now closer. If the images had separated, the ship further away.