TO KNOW THE STARRY HEAVENS 



171 



The Occultation of Uranus. 



The moon, which passes completely 

 around the sky in one month, following- 

 nearby along the path A \' B, Fig. i, 

 will pass over both Uranus and that 

 star at C on the evening of October i6. 

 As seen from Washington, the planet 

 will disappear at the eastern (dark) 

 edge of the moon at 9 hours 40 minutes 

 P. M., Eastern Standard Time, and will 

 reappear at 10 hours 43 minutes. The 

 star will disappear at iihours 34 min- 

 utes and reappear at 12 hours 34 min- 

 utes, but at the emergence the moon 

 will have set throughout the eastern 

 states. 



These exact times, however, apply 

 ■only for observers at Washington, at 

 another station they will be very differ- 

 ent. The observer should therefore 

 find Uranus in his telescope in the 

 early evening" and having noted the 

 relative positions of the planet and the 

 moon, he should estimate as closely as 

 "he can the time when the occultation 

 will occur, remembering that the moon 

 requires one hour of time to move 

 eastward a distance equal to its own 

 diameter. As the planet has a disc, he 

 will see this object gradually fade away 

 as the dark edge of the moon pauses 

 •over it ; the disappearance of the star 

 will, on the contrary, be absolutely 

 instantaneous. This fact shows us that 

 •our satellite has no appreciable atmos- 

 phere upon its surface. 



The Hunter's Moon. 



The full moon of October 22 is the so- 

 called Hunters' Moon. On this evening 

 the moon will rise almost exactly at the 

 east point of the horizon while on the 

 following evening it will rise much 

 farther north and only about a half 

 an hour later than on the evening 

 before. For four or five evenings the 

 moon will thus move so rapidly north- 

 ward on the heavens that this motion 

 will partly overcome it usual retarda- 

 tion in rising. It will rise but about 

 one half hour later on each successive 

 evening instead of about one hour later 

 as is usual. These nights will there- 

 fore have an unusual amount of moon- 

 light. The efifect was even more marked 

 at the time of the Harvest Moon, which 

 occurred this year on September 23, — 

 the exact dav of the AiUumnal T^ciuinox. 



Retrogradation of Stars. 



Pattersonville, New York. 

 To the Editor : 



I noticed in your August number of 

 The Guide to Nature that you speak of 

 the planet Jupiter retrograding. What 

 is the cause of this motion? 

 Very truly yours, 

 Charles H. Patterson. 



University of Pennsylvania, 

 Philadelphia. 



The outer planets all move Cjuite 

 steadily eastward among the stars, 

 completing the circuit of the heavens 

 in a greater or less time depending 

 upon their distances from the sun. 

 Thus Jupiter moves completely around 

 the stm in 11.86 years, Saturn in 29.46 

 years, Uranus in 84.02 years, and Nep- 

 tune in 164.78 years. These numbers 

 represent the times that the planets 

 occupy in themselves actually passing 

 around the sun. But we must remem- 

 ber that we do not view them from 

 the sun itself but from our moving 

 earth, which world is itself going around 

 the sun once each year. The resulting 

 apparent motions can readily be imi- 

 tated by turning one wheel inside of 

 another, much larger one and noticing 

 how^ a point on the inner wheel some- 

 times passes a point on the outer one 

 and how sometimes these two points 

 are moving in the same direction. 



When the earth is between Jupiter 

 and the sun it is moving forward so 

 much more rapidly than Jupiter that, 

 as seen from the earth, the larger planet 

 seems to move backward on the sky. 

 and this backward motion continues 

 until the earth is about sixty degrees 

 ahead of the position of conjunction. 

 WHien, however, the earth gets on the 

 opposite side of the sun, both its own 

 motion and that of Jupiter make the 

 latter planet appear to move forward : 

 hence at these times the outer planets 

 run rapidly eastward. 



Figures illustrating this may be 

 found in almost any astronomy, but 

 the best way is to study it from a small 

 model. A so-called planetarium shows 

 it beautifully, but it may also be clearly 

 shown l)y merely drawing two concen- 

 tric circles of different sizes, moving 

 a point around each of the circles, let- 

 ting the point on the inner circle move 

 the faster, and noticing, (or indicating 

 l)v drawiiiQ- lines between the two 



