March, 1902.] 



KNOWLEDGE 



57 



century, divided these maguitudes, for the first time, into 

 thirds." Thus a star sliij;iitly less than an average star 

 of the second magnitude he called 2 — 3, that is nearer in 

 brightness to 2 than to 3 ; one a little brigliter than the 

 third he recorded as 3—2, or nearer to 3 than t<> 2, and so 

 on. This method has been followed by Argelander, 

 Behrmann, Heis, and Houzeau, but in the photometric 

 catalogues of Harvard, Oxford, and Potsilaui, the magni- 

 tudes are measured in decimals of a degree. Tliis has 

 been found necessary for greater accuracy, a.s the heavens 

 contain stars of all degrees of brightness. 



The term " magnitude " means the ratio Ijetween the 

 liifht of a star of a given magnitude and that of anotiier 

 exactly one magnitude fainter. Tliis ratio has been 

 variously estimated by different astronomers, and ranges 

 from 2155, found by Johnson in 1851, to 306, assumed by 

 Pierce in 1878. The value now universally adopted by 

 astronomers is 2-512 (of which the logarithm is 0'4). 

 This number is nearly a mean of all the estimates made, 

 and agrees witli the value found by Pogsou in 1854 by 

 means of an oil flame, and by Rosen with a Ziilluer photo- 

 meter in 1870. It simply means that an average star of 

 the first magnitude is 2512 times the brightness of a star 

 of the second magnitude; a star of the second, 2-512 times 

 brighter than one of the third, and so on. This makes a 

 star of the first magnitude just lUO times bright^'r than 

 one of the sixth. 



There are seveiul stars brighter than an average star of 

 the first magnitude, such as Aldcbaran. These arc Sirius, 

 which is nearly 11 times brighter than Aldcbaran (accord- 

 ing to the revised measures at Harvard) ; Canopus, the 

 second brightest star in the heavens, and about two 

 magnitudes brighter than Aldebaran ; Arcturus. Capella, 

 Vega, Alpha Centauri, lligel, Procyon, Alpha Eridaui, 

 Bet;i Centauri, and Alpha Orionis. Al-Suti rated 13 stars 

 of the first magnitude, visible at his station in Persia, and 

 Halley enumerates Iti in the whole sky. According to the 

 Harvard photometric measures, there are 13 stars in both 

 hemispheres brighter than Aldebaran, which is rated 107. 

 As average stars of the different magnitudes the follow- 

 ing may Ix' taken as examples, derived from the Harvard 

 measures: First magnitude, Aldebaran and Spica ; second 

 magnitude, /3 Aurigai and /3 Canis Majoris ; third magni- 

 tude, I Aurigaj and /? Ophiuchi ; fourth magnitude, 

 $ Herculis and i Draconis ; and fifth magnitude, p Ursaj 

 Majoris and m Sagittarii. Stars of about the sixth magni- 

 tude are, of course, numerous, and lie near the limit of 

 naked-eye vision for average eyesight, although on clear 

 moonless nights still fainter stars may be " glimpsed " by 

 keen-eyed ol>servers. 



The stars have been divided into groups and constella- 

 tions, now chiefly used for the purpose of reference, but 

 in ancient times they were associated with the imaginary 

 figures of men and animals, etc. The origin of these 

 constellation figures is doubtful, but they are certainly of 

 great antiquity. Ptolemy's constellations were 48 in 

 number, but different writers from the first century B.C. 

 give different numbers, ranging from 43 to 62. Bayer's 

 Uranometria, pul)lished in 1603, contains 60, 12 new 

 constellations in the Southern Hemisphere having been 

 added by Theodorus to Ptolemy's original 48. 



The figures representing the constellations were originally 

 drawn on sf)heres, or celestial globes as they are now called. 

 The ancient astronomers attributed the invention of the 

 sphere to Atlas. It seems certain that a celestial sphere 

 was constructed by Eudoxus in the fourth century b.o. 

