LESSONS IN A MY. 





.IV. VIII. 



WCBMION OF THK :KH NUTATION SOI. 



's '"i.i ITVDI AM, tO 



MUTH INSTRUMENT PBOBLKM8 WITH IIIK OLOBKH. 



Ir we look to the celestial globe wo shall find that, though tho 

 mark r signifying tho commencoment of the mgn Ariel is placed 

 nt tli.' intersection of the equinoctial and tho ecliptic, yet the 



portion of the zodiac commencing at that nign i* m reality 

 occupied l,y tho con.sU'llation I'tsfra The tara forming Aries 

 are im>\i-.l 'M to tho east, occupying tho place assigned to 

 Taurus, and, in tho same way, Taurus ami all 

 the other zodiacal constellations are moved 



_'ll to tin- 



The reason of this is the precession of tho 

 equinoxes which has already been referred to 

 as having boon discovered by Hippurchns. 

 The points of intersection of tho equator and 

 ecliptic, or, as they are frequently termed, 

 do not rcniiiin constantly in tins 

 same place, lint uro slowly moving towards 

 the west, that is, in a retrograde direction. 



This was first observed by noticing that 

 tho right ascension of all stars was slowly 

 and uniformly increasing : this could only 

 be accounted for in one of two ways either 

 they must all be slowly moving for- 

 wards, or tho point from which we 

 niea-om} their right ascensions must 

 be moving backwards. The latter of 

 these explanations, being by far the 

 most simple, has been adopted. The 

 rate of this motion is but slow, so 

 that its effect on the position of the 

 stars from year to year can only bo 

 ascertained by the most careful and 

 delicate observations. When, however, 

 we compare the position of a star with 

 that assigned to it by observers a few 

 centuries ago, we soon become con- 

 scious of the change. The most care- 

 ful observations fix the annual amount 

 of this motion at 50'2", so that the 

 time occupied by the nodes in making 

 a complete circuit of the heavens 

 would bo a little more than 25,800 

 years. By reckoning back it is found 

 that the constellations and the signs 

 of the zodiac corresponded with one 

 another about the year 370 B.C. 



The explanation of this phenomenon re- 

 quires a higher degree of mathematics than 

 could be introduced into these lessons. We 

 may, however, state generally that it arises 

 from the accumulation of matter at the earth's 

 equator. On account of this the sun and 

 moon exert a more powerful attraction on 

 that part of tho earth than on the poles, and 

 thus the position of the axis is slightly 

 altered and tho changes spoken of are pro- 

 duced. 



In addition to this motion of the pole, 

 there is another of much smaller amount, 

 which is known as nut at inn, or the nod- 

 ding of the pole. It arises from the fact that the earth's dis- 

 tance from the sun varies at different times of the year, and 

 thus the amount of precession varies slightly from day to day. 

 The effect of this variation is to cause the pole to describe in 

 the course of about eighteen and a half years a very small 

 ellipse, the longer axis being about 18V 1 and the shorter nearly 

 14". This motion, combined with the other, produces a slightly 

 wavy or undulating movement of tho pole ; it is only, however, 

 in very accurate observations that this has to be considered. 



One important effect of the precession of the equinoxes is to 

 change the position of the pole-star. That at present known by 

 this name is distant about 1 .} from the true pole ; its distance 

 is, however, gradually diminishing, so that in tho coarse of 

 years it will bo within half a degree, and it will then commence _ 



104 N.E. 



toreet'l In about 12,000 yean tit* brilliant 



in the constellation of the Lyra, will be very do*e to U* i-,U, 



and serve M pole-Ur. 



A utrauge circuinnUnce has been discovered in connection 

 with tiiis change in the portion of tb pole-ster. Nearly 4,000 

 yearn ago the sU; he third in magnitude in the con- 



HtolUtion Draco, WM very near the pole. About this period 

 many of the pyramid* of Egypt were built, and it U bond that 

 several of these have openings on their north sides inclined at 

 an angle of about 20 to the horizon, Tbe*e paaiegee are all 

 directed toward* the meridian, and their inclination is each that 

 an observer standing at tho bottom of any 

 one of them would have been able to set the 

 pole-star at the time of its crossing the meri- 

 dian below the pole. Thin corroborates the 

 calculations of astronomers, and at the samo 

 time confirms the idea that the ancients were 

 attentive observers of the motions of the 

 stars. 



Fig. 11 represents the position of the globe 

 to an observer in the latitude of London, the 

 angle at which the axis N is inclined to the 

 horizon H H' being ju*t 51 J ; z is the zenith, 

 and N' the nadir. The great circle A o B i 

 represents the equator, and to or, inclined 

 to it at an angle of 234, the ecliptic, o and 

 I being the nodes ; o is sometimes 

 called the ascending node, as at this 

 point the sun passes north of the 

 equator, and I is the descending node. 

 There are also two other points K 

 and D in the ecliptic specially distin- 

 guished, and known as the toltlilial 

 points. These are situated midway 

 between the nodes, and are at the 

 commencement of the signs of Cancer 

 and Capriconras. The term toUlitial 

 is derived from sol, the sun, and ttzrc, 

 to stand, and is applied to these points 

 because when tho sun reaches them it 

 has attained its greatest north or 

 south declination, and appears to stand 

 still for a few days before commenc- 

 ing to retrace its steps. Two great 

 circles are drawn on the celestial glob* 

 passing through the poles, the one, 

 N a s I, passing likewise through the 

 equinoctial points, and tho other, 

 N e s D, through the solstitial points, 

 and these are known as the equinoc- 

 tial and solstitial colurcs. They divide the 

 ecliptic into four equal portions, and mark 

 the divisions of the seasons of the year. On 

 most globes there is on the wooden horizon a 

 circle in which the days of the month are 

 marked, so that by it the sun's actual place 

 in the ecliptic can be found for any day of 

 tho year, and by reference to this we shall 

 find that the days on which the sun is at the 

 solstices are the 21st of June and the 21st of 

 December, and these are respectively the 

 longest and shortest days. 



Two small circles, c D and s r, parallel to 

 tho equinoctial, and passing through the sol- 

 stitial points, are known as the Tropic*, that to the north being 

 distinguished as tho Tropic of Cancer, while tho southern one is 

 called the Tropic of Capricorn. These are, hcwevcr, of more 

 importance in the use of the terrestrial globe than in that of 

 the celestial. There are also two circles, K L and M o, situated 

 at a similar distance from the poles, which mark the limits of 

 the polar regions, from which the sun U sometimes hidden for 

 more than a complete day. These are known as the Arctic and 

 Antarctic circles. The shading in Fig. 12 represents tho extent 

 of the earth's surface which is illuminated when the sun has 

 attained its greatest south declination, at which time, an will be 

 seen, the Antarctic circle U completely illuminated, while the 

 sun is altogether hidden from tho Arctic. 

 The most usual way of describing the position of any star in 



