178 



KNOWLEDGE. 



[AuousT, 1901. 



Accepting those figm-es, and adjusting the remaining 

 magnitudes to suit a total of 100 millions down to the 

 17th magnitude, we obtain the following table: — 



MagnitTide. 



Sirius aud Canopus. 

 Aroturus. 



Oipella, Vi'gi, 

 a Centauri, Rigi'l 

 and Procvou. 



To this magnitude, 

 fiV, tlic numbers 

 of the stars in 

 each class have 

 been assumed 

 from Houzeau's 

 results. Thenum- 

 bersinthe fainter 

 magnitudes have 

 then been rough- 

 ly adjusted so 

 as to mate the 

 total about 100 

 millions. This 

 may seem a rather 

 arbitrary pro- 

 ceeding, but there 

 is evidence to 

 show that the 

 stars really "thin 

 out " below the 

 11th magnitude. 



It will be noticed that the numbers in column 4 of 

 the above table rapidly diminish for the fainter magni- 

 tudes. If there were 100 millions of stars of the 20th 

 magnitude their combined light would be only equal to 

 that of a single star like Arctiu'us. From this it is cleai' 

 that the light of all the stars below the 17th magnitude 

 may be safely neglected. 



In addition to the stars there are a large number of 

 nebulse scattered over the surface of the heavens, but the 

 majority of these are such faint objects that their com- 

 bined light must be inconsiderable. Assuming a total 

 number of 120,000 nebulfe and an average brightness 

 for each equal to that of a star of the 8tli magnitude. 

 we have their combined light equal to 30 stai-s of zero 

 magnitude. Hence the tot-al light of all the staxs and 

 nebulae in both hemispheres would be equivalent to that 

 of 589 stars of zero magnitude like Aixtui-us. This 

 estimate, of course, includes the Milky Wav. 



Now to find what fraction this is of moonlight we 

 must consider some estimates w-hich have been made of 

 sunlight and moonlight. Huygens in the 17th centtuy 

 found the sun 7.56,000,000 times brighter than Sirius; 

 Michcll in 1767 found 9,216,000,000; Wollaston in 

 1825-6, 20,000,000,000; Von Steinheil in 1836, 



light of Sirius. 



3,840,000,000; G. P. Bond in 1861, 5,970,500,000; and 

 A. Clark found 3,600,000.000. The mean of all these 

 rather discordant measures is: — 



Sun's light = 7,230.000,000 times 

 Modern photometric measures make Sirivis about four 

 times the brightness of Arcturus. and hence we have 



Sun's light =28,920,000,000 times light of zero star. 



Comparing sunlight with moonlight. Bouguer found 

 the sun 300.000 times the brightness of full moonlight: 

 Euler found 374.000; Wollaston 801,072; G. P. Bond 

 170,980. and Zbllner 618,000. The mean of these is 

 512,810, but ZoUner's estimate of 618,000 is the one now 

 generally accepted. Assuming this value, we have- 



Moonlight = 



28.920,000,000 



618,000 



Moonlight = 46,800 times light of star of zero magui- 

 tude. 



Hence starlight =-'''— = — of moonlight. 



^ 46,800 80 ° 



This result gives for one hemisphere (which is all that 

 is visible from one place at one time) 



Starlight = — -- of moonlight, 



"^ loO 



And this is probably not far from the truth. 



An examination of the table will show that the com- 

 bined light of the stars below 6h magnitude is con- 

 siderably greater than the light of those above that 

 magnitude, so that if all the stars visible to the naked 

 eye were extinguished we should still have nearly the 

 same amount of starlight. 



CONSTELLATION STUDIES: 



By E. Walter Maunder, f.r.a.s. 



VIII.— THE ARCHER AND THE WATER- 

 BEARER. 

 The constellation of Sagittarius, when on the meridian, 

 is almost entirely above our English horizon, but it lies 

 so low that it is perhaps less familiar to us than any 

 other of the zodiacal signs, for though Scorpio does 

 indeed lie lower still, its brilliance has made it better 

 known. Still, there is no difficulty in recognizing it on 

 a very clear night, at its culmination, which takes place 

 at midnight at the end of June. The old rhymester 

 directs us — - 



" From Deneb, in the stately .Swan, describe a line south-west 

 Through bright Altairin Aquila, 'twill strike the Archer's breast." 



Or, most strictly speaking, his shoulder, marked by the 

 bright star, Sigma. A little in advance of Sigma are 

 five bright stars in an undulating line on the east-em 

 branch of the Milky Way. which here sufTers one of its 

 numerous divisions. Proceeding from the most 

 northerly of these downwards, they are lettered Mu, 

 Lambda, Delta, Epsilon, Eta, and mark the position of 

 the Archer's Bow. A pair of stars, both bearing the 

 letter Gamm.a, a little in advance of Delta, marks the 

 point of the Arrow which the Archer is discharging at 

 the Scorpion, whilst Zeta, a bright stai- a little below 

 Sigma, marks the wing of the Arrow. A little triangle 

 of stars, Xi, Omicron. Pi. mark the neck of the figure, 

 and practically exhaust the list of its brighter stars. 

 Alpha and Beta, the latter a wide double star to the 

 eye, are in one of the hind legs of the Man-horse, but 

 are below oiu- English horizon. 



But though Sagittarius is not. a distinguished constel- 

 lation, viewed as a whole, it is very rich in objects of 

 the gi-eatest interest in opera-glass or telescope. Mu, 

 the upper horn of the Bow, is the centre of a region 



