The present writer has successfully used a similar plan in measuring position angles of a Centauri with the heliometer, viz.
The Hercules cluster is of this form; another example is Centauri, in which over 6000 stars have been counted, comprised within a circle of about 40' diameter.
These clusters present many unsolved problems. Thus Perrine, from an examination of ten globular clusters (including Messier 13 and Centauri), has found in each case that the stars can be separated into two classes of magnitudes.
Henderson at the Cape of Good Hope measured the parallax of a Centauri, but his resulting value 1" was considerably too high.
So far as is known a Centauri is our nearest neighbour.
For one or two of the more famous stars such as a Centauri the probable error is less than so oi"; but for others in the list it ranges up to X0.05".
The method is of very limited application, for in general the orbital velocity of a visual binary is far too small to be found in this way; one of its first applications has been made to a Centauri, with the result that the parallax found in the ordinary way is completely confirmed.
Gellius and Ausonius state that he composed an Erotopaegnia, and in other sources he is credited with Adonis, Alcestis, Centauri, Helena, Ino, Protesilaudamia, Sirenocirca, Phoenix, which may, however, be only the parts of the Erotopaegnia.
Thesun's nearest neighbour is a Centauri, which is separated from it by 270,000 times the earth's distance, a space which it would take light four years to traverse.
Thomas Henderson (1798-1844) had indeed measured the larger displacements of a Centauri at the Cape in 1832-1833, but delayed until 1839 to publish his result.
Several estimates have been made which agree well together; whether direct use is made of known parallaxes, or comparison is made with binaries of well-determined orbits of the same spectral type as the sun, in which therefore it may be assumed there is the same relation between mass and brilliancy (Gore), the result is found that the sun's magnitude is - 26.5, or the sun is Io n times as brilliant as a first magnitude star; it would follow that the sun viewed from a Centauri would appear as of magnitude 0.7, and from a star of average distance which has a parallax certainly less than o 1 ", it would be at least fainter than the fifth magnitude, or, say, upon the border-line for naked-eye visibility.