THE SUN'S DISC.] 



ASTRONOMY. 



927 



years and 105 years, and the two next times will occur on 

 Dec. 8, 1874, and Dec. 6, 1882. Since Venus comes 

 nearly to the same point of the heavens every eight 

 years, it may be expected that if it transits over the sun 

 at one given epoch, it will pass over it on the eighth 

 following year, and this generally takes place. As 

 Venus, however, changes her latitudes during this period 

 of eight years, it is impossible that three can follow each 

 other in succession, as it must then pass beyond the disc 

 of the sun, which is only 32* in diameter ; and as they 

 can only take place when the plane is very near its nodes, 

 it follows that for some centuries this will occur either 

 in the months of June or December. 



The solid mass of the sun is found to be 354,936 times 

 greater than that of the earth ; and, comparing this with 

 its volume, we find that the latter is only equal to one- 

 fourth of that of the sun.* The gravity on the surface 

 of the sun is 28 "36 times greater than that on the surface 

 of the earth ; or a body which weighs one pound on the 

 surface of our planet, weighs twenty-eight one-third 

 pounds on the surface of the sun ; and, consequently, a 

 body will fall with twenty-eight one-third times the 

 velocity during the first second of time. 



FORM OP SDN'S Disc. In measuring the disc of the 

 sun with the heliometer, or any graduated instrument, 

 it is necessary to take precautions that it be favourably 

 situated, and not too near the horizon, where the re- 

 fraction has a very sensible effect in elevating the lower 

 part >{ the disc in a greater degree than the upper. 

 \Vhen the sun is 45 above the horizon, the difference 

 between the vertical and horizontal diameters only 

 amounts to 1"; but when it touches the horizon, the 

 vertical diameter is one-sixth part of the whole diameter 

 less than the horizontal. The figure which the sun 

 takes under these circumstances will be seen by Figs. 

 17 and 38, which have been constructed with exact pro- 

 Fig. 17. rig. 18. 



portions. The refraction causes the sun, when really at 

 IS, to appear at S', with its apparent lower limb touching 

 the horizon. This is the usual effect of refraction ; but 

 at the horizon, many irregular local causes tend to pro- 

 duce changes, and it is generally found to be serrated, 

 and with a continual whirling motion, due to the irregular 

 motions of the atmosphere. It appears strange that the 

 discs of the sun and moon, which, if anything, are 

 smaller under those circumstances than when at a great 

 altitude, are commonly remarked to be larger at the 

 horizon than elsewhere. The ancient astronomers sought 

 to explain this optical illusion, by the effect which the 

 atmosphere had upon the luminous rays of the sun. It 

 is, at the present time, more naturally attributed to the 

 effect which a comparison with the various objects seen 

 at the horizon produces ; and, as we suppose the sun to 

 be removed an immense distance beyond those bodies, 

 its diameter appears greater than it really is. A similar 

 effect takes place when the apparent areas of the con- 

 stellations are compared at the zenith and the horizon ; 

 This may be put in the following form : To obtain a body having an 

 attractive force equal to that of the gun, would require that 354,936 

 pheres each an lartre, and having the same specific gravity, aa our earth, 

 should be combined. The MASS of matter, however, forming the BUR, 

 occupies four times the BULK, or "PACE, of such imaginary spheres ; and 

 hence, the volum* of the sun is equal to about l.lOj.MS times that of the 

 earth. ED. 



in the latter position, they appear considerably mora 

 extensive than they really are. It is, probably, due to 

 a similar cause, that if we attempt to measure an altitude 

 of 45 from the horizon, or the point half-way between 

 the zenith and horizon, it frequently happens that when 

 we come to measure it with an instrument, that which 

 we consider to be the central point is situated some 

 degrees nearer to the horizon than it appears to be. 



TELESCOPIC APPEARANCE or THE SUN. An observer, 

 viewing the sun's surface with an instrument of moderate 

 power, would probably not perceive any marked differ- 

 ence of light and shade over its general surface ; but 

 with higher powers, and a better telescope, a steady 

 mottled appearance would become apparent, dark and 

 light specks being softly intermingled, but not strikingly 

 apparent when seen for the first time, or under favour- 

 able circumstances. Whilst the growid is riot uniformly 

 bright, there are very frequent portions which are de- 

 cidedly brighter than the general surface, and others, 

 which are much more remarkably darker, being as dark 

 and black as the surrounding heavens, when viewed with 

 the coloured glasses made use of. It is by means of 

 these fleeting maculae and faculce, as they have been 

 called, that nearly all our knowledge of the solar nature, 

 its rotation, and the position of its axis, as well as the 

 structure of its lurid and changeable atmosphere, is 

 derived. These spots have constantly been perceived 

 ever since the discovery of the telescope, and there is 

 little doubt but that they were frequently seen before 

 that time with, the unaided eye, as they have occasionally 

 been seen since. They sometimes cover large portions of 

 the sun's surface, and are strikingly apparent from the 

 contrast they afford with the bright surface surrounding 

 them. 



By whom these spots were first seen, after the invention 

 of the telescope, is a matter which has found its way 

 into the debateable land of scientific 

 history. It would be most natural to 

 suppose that Galileo, who had first 

 scanned the heavens with the optic 

 tube of his invention, and who was so 

 capable of distinguishing every phe- 

 nomenon which was apparent, would 

 have been the first to notice these irre- 

 gular appearances ; but the first pub- 

 lication of them is due to Fabricius, 

 who perceived them early in the year 

 1611 ; and his work, published in the 

 same year, has a dedication bearing 

 date June 13, 1611. In so far, how- 

 ever, as the mere observation of the 

 spots is taken into account, they were 

 seen by our countryman, Harriot, some 

 time previous to any date fixed upon by 

 the regular claimants to the discovery. Galileo appears 

 to have seen them in April or May, 1611. Soheiner 

 informs us that he perceived them during the same 

 months, but he did not take them into consideration 

 before the October of that year. 



In the annals of China, it would appear that a large 

 spot was visible on the sun in the 'year 321 of our era. 

 In the year 807, a large spot was visible on the sun for 

 the space of eight days, which was supposed by many to 

 be a passage of Mercury over the sun's disc ; but the 

 length of time during which it remained visible is, of 

 course, quite incompatible with such a supposition. 

 Large spots which were seen by Averrhoes, Scaliger, and 

 Kepler, were likewise supposed to be passages of this 

 planet ; but when it is remembered that a spot of the 

 dimensions of Venus, with a diameter of five times that 

 of Mercury, could not be detected on the sun's disc, we 

 may be quite certain of their nature. Since that period, 

 they have been frequently seen with the naked eye. It 

 was, however, from telescopic observation of these spots, 

 that the fact of the rotation of the sun on itg axis was 

 made apparent ; and Fabricius was the first who surmised 

 that they adhered to the sun, judging from their slow 

 motion when they arrived at the edge of that luminary. 

 This rotation, and its duration, were subsequently con- 



