April 1, 1890.] 



KNOWLEDGE. 



119 



there are two points on which I should like to address a 

 few words to you. (1st.) The various cases where two or 

 more distinct values are given for the parallax of a star, 

 the observer, method, date, publication, &c.. being the 

 same, and there being nothing to show that a redetermina- 

 tion has been made, as in the case of y Cassiopeiae, where 

 the values (neglecting probable errors) of 0"-14, +0''007, 

 0"-025, and + 0"'050 are given. Are these to be regarded 

 as four distinct determinations, each from oliservations 

 extending through one year ? In the case of Polaris, the 

 values + 0"-073 and + 0"-079 are given. From a con- 

 sideration of your note on p. 62, I conclude that Mr. 

 Sadler's list, though m type, was open for new results at 

 least as far as November 7, 1889 ; yet in the number of 

 Xiiture for that date (vol. xli.. No. 1015, p. 19), in mention 

 of Professor Pritchard's Researches in Stellar Parallax, 

 the value given for Polaris is +0"-052±0"'011 ; and as far 

 back as July 5, 1888, it is stated in Nature (vol. xxxviii.. No. 

 975, pp. 227-8), with regard to the Oxford observations, that 

 the determinations of 61' and 61- Cj/i/iii, /x Ca usii ipei a e, -And 

 Polaris " may be regarded as nearly completed," and the 

 result for Polaris is given as +0"-052±0"-0311, of which 

 I take the above-mentioned result of +0"-052±0"-011 to 

 be the completion. Adding this value to the two given in 

 the list, we have for Professor Pritchard's results the 

 values + 0"-073, +0-079, and + 0"-052. To take an example 

 other than the Oxford observations, I may refer to p. 66, 

 where Briinnow's (1870-71) result for Vega is given as 

 + 0-131 and +0"-188. /? and a ( 'axsiapeiae are also cases 

 in point, and for these two stars and y Cassiopeiae the same 

 date, viz. 1888, is given as the time of observation, from 

 which I understand (for in other cases the number of 

 years is given through which the observations extend) that 

 these stars have only been observed by Professor Pritchard 

 through one year ; yet if this be so, how have the widely 

 differing determinations been obtained ? I venture to hope 

 that if the points admit of a brief explanation Mr. Sadler 

 or yourself will be good enough to devote a few lines to 

 the subject. — Yours most faithfully, 



" y Draconis." 



Galashiels, N.B. 



[Your correspondent (who. I regret, does not give his 

 name) is perfectly correct in his criticism. There is a 

 difference in the results which have been published by 

 Professor Pritchard in tlie ^lonthhi Xotiees and in the 

 Oxford observations. The difference between these and 

 the results commimicated to Xatiire (before the publica- 

 tion of the Oxford determmations) may be due to the fact 

 that in the case mentioned of Pularis the parallax as 

 derived from only one of the comparison stars is given. 

 Possibly Professor Pritchard may not be himself respon- 

 sible for this. It is one of the disadvantages of premature 

 announcements that incomplete results, which subse- 

 quently receive modification, are pubhshed. In the case 

 of y Cassiopeiae, 0"-ll is misprinted in my paper for 

 0"-014. The same stars are not always employed in the 

 two determinations. As I have said above, I presume that 

 the writer of the articles in Xatiire has taken the parallax 

 of Polaris as determined from the brightest of the four stars 

 observed at Oxford (B. I).+8H°, No. 4, 6-7 mag., in 

 Monthlij Notices, 6-84 mag. in Oxford University observa- 

 tions). The parallax of Polaris from the star is given in 

 the Montlil;/ Xotiecs as +0 '0429 + -015. and in the 

 Oxford University observations as +0' •052±0 -Oil. In 

 the case of !>(/(/, the differences in Briinnow's results are 

 due to his accepting or rejecting certain constant errors. 

 " y I)raconis" must refer to the original papers for infor- 

 mation of this kind, which it was impossible to give in the 

 limits of a magazine article. — II. Sadler.] 



THE FACE OF THE SKY FOR APRIL. 



By Herbert Sadler, F.E.A.S. 



