SCIENCE-GOSSW. 



187 



CHAITKKS li)K VOING ASTRONOMKKS. 



Bv Frank C. Dbnnrtt. 



THE SUN. 



( Continued from fagi 1$$.) 



Thk (lark spots or maculae usually consist of two 

 portions, A sccMiiingly black cenlrc, known as llie 

 umbra, surroiindcii by a gity portion, or penumbra. 

 Kurlhcr study will reveal ilie diet that the uuilir.i in 

 itself is not black except by contrast .igainst tlic 

 brilliance of the disc. I'lirthermure, it will often be 

 found that within the borders of the umbra there are 

 one or more still darker portions known as nuclei. 

 .Sometimes one penumbra will contain two or more 

 mubrae. The penumbra is not an evenly grey 

 surface, but is often seemingly made up of fine 

 filiaments, as will be shown later on. The m.aculae 

 vaiy in size from the most minute "pores" to 

 enormous spots, having sometimes a diameter of very 

 many thovisands of miles. Occasionally they appear 

 singly on the disc, but more frequently as members of 

 a group. \'ery often the members of a group will be 

 found to be ranged in a curve figured not unhke the 

 letter S, a similar curve fre<|uently showing itsell in 

 the pl.acing of the umbrae in large spots. .Some 

 spots and groups arc remarkable for their persistence, 

 others for their evanescence. Some may be observed 

 through several rotations of the sun, whilst others 

 seem to appear, or disappear, almost under the 

 observer's eye. Sir William lierschel was once 

 looking at a group, when he turned his head for a 

 moment, then on looking into the eyepiece again 

 found that the group had disappeared. Other 

 ob.servers have noticed ei|ually rapid changes. Con- 

 siderable alteration may often be noted in the appear- 

 ance of a large spot even at short intervals. Occa- 

 sionally it is found that extensive portions of the 

 umbra of a large spot inste.id of appearing to be 

 black, are indeed of a brown or amber tint. I have 

 .seen this with a telescope of barely 2 inches 

 aperture. \'ery frequently bridges of brilliant matter 

 are seen crossing the innbra and sometimes the 

 penumbra as well, and rapid changes may often be 

 noticed both in their appearance and direction. 



Sun spots are by no means equally frequent at all 

 times. .Some years not a single day passes when the 

 sun is entirely free from spots : at other times more 

 than a hundred of the dates on which the sun is 

 observed have the record of no spots. Careful 

 study of oliservations has led to the conclusion that 

 sun spots have a "period," which Wolf makes il'i 



K= 



Diagram of Projection Ari»AR.\Tcs. 



years, and otherobserversconfirm the same. This is the 

 mean periixl, because some variation is noticeable in the 

 length of time separating the maxima, or periods when 

 most spots are visible. Near the time of minimum it 

 is noticed that the sun spots are usually nnich nearer 

 to the Kijualorthanat the time of maximum. 



The sun's equator is not in the same plane as that 

 i>f the earth'sorbit, but inclined at an angle of 7' 15'. 

 In consequence of this the spots do not always 

 appear to travel in the same direction across the disc. 

 Early in June and December they seem travelling in 

 straight lines, but in September and March their 

 path appears convex towards the south and north 

 respecti\ely. Again, in June the pathw.ay of a spot 



ap|jears to gradually rise as il travels from cast to 

 west, whilst in Decenil)er il sinks. , 



When u spot or group is pitssing on or off the disc, 

 it appears to travel much more slowlv than when near 

 the middle of the disc, because of tfie globular Uixm 

 of the sun. Owing to the same cause when a spot is 

 close to the limb it only appears as a dark line, but 

 as il travels on to the disc the thickness of the line 

 increases and the six)t in turn appears oval, rounded, 

 and often elongated in east and west ; .similar eft'ects, 

 but in the opposite order, happening as spots advance 

 towards the western limb. Cognisance of this fore- 

 shortening, as it is called, has to l«: taken when 

 determining the size of spots or groups. 



The readiest method of measuring sun spots or 



SOUTH. 



NORTH. 



Su.s Spots of September, i8q6. 

 groups is that of projection, one form of apparatus for 

 which purpose is illustrated in the di;igram. 



T is the telescope, A a collar round lhi> telescope 

 body carrying a flat ring of metal about 6in. in 

 diameter to act as a screen to shade the surface U, 

 and strong enough to carry two or three telescopic 

 rods, B B, carrying a light frame C, which carries a 

 white screen D, on which the image falls, and which 

 may well be a disc of ground opal gla.ss. Two 

 methods are .available for measuring. The foimer is 

 to have a line drawn across the middle of the screen, 

 and then to note the exact time occupied by the spot 

 or group in crossing the line. The better way. how- 

 ever, is to have two screens fitting at D, one plain, 

 the other 5in. in diameter divided into half-inch 

 squares, one or two being subdivided into quarter- 

 inch squares. To measure a spot or group make the 

 projected image correspond, when quite sharp, with 

 the 5in. circle, which may be accomplished by aid of 

 the telescopic rods B B. When this is done the 

 divisions correspond to tenths and twentieths of the 

 sun's diameter, so that il is easy lo determine the 

 dimensions of a group. Of course the scale disc 

 should be adjustable, .so that one set of the lines on 

 the disc can be set parallel with the longest diameter 

 of the group. 



A wonderful group of spots, none of them how- 

 ever of great size, appeared on the sun in Septem- 

 ber, 1S96. They are shown on the accompanying 

 reduction of a Greenwich photograph, reproduced 

 by the kindness of the Astronomer Royal. 

 ( To fie tonlinncd. ) 



