Oct. 27, 



KNOWLEDGE 



355 



A telegram from the Director of the Athens Observa- 

 tory announces that a comet had been seen on October 8, 

 four degrees south-west of the great one, and having the 

 same motion. JIas the comet been partially broken up in 



its passage through its perihelion 1 Ifc is strange that, 

 tjefore the Biela comet broke up, Dr. Hind noticed that it 

 appeared somewhat elongated, or pear-shaped. 



The pictures illustrate the appearance of the head (Fig. 1) 

 and the whol<> comet, as seen by the nuked eye, during the 

 last week of September. 



The comet can still be seen, and well seen, towards the 

 south-east in the morning hours, from about four. We 

 shall give next week a map, showing its position and course 

 (past and future) among the stars. 



TRANSITS OF VENUS. 



By the Editor. 



ALTHOUGH not belonging to the more interesting 

 relations connected with the occurrence of transits, 

 the laws according to which the successive conjunctions of 

 the earth and Venus recur, are worth e.xaraining, and they 

 artord a useful exercise for the student of astronomy. 



Let us, in the first place, regard the two path? as circles 

 around the sun as centre, and traversed wirli uniform 

 velocity — namely, with the respective mean velofities of 

 the earth and \'enus. 



Let S (Fig. 1) be the Sun, E Ej and V V, the paths of 

 the Earth and Venus, and let us suppose that the planets 

 start from conjunction on the line S V E, Venus at V and 

 the Earth at E. Then we know that the earth completes 

 the circuit of her orbit in r)f).")'2.')r)4 days, while Venus's 

 period of revolution amounts to 224 -7008 days. Accord- 

 ingly, when Venus has completed one revolution, and is 

 attain at V, the Earth has traversed a portion of her orbit 

 which is to thewhole circumference as 224 7008 to 36.")-2.'>64 ; 

 thus the Earth is as at f. The remaining part of her orbit, 

 orf E, is traversed in 140o."i.")t> days, and then the Earth 

 is again at E, while Venus has got to a point i' in her 

 orbit, such that the arc V v bears to the whole circum- 

 ference the proportion 140 .'i.'ir)!) to 224 7008. The part 

 r V is traversed therefore in 84 1448 days ; by which time 

 the Earth has got to e, 84 14 48 days' journey from E. It 

 is clear that all this time the planets have not been in con- 

 junction ; * but it is also clear that they are now drawing 

 near to conjunction, and that before Venus is at V again 

 they will i)e in conjunction. For Venus will require 

 224 7008 day« to "get to V, whereas the earth will 

 require 281-ril6 days (obtained by taking 84 1148 days 

 from the year), so that Venus now at V, and thus 

 behind the earth at »■', will pass in advance of the earth 

 before reaching V again. In order to determine whero 

 the two planets will be in conjunction, it is only necessary 

 to notice that, at starting, Venus might be regarded as 

 having one complete revolution to gain upon the Earth, 



Illy lioon fur tin 

 il'tlie two plnm 



of innkiiie this cloar that I 

 uc l)V lire, as alicive. 



