ASTRONOMY. 



[DI8TA.HOB or THE BOS. 



d. k. m. 



Spring 93 30 69 



!3 14 13 



.am 811 1H to 



\\II.UT 89 2 



The sun, then foro, remains nearly eight days longer 

 in the northern than in the southern hemisphere. By 

 taking into account the rate of the progressive motion of 

 the apsides, M N, which is 61' fl per annum, according 

 K.tli to theory and observation, the time when the line 

 of s .Utices, D B, coincided with the line of apsides may 

 be determined ; and it has been concluded that this 

 occurred in the year 1200. At that time the autumn 

 and winter quarters were of equal length, and likewise 

 tin- i-j.i m:,' and summer ; but the summer was longer 

 than the winter. 



DISTANCE or TH SUN. Before we make any very ac- 

 cur.ito progress in the motion of the sun in its orbit, wo 

 : tint discover the sun's actual distance, or the angle 

 which tli earth subtends at the sun. This angle is 

 termed the parallax, and it is obtained by observing a 

 traiinit of Venus, across the sun's disc, in the manner 

 illustrated in the following diagram (Fig. 16). 



The relative distances of the earth and Venus from the 



Fig. 16. 



tin's centre are accurately known. Let us suppose that 

 when Venus is situated between the sun and the earth, 

 its distance from the former is 073, the earth's distance 

 being 1-00. The earth being situated at T, and two ob- 

 servers stationed at the opposite diameters A and B, the 

 observer at A will perceive Venus, V, crossing the sun's 

 disc from c to d, whilst the observer at B will see it 

 passing from etaf. The breadth of this zone, a b, will 

 be considerably greater than the diameter of the earth, 

 A B, in consequence of the greater proximity of the 

 planet to the earth, V S being 27 times greater than the 

 line V T, and consequently the line a b is 27 greater 

 than the diameter of the earth, A B. If the angular 

 measure a 6 could be obtained, this would therefore be 

 27 times greater than the earth's apparent diameter as 

 seen from the sun, or 5-4 times the sun's horizontal 

 parallax. This angular measure can be found by the 

 observers A and B taking the distance of the limb or 

 centre of Venus from the sun's limb. The line a 6, as 

 here represented, is drawn much greater than it really 

 is, for the resulting diameter of the earth is only 8.} 

 seconds of arc ; and consequently the breadth of this 

 tone is only 46" -4, or about three-quarters of the dia- 

 meter of Venus at the time of inferior conjunction, or 

 j*j of the mean diameter of the sun. 



Although th calculations of the parallax of the sun, deduced from 

 the transit* of Venus in 1761 and 1769, were subsequently made with all 

 po-ible can by Proirawr Kncke, affording 8" 5776, still there was 

 reason to doubt the accuracy of the data on which that value was founded. 

 It was, therefore, extremely desirable to check the re*ult thus ob- 

 t mi. d ; and beyond the reasons Just mentioned, the motion of the nodes 

 of Venus, oonsidered in relation with the other planet*, the speed of light, 

 a* dru-rniined by M Foueault, and other question., required more accu- 

 ral* solution of the inquiry. 



In ISM, Profeeaor Airy called the attention of observer* to the oppo- 

 sitions of HITS, occurring in 19W and 18J. The former year was un- 

 1. owing to the state of the weather precluding observation.! ; but 

 in IS81 sufficient data were gathered to show that the value 8"'i776 was 

 underrated. In that year, observations of the parallaxc* of Mara at 

 Greenwich and Wiuiamsiown. gave "-9Jj whi t how of PulV.va and 

 UM Cap* of Uood Hop., reiulwd In S"-97. Bubwquently, M. Leverrier 

 arrived at the conclusion, that a" -a.! would represent the solar parallax 



If this modinrd value be adopted, c.m...l.Tble dinVrrnen wi:l arise in 

 Ik* estimate of the distance*. 4 i,al between the sun 



and the earth BUM b* reduced by about 4,090,000 miles, affording a 

 MB distance of N.UO.OOO mil... The mean distance from the sun" of 

 KVSSH. f t f*' wooM "meanuemly be, In round numbers Mercurv, 

 100.000; Venus, M.400,000; fcsr*. 1S9.I100.000: Jupiter, 474.000.000; 

 -turn. 171.000.000; Uranus. 1.7M.OOO.OOO ; and Neptune. 2,742,000.000. 