SUN-SPOTS have been rare of late, but at the time of 

 writing a fine double one is entering on the solar 

 limb. The Zodiacal Ught should be looked for 

 in the absence of moonlight during the second and 

 third weeks of the month. Mercury is invisible 

 during the first half of the month, being in superior eon- 

 junction with the sun at 8h. a.m. on the 9th. During the 

 latter half of the month the planet will be more favourably 

 placed, setting on the 23rd at 8h. 42m. p.m. with an 

 apparent diameter of 6", and a northern declination of 

 19', and on the 30th at 9h. 24m. p.m., 2h. 5m. after 

 sunset, with an apparent diameter of 7", and a northern 

 dechnation of 22|°. While visible he passes from Aries 

 into Taurus, being about 20' s.p. the 4th magnitude star 

 8 Arietis on the 23rd at 8 p.m. ; at the same hour on the 

 25th he will be about 6' «.;'. the 6th ma.giiitude star 65 

 Arietis, and on the evening of the 28th about 2 south of the 

 Pleiades. Venus is an evening star throughout the month, 

 setting on the 1st at 7h. 27m. p.m. with a northern de- 

 clination of 7f°, and having a shghtly gibbous disc sub- 

 tending an angle of 10" : on the 30th she sets at 9h. Om. 

 p.JL with a northern declination of 20° and an apparent 

 diameter of lOj". During the month she passes from 

 Pisces through Aries into Taurus, but without approaching 

 any conspicuous star very closely. Mars is an evening 

 star, rising on the 1st at llh. 50m. p.m. with a southern 

 declination of 21° 21', and an apparent diameter of 14" ; 

 on the 30th he rises at lOh. 12m. p.m. with a southern 

 dechnation of 22° 39', and an apparent diameter of 18i". 

 During the month he passes through portions of Scorpio 

 and Ophiuchus, but does not approach any naked-eye star 

 very closely. He is stationary at midnight on the 22nd. 

 Pallas is in opposition to the sun on the 29th, when she is 

 at a distance of 178^ milhons of miles from the earth, and 

 appears as a 7-4 magnitude star. Her chameter is given, 

 from photometrical considerations, by Argelander at 158 

 miles, Pickering 167, Stone 171. Lamont, from measm-e- 

 ments of the chameter of the disc, found 630 miles ; 

 Schroter exceeded all bounds by giving one of 2,030 mOes. 

 During the month she passes fi-om Serpens into Corona 

 Borealis. On the night of the 3rd, about midnight, she is 

 4' due north of the 6-0 magnitude star t* Serpentis, and on 

 ' the 8th about 15' n.f. to the 6-0 magnitude star t" Ser- 

 pentis. On the night of the 10th she is 27' due south of 

 the 4i magnitude star i Serpentis. .Jupiter is a morning 

 star, rising on the 1st at 3h. 37m. a.m., with a southern 

 declination of 18|°, and an apparent diameter of 33.i". 

 On the 30th he rises at Ih. 54m. a.m. with a southern 

 declination of 18°, and an apparent diameter of 36^". The 

 following phenomena of the satellites occur while the 

 planet is more than 8° above, and the sun 8° below the 

 horizon. A reappearance from eclipse of the third satellite 

 at 4h. 47m. 36s. a.m. on the 2nd. An egress from transit 

 of the third satelhte at 4h. 6m. a.m. on the 13th. A re- 

 appearance from occupation of the second satellite at 3h. 

 29m. a.m. on the 24th, and an ingress of the first satelhte 

 on the disc one minute later. An ingress of the shadow of 

 the third satellite at 3h. 17m. a.m. on the 27lh. -lupiter 

 is in Capricornus throughout the month. On the mornings 

 of the 5th and 7th a very pretty double star, 6i and 7-0 

 magnitudes, 15}." apart, will be noticed in the field of view a 

 little north of the i)lanet. On the morning of the lOtli a 

 6J magnitude star will be about 7' north of tlie planet. On 

 the morning of the 15th the 6th magnitude star 19 Capri- 

 corni will be about 2i' north of the planet. Saturn is still 

 in a favourable position for observation, rising on the 1st 



