Visual measurements of the proper motions of 61 Cygni & Groombridge 1830 using an undriven 10-inch Dobsonian telescope

2018 July 19

Nick White

Introduction
To the unaided eye, the relative positions of the stars in the night sky appear fixed. However, based on a review of historical observations, it was recognised by Edmund Halley that the relative positions of stars as measured in the plane perpendicular to the line of sight change over time. This change in relative positions is a consequence of stellar motion around the centre of the Galaxy and is called ‘proper motion’.
The motion of the Sun and stars through the Galaxy means that millions of stars in the night sky show the effect of proper motion when scrutinised closely over a long enough period of time. However, the vast majority of stars are too distant for the effect to be apparent in amateur equipment over timescales of decades (or longer), and therefore the vast majority of stars appear fixed in their relative positions. Such ‘fixed’ stars are called ‘field stars’ for the purpose of this article.
In contrast to field stars, 61 Cygni (a double star) and Groombridge 1830 are sufficiently close to the Sun, and are moving sufficiently quickly through space, that their motions relative to the field stars can be measured by amateur astronomers using small telescopes and eyepieces such as an astrometric eyepiece.
Table 1 lists modern measurements of the proper motions of 61 Cygni and Groombridge 1830. The proper motion of 61 Cygni was first pointed out by G. Piazzi in 1804 and that of Groombridge 1830 was first reported by F. W. Argelander in 1843.
This article presents results of visual measurements of the proper motions of 61 Cygni and Groombridge 1830 (the target stars) made using an undriven 10-inch Dobsonian telescope and an astrometric eyepiece. The results are compared with those of Table 1 to provide an indication of the accuracy which may be achieved over a period of time of less than two years. (continued…)

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