Constellation Boundaries

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Nature Magazine, October 1948

Naming most of the constellations dates back to the remote ages, and their boundaries were either non-existent or ill-defined. Many early maps showed mythological figures, giants and warriors, a host of zoological creatures, and many inanimate objects or geometrical figures, all in haphazard arrangement. Bright stars were frequently located in heart, head or foot of some mythical creature. Antares was in the heart of The Scorpion; Regulus in the heart of The Lion; Aldebaran in the eye of The Bull; Betelgeuse in the right shoulder of Orion; Castor and Pollux marked the heads of Gemini, The Twins.

The constellation of Coma Berenices, Berenice's Hair, was added about 200 B.C. There were no more additions until the 17th century, when the astronomers Hevelius and Bayer and others filled in regions of the southern heavens, which had been uncharted up to that time, with many newly created constellations. Some of these were never generally recognized, or have been dropped or replaced by others on modern charts. Thirteen constellations in the southern hemisphere were added by La Caille a century later. He also subdivided the huge constellation of Argo Navis in the southern hemisphere into Malus, The Mast, now replaced by Pyxis, The Compass; Carina, The Keel; Puppis, The Poop, and Vela, The Sails.

The first boundaries of constellations were outlined by Bode at the beginning of the 19th century. These did not receive wide recognition, however. Later atlases and star maps showed different boundaries, and it was not unusual for a star to be listed in different star catalogs as belonging to two different constellations. This led to errors and confusion in the identification of some stars, and it was proposed from time to time to define more exactly the boundaries of the constellations, which was certainly good sense.

The astronomer B. A. Gould was the first to adopt a simple and convenient plan of defining the boundaries between constellations in his catalog of stars of the southern hemisphere (Uranometria Argentina mapas, 1877). The lines that Gould chose for these boundary lines were hour circles passing through the north and south poles of the celestial sphere and circles of declination, which are parallel to the celestial equator. They correspond to our meridians and parallels of latitude on the earth, and, as is well known, boundaries between states or countries are often coincident with these lines on the earth's surface.

If hour circles and circles of declination are used as boundary lines for the constellations, the position of a star in any constellation is permanently fixed with respect to the boundaries of the constellation. Due to the procession of the equinoxes, however, the positions of these boundary lines on the celes-tial sphere, as well as the positions of the stars, will not remain fixed in position with respect to the celestial equator and poles of the heavens, although the stars in a constellation remain fixed with respect to its boundaries.

The positions of the boundaries of the constellations in Gould's catalog of stars in the southern hemi-sphere was for the epoch 1875, January 1. In a period of fifty years slight changes would be noticeable on ordinary star maps and atlases in the positions of the boundary lines, as well as in the positions of the stars with respect to the equinoxes. Star atlases are revised from time to time to take care of these changes.

Some thirty years or so ago certain astronomers, among them E. Delporte at the Royal Observatory of Belgium at Uccle, were impressed with the fact that many advantages would result from having a uniform system of constellation boundaries adopted. There was at that time, and still is, an International Astronomical Union, which held meetings every three years. These meetings were interrupted by World War II but were resumed with the meeting this year at Zurich, Switzerland, on August II for the consideration of problems of general interest to astronomers of all countries.

It was as far back at 1923 that a National Belgian Astronomical Committee examined the question of defining new limits for the northern constellations. At the request of this Belgian committee The Inter-national Astronomical Union, which met at Cambridge, England, in 1925, took up the consideration of the question of revising the boundaries of the northern constellations.

The General Assembly of The International Union decided at this meeting that it would be useful to have such a revision made, and a Commission of Notations was appointed to consider the matter and to report at the next meeting of the Union, which was to be held at Leiden in 1928. At that meeting the recommendation of the committee that had been appointed to consider the matter was to adopt new boundaries for all of the constellations north of 12 ½ degrees south of the equator. The new boundaries were to be hour circles and circles of declination, as were those adopted by Gould for his southern constellations.

A grant to cover the cost of publication of a new atlas, which should show the new boundaries, was also approved. Mr. Delporte, who had given much time and thought to the problem, prepared a manuscript table giving the newly defined boundaries, and these were carefully examined by astron-omers of the Yale Observatory to make certain no change in the designation of any variable star would be made and that a minimum of stars would be moved to other constellations. Only a few minor changes were suggested for these reasons. Mr. Delporte then sent the manuscript of a volume entitled, Delimitation Scientifique des Constellations, to the General Secretary of The International Astronomical Union. It was accepted by the Union and published by the Cambridge University Press in 1930. Following the suggestion of the President of the Commission it included all of the 88 constellations, and extended from pole to pole so as to cover the entire heavens. Not only tables defining the limits of the constellations were given, but charts that showed the positions of stars down to the sixth magnitude as well as the positions of the boundaries.

A second book by Delporte was also published in 1930, bearing the title of Atlas Celeste. Charts of the constellations, showing the new boundaries with the positions for the equinox of 1875, which were those of the Gould Catalogue, also appeared in this publication. It contained, in addition, positions of all stars in the constellations down to 4 ½ magnitude for both 1875 and 1925 so that positions for later dates could be easily obtained. Positions of variable and double stars and nebulas and much other useful information was also given.

All modern star atlases now give the new boundaries of the constellations. The decision to establish new boundaries for 88 constellations covering the entire celestial sphere was painlessly brought about by means of The International Astronomical Union, which, in a period of only a few years, in which two meetings of the Union were held at three-year intervals, considered and adopted these changes and granted funds for publication of the results. Any confusion or question as to the exact position of any star in the heavens, and the constellation to which it belongs, has been eliminated by the adoption of these definitely determined boundaries.

There has been, as the result of the change of constellation boundaries, no alteration in the characteristic configurations of the various constellations, and no bright or well-known star has been displaced or removed from the constellation in which it has always been found.

During the month of October there is a dearth of bright planets in the evening sky. Early in the month Mercury is poorly placed, being very low in the west at sunset. It passes to the morning sky after inferior conjunction with the sun on October 19 and will, later in the month, be quite favorably placed for observation when it will be about 15 degrees above the eastern horizon at sunrise.

Venus is now a fine morning star as it rises several hours before the sun. It will pass close to Regulus on October 6. Mars is in Scorpio, quite near to Antares, and is so low in the southwest at sunset that it will be seen with difficulty. Jupiter is well over in the southwestern sky at sunset in Ophiuchus and it sets a few hours later. Saturn is in the morning sky a few degrees southeast of Regulus in Leo. Venus is also in Leo at this time, and the two planets will be in conjunction on October 8.