M.\RCH, 1911. 



KNOWLEDGE. 



101 



whilst the polar diameter is 2"' 7 smaller; this polar Hatteniiifj 

 is readily observed in telescopes powerful enontjh to see the 

 belts. 



In larger telescopes, markings on the belts may be observed; 

 these rotate with the planet and recur in the same position 

 every 9*' 55" which is the planet's rotation period. 



The following table gives the satellite phenomena \ isible in 



'*0c. D." ilenotes the fii.sappearaiice of the SalelHle liehind the disc, ami 

 "Oc. R." its reappearance: " Tr. I." the ingress of a transit across the disc, and 

 "Tr. E." its egress ; " Sh. I." the ingress of a transit of the shadow across the disc 

 and "Sh. E." its egress; " Ec. D." denotes disappearance of Satellitt: Ijy Eclipse_ 



Saturx : — 



Saturn is only observable for a short time each evening 

 during the month, as he sets at 10.28 p.m. on the first and at 

 8.45 on the 31st. The planet appears as a fairly bright star 

 looking nearly due West as soon as it is dark. Observed in 

 the telescope, the rin.g appears wide open, as we are looking on 

 the Southern surface at an angle of 1S\ The apparent 

 diameter of the outer major and minor axes of the ring are 

 39" and 12" respectively, whilst the diameter of the ball is 15". 



The Moon appears r.ear the planet on the evenings of the 

 4th and the 31st, whilst Venus appears in proximitv on the 

 29th. 



L'R.ANUS: — 



Uranus is visible in the early morning, rising about 4.15 a.m. 

 near the middle of the month. 



Neptune ; — 



Neptune is situated in Gemini about fhree-and-a-half 

 degrees S.E. of the star S Geminoruni. 



Near the middle of the month the planet is on the meridian 

 about 7.45 p.m., and sets .about 3.45 a.m. The planet is 

 diflicnlt to identify among the numerous small stars appearing 

 in the same field of view, and as he is practically stationary 

 this month he cannot readily be detected by his relati\ e 

 motion. Moreover, it requires a high power (about 300) and 

 .good definition to distinguish his disc. 



Meteor Showers: — 



Minima of .Algol occur on the llth at 10.21 p.m., and on 

 the 14th ,it 7.10 p.m. Its period is 2^ 20'' 49'", by which other 

 iiiininia may lie deduced. 



Double Stars. — 7 Leonis. X.'' 14'", N. 20" 22', mags. 2, 4; 

 separation 3"-S. In steady air. the prime requisite for double 

 star observations, this double m.ay be well seen in a 3-in. 

 telescope with an eyepiece magnifying about 30 to the inch of 

 aperture, but on most nights one with a power of 40 is better. 

 The brighter component is of a bright orange tint, w hilst the 

 fainter is more yellow. 



1 Leonis XI." 19"", N. 11 5'. mags. 4, 7*; separation 2"-2. 

 A pretty double of different-coloured stars, the brighter being 

 yellow, the other blue. This object requires a favourable 

 night and a fairly high power on small telescopes. 



a Leonis iRcf^nlits) has a small attendant about 180" 

 distant, magnitude 8-5, and easily seen in a 3-inch telescope. 



a Canuin \'enat (Cor Carolil XII.'' 52"', N. 38= 49', mags. 

 2-5. 6-5, separation 20". Easy double; can be seen with 

 moderately low powers, even in 2-in. telescopes. 



CORRESPONDENCE. 



COLOURS OX THK ECLIPSED MOON. 



Tv titc Edit,, 



)f " Knowledge." 



Sirs, — I think I remember a recent question in " Know- 

 ledge " to this effect : " What would be the distance of 

 the red light from the sun refracted through the earth's 

 atmosphere, regarding this as a lens stopped down by a large 

 central circular disc," and I think this had reference to the 

 colours on the surface of the moon in eclipse. Now the 

 action is quite unlike that of any lens because the density of 

 the air diminishes from the surface upwards. Regarding the 

 sun as a point of light, the focus for red rays passing close to 

 the surface of the earth would fall at a point somewhat 

 nearer than the moon's distance, and the difference of position 

 for any other coloured light would be very small, but the light 

 which passed through the outermost layer of the atmosphere 

 would fall only just inside the vertex of the earth's geometrical 



conical shadow nearly a million miles awa\-. and at every 

 point between these extreme distances there would be a focus 

 of some infinitesimal fraction of the light ; moreover, the sun is 

 not a point and the question is still more vague. The light 

 which passes through the earth's atmosphere would cover a 

 surface which, at the distance of the moon, would be very 

 much larger than the moon's disc ; the distribution of the light 

 over this surface would not be uniform, and would be very 

 slightly different for different colours, but this could not possibly 

 produce any recognisable colourdifference ; the colours observed 

 are due to a cause quite distinct from refraction. The green 

 colour often noticed at the edge of the shadow before and 

 after totality is no doubt due to the familiar subjective 

 complementary colour-effect of the reddish light in the shadow, 

 and the coppery colour in the shadow is due to the same cause 

 which gives us a blue sky and a red rising and setting sun, 

 namely, that light of short wave-length is much more reflected 