 TM dhuswtrr of the sun would be about MO.OOO mile*, and hi* volume 

 Mt*VtM uaae* that of UM earth. The BUM of our (lob* would b* in- 



It is not only, however, by observing micn> 

 the distance of Venus from the sun's limb that the rela- 

 tive distance a 6 may be deduced ; it can likewise be 

 determined by the tim* which the planet remains on th 

 sun's diac at the several stations, or the interval elapsed 

 between its entry at c and e to its disappearance at d 

 and / The motions of Venus are accurately known by 

 means of the tables, and thui the lengths of the ch 

 c d and e f, and therefore the distance a b also. It is 

 usu:il, in these cases of the passage of the planets Venus 

 and Mercury over the sun's disc, to note the time when 

 the limbs of the planet aud sun come in contact, and 

 also, after the ingress has occurred, to note the time 

 when they are last in contact. The same must be done 

 at its egress. The time* of interior contact can be per- 

 ceived much more exactly than the exterior. If the 

 transit be central, the duration of its passage may extend 

 from "h. 52rn. to 7h. 54m. ; and according as it is more 

 distant from the centre, the chord will be shorter, ami 

 the time in the same proportion. In the passage of 1769, 

 the duration at Wardhus was Ch. 29m. ; at Hudson's 

 Bay, 6h. 22m. ; at California, 6h. 14m. 3s. ; and by I'.i].- 

 tain Cook, at Tahiti, at Ch. O.jin. From those measures 

 the horizontal parallax of the sun was calculated by the 

 astronomers of the time at8*'5693 ; but the results dif- 

 fer considerably, the entrances being 8* '2 and 9* '2. 

 The small altitude of the sun in the northern latitude, 

 and its consequently bad definition, made it difficult to 

 estimate the exact moment of entrance on the solar 

 disc. Some observers saw it as much as twenty seconds 

 sooner than others ; but the effect of this would only en- 

 tail an error of one-sixtieth, or probably of only one- 

 half of that amount, on the total parallax ; so that in- 

 stead of 8*-5o93, weshonld have 8" 040 or 8" 4'.'*. 

 calculations have been performed anew with the utmost 

 accuracy, by Professor Encke, of Berlin. From the lirat 

 transit of June 5, 1701, that celebrated astronomer has 

 obtained the value 8"-490526 comprised between the 

 limits of 8*'429vJ13 aud 8* do 1237. The discussion of 

 the second passage of June 3, 1709, gives 8* '5770, not 

 greatly different from the above : it is the value now gene- 

 rally adopted by astronomers. The resulting mean dis- 

 tance of the earth from the sun would, consequently, be 



, or 24040 9 radii of the terrestrial equator, 



sin. 8--6776 1 



which corresponds to 82,007,200 miles, of CO to a de- 

 gree. To pass through this distance light employs MM. 

 l:i I. ">s., and in consequence we see tlie snn - 

 behind its real position. Tiie radius of the sun at 

 its mean distance being 960" '9, the real diameters of 

 the two bodies will be in proportion to their appa- 

 rent diameters, or as 8578 to 900-9, or the dian 

 of the sun is 112 O24 times greater than that of the 

 earth. Their volumes being proportionally as the cubes 

 of their radii, it would follow that the bulk of the sun 

 is 1,403,845 times that of the earth.* 



PASSAGE or VENUS ACROSS THK SUN'S Disc. This 

 phenomenon can only take place at intervals of eight 



creased by one-tenth; the circumference of onr orbit would be 599,191,000, 

 and the mean hourly velocity of the earth, 66,460 mile*. Each 

 would be similarly affected In iu mass, volume, &c., by changing th* 

 estimate of solar parallax from "-5778 to "-9J. 



No transit of Venus will occur, by which theae results can be tested, 

 until the years 1874 and 1H82. Meanwhile, the care bestowed on the cal- 

 culations, giving the new value, having been made by so eminent an 



l*tnS>MMI l- M, 1 - veu., r, c -Mill., tl.' Ill 1'. HI. 1. 1).'!!. '. I. -|l, It, It lint I III- 



fullest confidence of observers. Indeed, incidentally, they receiv < 

 siderable confirmation from tli<. hivwtigatioaa of M. foucauu 

 the velocity of light. That philosopher, from careful ex|H>rmient, pro- 

 pose* to reduce the estimated speed of light from 11)2,00(1 to ISi 000 

 miles per second. For those not acquainted with tlie value of such 

 data, in an astronomical point of view, we may ob-ervc, that in an 

 eclipse, we never see the rch|i>ed body at the moment or last contact or 

 emersion, obtained by calculation. It doe* not reappe.u lor som.' utin 1 , 

 owing to the fact that Its light has to travel ID the . y , unit, in uoing so, 

 ocruj!: taa. ity noting tlii- t.me which claptc* 



between the m .- the eclipse, 



to the observi nee in srcontK 



gives the distance of the body. <>i. 



of the body be divided by tlie number of i-etonds which elapse bi 

 the actual and apparent conciUKion of i d of li^lit. in 



mile* per second, is obta : - .li lines of J 



afford an illustration of this fact, and will be i ; i r< <t to ut uuge y* I, el 

 arf ., in connection with the law* of the aberration of light. * u. 



