2 88 



THE GUIDE TO NATURE. 



OTULW^STRdNoMY 



The Heavens in January. 



BY PROF. S. ALFRED MITCHELL, OF COLUM- 

 BIA UNIVERSITY. 



When will Halley's Comet be visible 

 in a small telescope? To this question 

 there is a very simple answer which 

 may be summed up in one syllable, now. 

 If one has a six-inch telescope, is gifted 

 with moderately keen eyesight, and has 

 good clear weather, the comet should be 

 visible and can be located without much 

 difficulty. Early in December the Smith 

 College observatory reported that Hal- 

 ley's comet had been seen there through 

 a three-inch telescope. Thus for posses- 

 ses of telescopes smaller than six-inch, 

 this interesting comet should prove a test 

 of eyesight. Amateur astronomers who 

 are not fortunate enough to possess a 

 telescope, but who wish to see the comet 

 with the naked eye, will probably have 

 to wait another three months till after 

 the comet passes by the sun in March, 

 and comes out from the sun's rays in 

 April. The comet will then rise before 

 the sun, and consequently can be seen 

 only shortly before sun rise — a much less 

 convenient time of studying the heavens 

 than in the early evening. The comet will 

 then rapidly increase in brightness, and 

 will* reach a greater and greater angle 

 from the sun till May 15, when it sud- 

 denly changes its position again and will 

 draw in quickly towards the sun. The 

 comet is due to cross the face of the sun 

 on May 18.. at 9 P. M., Eastern standard 

 time. This will be 6 P. M. Pacific time, 

 and the sun will not have then 

 set. Whether the comet will be big 

 enough to be seen in transit is an interest- 

 ing question. There is no authentic ac- 

 count of a transit ever having been seen 

 to have actually taken place, though 

 many, notably the Biele comet in 1826, 

 were calculated to have done so. 



Of course, everyone knows that Hal- 

 ley's comet moves in a great ellipse. 



stretching out beyond the orbit of Nep- 

 tune. Of all the comets with periods 

 less than 80 years, Halley's is the only 

 one which moves with a retrograde mo- 

 tion, that is, in the direction opposite to 

 the motions of all the planets about the 

 sun. This is the comet's own motion, 

 what is its apparent motion as seen from 

 the tiny earth, which itself is in motion 

 about the sun? The movement among 

 the stars is the resultant of the other 

 two motions. At the present time the 

 comet is traveling westward among the 

 stars, the sun in the opposite direction, 

 eastwards. The angle between them will 

 continually decrease till about March 25, 

 when the sun will pass between us and 

 the comet. By referring to a diagram 

 giving the paths of the earth and comet, 

 it will readily be seen that the comet 

 becomes a moving object and increases 

 its angular distance from the sun. On 

 April 19 the comet is at perihelion and 

 closest to the sun, and consequently is 

 moving with its greatest speed in its 

 orbit. Five days later it is headed in the 

 direction of the earth, but the world slips 

 by unharmed. The comet now ceases its 

 retrograde motion in the sky, and begins 

 to move direct or eastwards, at first 

 slowly and then with quickened pace. On 

 the evening of May 18 the comet comes 

 within 14,000,000 miles of the earth, and 

 this near approach causes a tremendous 

 apparent motion among the stars, and in 

 the next twenty-four hours and for sev- 

 eral days thereafter it moves eastward as 

 much as 15° each night. Consequently 

 on May 19 there will be no need to ask, 

 "Where is the comet?" All may see it 

 as a magnificent object appearing im- 

 mediately after sunset. It will be higher 

 and higher up in the western sky each 

 day when the sun sets. We have not had 

 a brilliant comet in northern latitudes 

 since 1882, and it is altogether probable 

 that Halley's will be just as magnificent, 



