COMETS 233 



utilised by astrologers and others as the emblem of a 

 star. However they arose, neither the weird and astonish- 

 ing representations of mediaeval times nor the geometrical 

 decorative " stars " of later date seem to have any relation 

 to an attempt to represent a star as it really appears to 

 the human eye and the interpreting brain behind it. 



The orbits of comets, says Professor Turner, of Oxford, 

 in a delightful lecture delivered in Dublin in the summer 

 of 1908, from which I have culled many interesting facts 

 and presented them to my readers, " differ from those of 

 the planets in being far more highly elliptical. Our own 

 path round the sun is nearly a circle, so that our distance 

 from him remains nearly the same all the year round ; but 

 the distance of a comet from the sun varies greatly from 

 ' perihelion,' when it is near, and consequently bright, to 

 ' aphelion,' when he is so distant and faint that we lose 

 sight of him." The sun is not at the centre of the ellipse 

 described by a comet's path, but is quite near to one end 

 of it, so that comets approach the sun far more closely 

 than do the planets, some taking so close a turn round the 

 sun that the heat from it to which they are exposed is 

 2000 times as great as that which the earth receives. If 

 the orbit of a comet is really elliptic, then there at last 

 comes a time, though it may be only after thousands of 

 years, when the comet, having rounded the sun at close 

 quarters, and journeyed off into space; has his journey 

 brought to a turning-point at the other end of the ellipse, 

 and begins to draw near again, advancing towards the 

 sun. The length of the orbit of Halley's comet is about 

 3255 million miles, and the breadth at its broadest is 

 about 800 million miles, and he takes about thirty-eight 

 years to travel the full length (along the curve) and thirty- 

 eight years to come back again ! Other comets have 

 other lengths and breadths of orbit, and take longer or 

 shorter periods to go round. But the conditions of attrac- 



