THE EARTH AND COMETS' TAILS. 



By R. T. A. Innes, F.R.A.S. 



(Plate 10). 



In spite of the unreserved predictions of astronomers the 

 Earth did not pass through the tail of Halley's Comet on the 

 18-19 Alay, nor subsequently. The tail as seen in the morning 

 sky, previous to the transit of the comet across the sun's disc, 

 appeared like a long and straight beam of light stretching from 

 the horizon to Aquila. It was noticed from day to day that 

 the tail was practically fixed in position in the sky. We rather 

 expected the tail to get nearer to Venus and vSaturn as the 

 comet approached the ecliptic, but it remained stationary. On 

 the morning of transit, 18-19 May, the tail was unchanged, but 

 a second branch to the South was now noticed. It joined the 

 Northern branch to the East of the Square of Pegasus. 

 Unfortunately this Southern branch was near the zodiacal light 

 and only distinguished from it with difficulty. Both of these 

 tails were seen morning" by morning including this morning 

 (22nd May, 1910, Civil day), but they have diminished in 

 brightness and were difficult to see. Further observation of 

 these will be impossible because of the Moon remaining above 

 the horizon until after dawn during the next 10 days. The 

 whole Eastern horizon where the tails meet, and where the zodiacal 

 light is, was suffused with a dim and indefinite glow which 

 was particularly noticeable on the 18-19 and 20-21. This glow 

 was not so definite in boundary as the zodiacal light. When the 

 comet was seen on the evening" of the 20th we were surprised 

 to see it had the ordinary tail pointing away from the Sun as 

 usual. It had been noticed for several days that in the 

 neighbourhood of the vSun the sky was not so blue as usual, but 

 this was the case even a week before the transit and is probably 

 merely a meteorological phenomenon. This brief summary of 

 the facts will suffice here, the observations in detail will be 

 published elsewhere. 



We have now to explain the reason why the Earth did not 

 pass the tail of the comet and why the tail broke up so that 

 some of it was left in the morning" sky where it remains and 

 is slowly losing" its luminosity, and some (or another tail) 

 appeared in the evening sky. It is well known that a comet 

 under the Sun's radiant action (I do not attempt to define it 

 more closely) expels corpuscles towards the Sun which the 

 Sun repels, and these luminous corpuscles form the tail. This 

 process goes on even when (as in the case of Halley's Comet) 

 the distance between the comet and the Sun exceeds the 

 distance of the Earth from the Sun. If the nearer planets do 

 not show tails it is because these corpuscles have been shed 

 by the planets ages ago. In short, a comet and a planet under 

 the radiant action of the Sun, and the Sun itself all repel these 

 corpuscles. This being so, it is impossible for the Earth to go 

 through the tail of a comet — it simply repels the tail and as a 

 consequence, instead of a passage through it, a disruption near 

 the time of passage must occur, one part being left in the (in 

 this case) morning" sky, whilst a new one is developed in the 

 evening sky. Here I might remark that on the evening" of the 

 20th the measured length of the new tail was 19 degrees, on 

 the 21 St 32 degrees and on the 22nd it was 40 degrees. Again, 



