21.
Star charts. For the present we possess two great photographic star charts, embracing the whole heaven:—The Harvard Map (H. M.) and the Franklin-Adams Charts (F. A. C.).
The Harvard Map, of which a copy (or more correctly two copies) on glass has kindly been placed at the disposal of the Lund Observatory by Mr. Pickering, embraces all stars down to the 11th magnitude. It consists of 55 plates, each embracing more than 900 square degrees of the sky. The photographs were taken with a small lens of only 2.5 cms. aperture and about 32.5 cms. focal-length. The time of exposure was one hour. These plates have been counted at the Lund Observatory by Hans Henie. We return later to these counts.
The Franklin-Adams Charts were made by an amateur astronomer Franklin-Adams, partly at his own observatory (Mervel Hill) in England, partly in Cape and Johannesburg, Transvaal, in the years 1905-1912. The photographs were taken with a Taylor lens with 25 cm. aperture and a focal-length of 114 cm., which gives rather good images on a field of 15 × 15 square degrees.
The whole sky is here reproduced on, in all, 206 plates. Each plate was exposed for 2 hours and 20 minutes and gives images of the stars down to the 17th magnitude. The original plates are now at the observatory in Greenwich. Some copies on paper have been made, of which the Lund Observatory possesses one. It shows stars down to the 14th-15th magnitudes and gives a splendid survey of the whole sky more complete, indeed, than can be obtained, even for the north sky, by direct observation of the heavens with any telescope at present accessible in Sweden.
The F. A. C. have been counted by the astronomers of the Lund Observatory, so that thus a complete count of the number of stars for the whole heaven down to the 14th magnitude has been obtained. We shall later have an opportunity of discussing the results of these counts.
A great star map is planned in connection with the Paris catalogue mentioned in the preceding paragraph. This Carte du Ciel (C. d. C.) is still unfinished, but there seems to be a possibility that we shall one day see this work carried to completion. It will embrace stars down to the 14th magnitude and thus does not reach so far as the F. A. C., but on the other hand is carried out on a considerably greater scale and gives better images than F. A. C. and will therefore be of a great value in the future, especially for the study of the proper motions of the stars.
22.
Distance of the stars. As the determination, from the annual parallax, of the distances of the stars is very precarious if the distance exceeds 5 sir. (π = 0″.04), it is only natural that the catalogues of star-distances should be but few in number. The most complete catalogues are those of Bigourdan in the Bulletin astronomique XXVI (1909), of Kapteyn and Weersma in the publications of Groningen Nr. 24 (1910), embracing 365 stars, and of Walkey in the “Journal of the British Astronomical Association XXVII” (1917), embracing 625 stars. Through the spectroscopic method of Adams it will be possible to enlarge this number considerably, so that the distance of all stars, for which the spectrum is well known, may be determined with fair accuracy. Adams has up to now published 1646 parallax stars.