LESSONS IN A8TEONOMT. 



one. When the Moon thus intervene* between the Earth and 

 the Sun wo have the phenomenon of a Molar oclipue, the Moon 

 rutting off the rays of the Sun, and causing iU own dark 

 Hhodow to pasa over a portion of the aarfaoe of the Earth. 

 I explain this. In it a ropruHonU the Bon, M the 

 M....II, and K tho Karth. The siios and distances aro of ooorae 

 thrown out of proportion, in order to render the figure more 

 As tho Moon travels on in its orbit, it come* into 

 tho position shown, and 

 then completely hide* tho 

 Sun from that part of 

 the Earth which hap* 

 pens to bo in tho lino 

 of ita ahadow. Tho con- 

 tral dark oone of ahadow 

 ia called the umbra, tho 

 Sun'a raya being entirely 

 cut off from it. Around 

 this ia a apace from 

 which only a portion of 

 tho Sun's raya is ex- 

 eluded, so that the ahadow 

 is less dark. This part 

 is known as the penum- 

 bra (f), and, aa will be 

 aeon, the raya from aomo 

 parts of the Sun pene- 

 trate this, BO that it is in reality a gradual shading off of tho 

 umbra. Those parts of the Earth's surface over which the 

 penumbra passes will see a partial eclipse of the Sun, only a 

 part of his disc being hidden by the Moon, aa represented in 

 Fig. 34. The nearer they lie to the lino whore tho central 

 shadow passes, the greater the portion of his disc which will 

 bo hidden. 



The diameter of the umbra on the Earth is never large, and 

 averages about 150 miles, so that it is only along a tract of 

 surface of this width that the eclipse will be seen as a total 

 one. Its exact size varies with tho distance of the Moon from 

 the Earth. When the 

 Moon is at its greatest 

 distance, the umbra does 

 not reach tho Earth 

 at all. Tho Moon then 

 appears to be smaller than 

 the Sun, and does not com- 

 pletely hide it, but leaves 

 a bright ring all round. 

 This is known as an 

 annuZar eclipse, and the 

 appearance it presents is 

 shown in Fig. 35. An 

 eclipse of this kind oc- 

 curred in England on the 

 14th of March, 1858, and 

 excited great interest. In 

 most places, however, the 

 weather was very unfavour- 

 able for its observation. It 

 h:is been calculated that 

 the greatest duration of 

 an annular eclipse at the 

 equator cannot exceed 12 J 

 minutes; a total eclipse 

 cannot continue so long 

 as this, since when the 

 eclipso is total tho Moon 



is nearer tho Earth, and its motion is therefore more rapid. 

 This duration depends, of course, on the comparative diameters 

 of the Sun and Moon. Now tho greatest apparent diameter 

 of the Moon is 3S 7 31", and the least of tho Sun is 31' 30", 

 the difference being 2" 1". The eclipso can therefore con- 

 tinue total while the Moon is passing over this arc, but no 

 longer. 



The occurrence of a total eclipse at any given place is, as may 

 be inferred from the above statements, a very rare event. None 

 was seen in London between 1140 and 1715. When seen, they 

 are most grand and wondrous phenomena, and consequently 

 astronomers usually travel, if necessary, a considerable distance, 



Fig. 36. ECLIPSE OF THE SUN, JULY 18, 1860. 



o M to be situated on the Una of totality. Aa the moon, which 

 ia iUolf invisible, advance* over and partially conceal* th no. 

 u dull hue teem* to creep over the face of all Nature ; bat it w 

 not till near the time when the ran is entirely hidden that the 

 darknoaa become* intense. It ia then nanally so dark that a 

 lantern ia required to enable any obaei raUuu* to be recorded, 

 or the time to be read from a chronometer. Many animals 

 seem to be affected by it ; birds harry to their nesU, sheep lay 

 down, flowers dose, and 

 the whole face of Nature 

 present* a most sad and 

 uncommon appearance. 

 Not 'infrequently, too, a 

 great and sadden fall of 

 temperature occur*. The 

 brighter stars aro also 

 frequently soon. 



Juat before the moment 

 of total obscuration, a 

 brilliant halo ia seen sur- 

 rounding the dark body of 

 the moon ; this ia known 

 as tho corona, and is 

 usually believed to be tho 

 snn'a atmosphere rendered 

 visible by the obscuration 

 of its direct raya. Obser- 

 vations made daring tho total eclipse of August 17, 1868, 

 showed that this atmonj hero was not self-luminous. Bright 

 rays aro also seen at tames darting out beyond the corona, 

 usually in the form of a cross. When the moon is just on 

 tho point of hiding the hut streak of the aun, the narrow 

 crescent appears to be divided into a number of separate beads 

 or drops, which seem sometimes to be in motion. This pheno- 

 menon ia known as " Baily 'a Beads," after Mr. Francis Baily, who 

 was the first to describe it particularly. No very satisfactory 

 explanation of this appearance has yet been assigned, bat some 

 observers in India during tho recent eclipse stated that it wae 



evidently produced by in- 

 equalities on the surface of 

 the moon. 



So great is the im- 

 portance attached to the 

 correct observation of a 

 total eclipse, that special 

 expeditions have been sent 

 out during recent yean to 

 many parts of the world 

 to observe them. Many 

 superior pieces of appa- 

 ratus have been lent or 

 procured for the purposes 

 of these expeditions, and 

 several most important 

 discoveries have thus been 

 made. 



A remarkable feature 

 in connection with total 

 eclipses of the sun, and 

 one that has attracted 

 much attention of late 

 years, is the existence of 

 a number of rose-coloured 

 protuberances around the 

 dark body of the moon 

 when the aun is completely 



hidden. These red flames appear at times to be in motion ; 

 they are, however, persistent enough to enable it to be clearly 

 seen that they belong to the solar atmosphere, the dark body of 

 the moon being observed to cover them as it moves onward. 



A general view of these remarkable objects is given in Fig. 

 36, which represents the eclipse of the 18th of July, 1860. In 

 the eclipse of August, 1868, one in particular was remarked, 

 having on elevation of 3* 18", or a tenth of the son's diameter. 

 This presented the appearance of a horn spirally twisted, and 

 was distinctly shown in the photographs taken by an expedition 

 sent out to India to make observations. Its shape and appear- 

 ance were very remarkable. 



