October 2, 1893.] 



KNOWLEDGE 



181 



Mte* 



m^ AN ILLUSTRATED ^V 



MAGAZINE OF SCIENCE 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: OCTOBER 2, 1893. 



CONTENTS. 



PAGE 



The Life-History of a Solar Eclipse. By E. Waltee 



llArNDEB ... ... ... ... ... 181 



Whalebone and Whalebone Whales. By K. Ltdekkeb, 



B.A.Cantab 184 



Galls and their Occupants— IV. By E. A. Butleb ... 186 



What is the Suns Photosphere .=■ By A. C. Rantakd 189 



Science Notes 191 



Letters :— Geo. E. Hale ; C. W. Sweeting ; P. H. Glew 192 

 Lightning Photographs and Some Photographic 



Defects. By A. C. Kanyaed 193 



The Constitution of Gases. By J. J. Stewaet, B.A.Cantab. 195 



The Earth in Space. By William Schooling, F.R.A.S. 196 

 The Face of the Sky for October. By Hebbebt 



Sadler, F.R.A.S 198 



Chess Column. By C. D. Locock, B.A.Oxon 199 



THE LIFE-HISTORY OF A SOLAR ECLIPSE. 



By E. Walter Maunder, Sec. R.A.S., Superintendent of tlie 

 Physical Department, Boijal Observatory, Greenwich. 



SHOKTLY before the recent total eclipse, a paragraph 

 went the round of the papers to the effect that this 

 eclipse would be a repetition of one which occurred 

 some eight hundred and sixty years before our era, 

 and which was made use of to bring about a 

 revolution in the government of Assyria. 

 Seeing, however, that an eclipse of the sun 

 is caused simply by the three bodies — the 

 sun, moon, and earth — coming into the 

 same straight line, the moon being between 

 the other two, it seems at first that it is 

 absurd to speak of one eclipse being a 

 recurrence or repetition of another, unless 

 we go fm:ther, and regard every echpse as 

 a recurrence of any other, the same three 

 bodies being alone concerned in each and 

 all of them. 



There is, however, a very real significa- 

 tion attaching to the expression. Certain 

 ecUpses do stand in a special relationship 

 to each other, a relationship which they 

 do not hold to the general run. Eclipses 

 have their own special characteristics, 

 and it is easy to see how these arise. The 

 periods of the earth and moon are nearly, 

 but not quite, commensurable, and the 

 conditions which prevail at one eclipse are reproduced very 

 closely, but not precisely, after a considerable interval of 

 time. 



To go more into detail. Since the orbit of the moon 

 has a slight tilt, and does hot coincide with the plane of 

 the earth's orbit, it follows that, instead of having a solar 

 and a lunar eclipse every month, an eclipse only takes 

 place when the new or full moon falls near one or other of 

 the nodes, the two points where the plane of the orbit of 

 the moon intersects the plane of the orbit of the earth. 

 Next, the time which the moon takes in moving from one 

 of her nodes round to the same node again is not the 

 same as that which she takes in travelling from conjunction 

 to conjunction, that is, from new moon to new moon again. 

 Or, to use the technical terms, a " draconic " month is 

 not the same as the " synodic." The synodic month, or 

 lunation, the " month " in the ordinary sense of the word, 

 is 29 days 12 hours 44 minutes ; the draconic month, the 

 return to the node, is accomplished in 27 days 5 hours 

 6 minutes, or two days and nearly eight hours sooner. If 

 we have a total solar eclipse at one new moon, the moon 

 being in conjunction and at her node at the same moment, 

 then when she reaches the same node the next month she 

 will still be 2^ days from " new," and when " new " wiU 

 have passed her node by about 30°. Thus in Fig. 1, M 

 represents the moon as new exactly at the node, and 

 therefore as centrally eclipsing the sun. When she is 

 new the following month she will be at M,, and as the 

 figure illustrates the descending node, will be below the 

 Sim, So, as seen from the earth at conjunction. No eclipse 

 can therefore take place ; indeed, if the moon has passed 

 her node by as much as 17° she will in general escape 

 eclipse, and under special circumstances a distance of even 

 15J° may leave her clear of the sun. 



The next month will see the moon about 60° from her 

 node when " new " ; another month the distance will be 

 not quite 90°. The conditions for an eclipse are, there- 

 fore, more unfavourable than ever. But in six months' 

 time, when the distance from the first node is a Uttle less 

 than 180°, the moon is necessarily near the opposite node, 

 and an eclipse follows. Then for six months more there 

 is no eclipse, until, at the end of twelve mouths, we find 

 that once again conjunction occurs nearly at the node. 

 Twelve times the synodic month is 354 days 8 hours 

 48 minutes 34 seconds, or not quite fifteen hours longer 

 than thirteen times the draconic month, 353 days 18 hours 

 12 minutes 48 seconds, and the moon is only about 8° 

 from the node at the " new." But since twelve lunar 



J^S 2 



months equal 354 days, the new eclipse occurs eleven days 

 earlier in the year than its predecessor ; and since the 

 mean hmit for the distance from the node is 17°, it is 