 Strabo speaks of one made by Krates about the year 

 130 B.C., and according to Ovid, Archimedes had con- 

 structed one at a considerably earlier period. None of 

 these ancient spheres have been presei-ved. There is, how- 



ever, in the Vatican a fragment in marble of a Greco- 

 Egyptiiin i>lanisphere, and a globe in the museum of 

 Arolsen, but these are of much later date. Our know- 

 ledge of the original constellation figures is derived from 

 the accounts given by Ptolemy and his successors, and 

 from a few globes which only date back to the Arabian 

 period of astronomy. Among the Arabian globes still 

 existing the most famous is one made of copper, and pre- 

 served in the Borgia M useuni at Velletri in Italy. It is 

 supposed to have been made by a person called Caisar, 

 who was executed by the Sultan of Eygpt in a.d. 1225. 

 The most ancient of all is one discovered some years ago 

 at Florence. It is supposed to date back to a.d. 1081, and 

 to have been made by Meucci. There is also one in the 

 Farnese Museum at Naples, made in a.d. 1225. Of 

 modern celestial globes the oldest is one made by Jansson 

 Blaeu in 1603. This gives all the constellations of the 

 Southi'vu Hcinis|ihere as well as the Northern. 



Ptolemy's tigun;s of the constellations were restored by 

 the famous painter Albert Durer, of Niiremberg, in 1515. 

 The figures on modern globes and maps have been copied 

 from this restoration. Durer's maps are now very rai*e. 



In 1603 an atlas was published by Bayer. This was the 

 first atlas to show the southern sky, and the first to 

 designate the brightest stars by the letters of the Greek 

 alphabet. Flamsteed published an atlas in 1729. Maps 

 and catalogues of the lucid stars have been published in 

 recent times by Argelander, Behrmann, Heis, Houzeau, 

 Proctor, and others. Of these Heis' is, perhaps, the most 

 reliable, at least so far as accurate star magnitudes are 

 concerned. Houzeau shows both hemispheres, all the stars 

 having been observed by himself in Jamaica and South 

 America. Behrmanu's maps are confined to the Southern 

 Hemisphere, between the South Pole and 20 degrees south 

 of the Equator. The maps of the Uranometria Argentina. 

 made at Cordoba in the Argentine Eepublic, show all the 

 southern stars to the 7th magnitude, but many of these 

 are beyond the reach of ordinary eyesight. 



It is a well-known fact that the planets Venus and 

 Jupiter are bright enough to form shadows of objects 

 on a white background. It has also been found that the 

 brightest stars, especially Sirius, are sufficiently brilliant 

 to cast shadows. Kepler stated that a shadow was formed 

 by even Spica, but I am not aware that this has been 

 confirmed by modern ob.servatious. 



There are some remarkable collections or clusters of 

 stars visible to the naked eye, of these the Pleiades are 

 probably the best known. To ordinary eyesight 6 stars 

 are visible, but MListlin, Kepler's tutor, is said to have 

 seen 14 with the naked eye, and some observers in modern 

 times have seen 11 or 12. Other naked-eye clusters are 

 the Hyades in Taurus, called Palilicium by Halley, and 

 the Pi-iesepe, or Bee-Hive in Cancer. Of larger groups, 

 the Plough or Great Bear, Cassiopeia's Chair, and Orion 

 are probably known to most people. 



Many of the lucid stars are double, that is, consist of 

 two components, but most of these are only visible in 

 powerful telescopes. There are, however, a few objects 

 visible to the naked eye as double, and these have been 

 called "naked-eye doubles," although not strictly double 

 in the correct sense of the term. Ptolemy applied the 

 term double to the star v Sagittarii, which consists of two 

 stars separated by a distance of fourteen minutes of 

 arc, or about half the apparent diameter of the moon. 

 According to Riccioli, Van dcr Hove saw two naked-eye 

 doubles, one in Capricornus, 5 to 5| minutes distant, and 

 the other in the Hyades, 4^ or 5 minutes apart. The one 

 in Capricornus was probably a, and the one in the Hyades 

 6 Tauri. The middle star in the tail of the Great Bear, 

 or handle of the Plough, has near it a pniall star, Alcor, 



