Feb. 25, I 



NA TV RE 



401 



In this way. Sometimes wlien we look at the sun it is a 

 beautifully pure hemisphere, almost equally illuminated all over 

 it with the exception of a darkening towards the edge, about 

 which more presently ; but at other times these conditions are 

 altered. It is more or less covered with what are called spots. 

 It is more than two centuries and a half ago since it was clearly 

 demonstrated that these spots belong to the sun itself ; and it 

 will be clear, therefore, that if these spots really do belong to 

 the sun, or, to be more precise, are phenomena occurring in 

 the photosphere — which is the part of the sun which we usually 

 see, the rotation of the sun will be demonstrated if these 

 spots are found to travel regularly across its disk ; whereas its 

 fixity will be demonstrated if we find that the spots do not 

 move at all. A very great deal of work has been done in this 

 direction, and it has been determined beyond all question that 

 the sun does rotate like the earth and like the other planets of 

 our system, and that it rotates from west to east, contrary to the 

 hands of a watch, as the earth does. 



We next have to consider the plane of this rotation. The 

 earth moves round the sun in a plane which we call the plane of 

 the ecliptic ; but we know that the earth does not r itate in this 

 plane. There is a difference of 23^° between the equatorial ""■^ 

 ecliptic planes. We therefore say that the earth's axis is incli 

 234° to the plane of the ecliptic. Further, if we wish to ki 

 the particular direction in which it is inclined, we must de 

 mine the longitude of the ascending node, and this done, we 

 determine the star towards which the earth's axis points. 



Well, can we get out these facts with regard to the sun ? '' 

 The sun does not rotate in the plane of the ecliptic as we m 

 first of all imagine that it would do. Its axis is inclined at at 

 7" to that plane, and its ascending node does not lie in 

 same direction in space as the ascending node of the earth, 

 it is distant from it some 73°. We have two very accu 

 determinations of these two sets of data. Carrington's 

 value gives — 



Longitude of node 73 40 



Inclination 7 'S 



We see, although the sun's rotation does not take place in 

 plane of the ecliptic, it does not take place in a plane 

 far removed from it. 



Sporer, a German observer, who has taken up this ques 

 since Carrington, makes some very slight changes. He giv 



Longitude of node 

 Inclination... • ... 



57 



The right ascension of the star towards which the sun's 

 points is 18 hours 14 minutes, and its declination 64° N. It 

 half-way between our own pole star, and the bright st; 

 Lyra;. 



We may now carry these considerations a little further. W 

 the earth passes through the nodes of the sun's equator, th; 



Sept 



Fic. 2. — Position of the Sun's axis, and apparent paths of the spots across the 

 disk, as seen from the Earth at ditTerent times of the year. 1 he arro\vs 

 show the direction in which the Sun rotates. The inclination of the axis 

 is exaggerated, so that the effect produced may be more clearly seen. 



to say, when the axis cf the sun is at right angles to the line 

 joining the centre of the sun and the centre of the earth, it will 

 be perfectly clear that the spots will appear to travel straight 

 across the disk. The two times of the year in which this occurs 

 are June 3 and December 5. The two poles of the sun at those 

 times are in fact on the sun's limb, and it is that condition really 

 which makes the path straight (Fig. 2). 



From June 3 to December 5 the north pole of the sun is 

 gradually moving earthward. It will be clear, therefore, that 



during that time the spots, instead of travelling in straight lines 

 across the disk, will gradually have their paths curved with the 

 convexity downwards. When we have got to December, from 

 that time to the next June it is the south pole that will be inclined 

 towards the earth ; and therefore the spots will then move with 

 the convexity of their paths upwards. 



So far I have said nothing about the period of the rotation 

 of the .sun. The question of the sun's rotation is not quite such 

 a simple matter as it might appear at first sight. Here we must 

 be quite honest to the first workers, and I must tell you that the 

 actual facts appear to have been clear to a man who lived 

 three centuries ago — Scheiner — who was the first to observe the 

 spots with any very great and continuous care ; he made what 

 appeared to him the extraordinary discovery that the spots which 

 were nearest to the sun's equator appeared to travel at a 

 quicker rate than those which were nearer the sun's poles. The 

 average time in which the spots appear to cross the sun is about 

 28 days. That you will understand is what we may term the 



Fig. 3, — Cur\-e showing the period of rota'tion^of the photosphere in different 

 latitudes north and south from Carrington's observations ; 851' of solar 

 longitude per diem = rate of rotation in lat. 15° N. The vertical lines 

 represent differences of 10' of longitude, + to the right, — to the left, of 

 the line cutting the curve in lat. 15° N. 



synodic period, because the observations are made from the 

 earth, which is moving in the same direction as the spots while 

 the observations are being made. Making the correction for the 

 movement of the earth, and getting the actual period, the 28 days 

 have to be brought down to something like 26 for an average 

 spot. Carrington, whose results for the plane of rotation I have 

 already stated, also paid very great attention to this point, 

 and to his work, and also to Sporer's, we owe very much of our 

 present knowledge on this subject. 



Carrington, from the observation of some thousand spots, 

 came to the conclusion that the photosphere in which these 

 spots are supposed to float really moves more rapidly at the 

 equator than it does away from it, in the manner that Scheiner 

 had suggested, in such a way that the movement at the equator 

 really takes place in, as near as may be, 25 days, perhaps 

 a little less ; but that in latitude 30° there is a slackening 

 off of a day and a half, so that it takes a spot in latitude 30° 

 north or south not 25 days, but 26i days to make its movement 

 right round ; if we go as high as latitude 45°, we have to add on 



