42 



♦ KNOWLEDGE ♦ 



[Jan, 19, 18fc3. 



THE FACE OF THE SKY. 



(From January liUh (o February 2nd, 1883.) 



TIIK Ptudent will maintuin Iiis watch on the Solar surface, ns 

 recommended on p. 11. 



Jlercury may bo detected with the naked eye on every clear 

 evening, after sunset, glittering above the S.W. by W. jjoiiit of the 

 horizon. The beginner who nioy employ a compass to determine 

 this point, must carefully note that the magnetic needle in this 

 country does not point to the true North, but 18° or 19° to the west 

 of it. Venus is still a most conspicuous object in the morning sky. 

 The major part of the constellations spoken of on p. 11 continue to 

 spangle our night sky ; but Pisces and Cygnus are disappearing in 

 the west ; and Cancer (Vol. I., p. 37G) Leo (id) Monoceros, Uydra 

 and Canis Major and Canis Minor (Jlap Vol. I., p. 383), are now 

 coming well into view in the Eastern half of the sky. Saturn 

 continues stationary (to the naked eye) to the south-east of 

 c Arietis. Jupiter describes a slightly retrograde path during the 

 fortnight, carrying him from east to west at 1° or 2'' above 

 Z Tauri. To-night (January I'Jth) his second Satellite will 

 reappear from eclipse at 6h. 41m. Os. to the right of the planet, and 

 somewhat below- his equator (as seen in an inverting telescope). 

 At 8h. 34m. p.m., Satellite I. will enter on to Jupiter's disc, fol- 

 lowed by its shadow at 9h. 20m. The Satellite will pass off at 

 lOh. 49m., and its shadow at llh. 36m. p.m. To-morrow night, the 

 20th, the same Satellite will be occulted at 5h. 43m. ; but instead 

 of emerging from behind the opposite limb of the planet, will 

 plunge into his shadow while still behind Jupiter, and will not re- 

 appear from eclipse until 8h. 44m. 5s. p.m. On the 21st, Satellite I. 

 will pass off Jupiter's face at 5h. 10m., as will its shadow at 

 Oh. 5m. p.m. On the night of the 22nd, the observer should direct 

 his attention to the egress of Satellite III. at 6h. 35m., or rather, to 

 the appearance of the Satellite itself, as soon as the evening becomes 

 dark enough, inasmuch as it has been seen on a good many occa- 

 sions itself as a dark spot, like its own shadow. Its real shadow 

 will not enter on to Jupiter's following limb until 7h. 15m. p.m., 

 and will leave the opposite one at lOh. Im. p.m. On the 24th, too, 

 the student should narrowly watch the transit of Satellite II., 

 which (or rather its shadow) has been seen to present some rather 

 abnormal phenomena. The Satellite will enter on to Jupiter's disc 

 at 9h. 52m. p.m., to be followed by its shadow at llh. 36m. The 

 egress of the satellite will occur 37 minutes after midnight ; that of 

 the shadow at 3h. 54ni. a.m. on the 25th. On the 26th, Satellite II. 

 will reappear from eclipse at 9h. 16m. 37s. p.m., and also the same 

 night, the ingress of Satellite I. will occur at lOh. 21m. p.m., and 

 that of its shadow at llh. 15m. The Satellite will leave the planet 

 at 12h. 36m. p.m. ; and the shadow at Ih. 31m. the next morning. 

 On the 27th, Satellite I. will be occulted at 7h. 30m., and reappear 

 from eclipse at lOh. 39m. 33s., p.m. The same Satellite will cross 

 Jupiter's face during the evening of the 28th, the sequence of phe- 

 nomena being, ingress of Satellite 4h. 48m., p.m. ; of shadow, 

 5h. 44m. I p.m. Egress of Satellite, 7h. 3m., p.m. ; of shadow, 8h. Om., 

 p.m. On the succeeding evening the same Satellite will reappear 

 from ecUpse at 5h. 8m. 30s. The chief feature of interest, how- 

 ever, on the 29th, will be the transit of Satellite III. ; which, for 

 the reason stated above, should be carefully watched. The Satellite 

 enters on to the planet's face at 7h. 25m. ; and passes off at 

 lOh. om., p.m. Its shadow will actually not enter on to Jupiter's 

 disc until llh. I5m., or Ih. lOra. after the Satellite casting it has 

 left that disc. It will pass off the preceding limb of the planet at 

 2h. 2m., a.m., on the 30th. A transit of Satellite II. will begin at 

 the not very convenient hour of 12h. 16m., p.m., on the 30th ; its 

 shadow following it at 2h. 14m., a.m., on Feb. 1, and its egress 

 occurring at 3h. Im. 



The Jloon is in Taurus on the 19th, passes through a portion of 

 Orion, and into Gemini on the 20th ; remains in Gemini during the 

 2l8t ; passes into Cancer on the 22nd, and travels to the eastern 

 limit of that constellation on the 23rd ; is in Leo on the 24th ; 

 travels across a portion of Sextans, and into Leo again on the 

 25th, not quitting it during the 26th. She enters Virgo on the 

 27tli J is there through the whole of the 28th and 29th ; moves into 

 Libra on the 30th, and is still there on the 31st and on the Ist of 

 the succeeding month. Her age at noon is 10'3 days on the 19th, 

 11'3 days on the 2Uth, and so on, being obviouslj- 23.3 days on 

 February 1. The occultations which occur at convenient hours 

 during one specified period are, unfortunately, all of small stars. 

 To-night (the 19th) UAC, 1651, a 6J mag. one will disappear at her 

 dark limb at 11 o'clock, at an agle from her vertex of 70°;* and 

 reappear at her bright limb, at an angle of 354°, at llh. 54m. p.m. 

 On the 20th. 71 Orionis, a 5k niag. star, will disappear at the dark 

 limb at 9h. 23m. p.m. 2" from the vertex, and reappear at the bright 



* See page 11. 



limb at an angle of 330" at 9h. 49ra. On the 23rd A' Cancri will 

 reappear at the dark limb (now the western one) at an angle of 

 214, at Gh. 3ui. ]>.ni. 60 Cancri, another 6 mag. star, will di.^-appi-ai 

 at the moon's bright limb on the same evening, at 9h. 49m., at an 

 angle of 37° from her vertex, and reappear at her dark limb at 

 llh. fim. p.m., at an angle of 233°. The remaining two oecnltationg 

 this month occur at hours too inconvenient for the amatenr astro- 

 nomer to involve the necessity of any details being given here. 



" Let Knowledge grow from more to more." — Alfred Texxvson'. 



lttttv& to t\)t ©tutor. 



Only a small proportion of Letters receii-ed can possibly be in- 

 serted. Correspondents must not be offended, therefore, should their 

 letters not appear. 



All Editorial communications should be addressed to the Editor of 

 Knowledge; all Bitsiness communications to the Pcblishebs, at the 

 Office, 74, Great Queen-street, W.C. If this is not attended to, 



DEL.WS AELSE FOR WHICH THE EdLTOR IS NOT RESPONSIBLE. 



All Remittances, Cheques, and Post Office Orders should be made 

 payable to Mes.srs. Wyman & Sons. 



The Editor is not responsible for the opinions of correspondents. 



No CO.MMCNICATIONS AKE ANSWERED BY POST, EVEN THOCGH STAMPED 

 AND DIRECTED ENVELOPE BE ENCLOSED. 



THE SUNDIAL OF AHAZ. 



[688] — The shif tings of shadows on the dial that Isaiali predicted 

 to sick Hezekiah, are liable to occur at any place, when these two 

 circumstances concur; 1st, that the upper atmosphere is in that 

 condition which causes two bright parhelia or mock suns to appear 

 on opposite sides of the sun ; and 2nd, that the lower air contains 

 drifting clouds, massive enough to hide often two of the three. 

 When the real sun and eastern mock sun are hidden, there is only 

 the western to cast shadows, which then coincide with what the 

 sun will cast an hour and a half later ; but if the clouds shift so 

 as to hide the west parhelion, and disclose the eastern, the shadows 

 instantly become such as the sun cast an hour and a half earlier. 

 The parhelia being always caused by i-ays refracted through two 

 faces of equilateral triangidar prisms or fibres of ice, their angular 

 distance from the sun is always the minimum deviation that such 

 a prism of ice produces on the brightest or yellow rays, which is 

 very near a fourth of a right angle ; so that if Hezekiah's diallers 

 divided the quadrant into forty, than which no number is more 

 likely, considering how constantly it recurs in the Hebrew Laws 

 and History (oftener, indeed, than any other above ten), the advance 

 or recession of the shadow would have to be ten of these parts. 



On March 29, 184S, these effects occm-red, had anyone been 

 looking, on every dial in the Isle of Portsea, and very probaldy of 

 much of Hampshire besides. The parhelia were present and bright 

 enough at about 11 p.m., and still better between 1 p.m. and 

 1.30 p.m. (not between 11 p.m. and 1 p.m., because, though the 

 atmospheric conditions were right, the sun had risen above the re- 

 quired limits of altitude), and the drifting clouds below were 

 present. 



By means of Maty's index to the first seventy volumes of the 

 " Philosophical Transactions," I find, between 1668 and 1750, 

 thirteen accounts of similar parhelia that would give these effects 

 if the clouds were present. Six of them were in England, and two 

 described by Hallcy (rare Inck, I should say, for one person to 

 witness two), and of the foi-mer, he says (Vol. XXIII., p. 1,128), 

 " it was plain that the vaponr which caused this appearance was 

 higher than the clouds, for they were seen to drift under the 

 circles." In Vol. XL., p. 59), Martin Folkes, V.P.R.S., describes 

 and figures another, of which he says that, at one time, " the 

 southern parhelion was so bright that, by taking the advantage of 

 a place where a chimney shaded the true sun, it cast a very 

 visible shadow." Obviously, then, the clouds jiresent in my case 

 and Halley's might have served as well as this chimney. 



AVhat llalley calls "the vapour which caused" these appear- 

 ances, Descartes had proved could be nothing but an atmosphere 

 fibrous, as it were, with ice prisms, such as, when turned at 

 random in all directions, produce the ordinary halo of 45° diameter, 



