their Appearance, Nature, and Origin. 3 
four hundred miles apart. Had the comet been at all near the 
Earth’s surface, the change in its bearings would have been very 
large. It was found to be negligible. 
Since the distance of comets is so great, it follows that their 
dimensions, in particular the length of their tail must be 
enormous. 
Tycho Brahe supposed that comets moved in circles, the circle 
being the “ perfect curve,’ and the only one worthy of a heavenly 
body, according to the notions of his day. : 
Kepler, one of Tycho Brahe’s pupils, supposed that comets 
moved in straight lines; and he looked upon them more or less 
as living creatures, travelling through space with will and 
purpose, “‘ like fishes in the sea.”’ 
Fic. 2.—ORBITS OF SHORT-PERIOD CoMETs. 
Kepler’s name is honoured by the discovery that planets do 
not move in circles, but in oval curves called ellipses, the sun 
being, not at the centre of the curve, but somewhat eccentrically 
laced. 
3 Newton showed that a comet entering the solar system from a 
large distance away, would move in a very elongated ellipse. If 
the original distance were infinitely great, the ellipse would be 
infinitely elongated. A curve of this kind is called by mathe- 
maticians a parabola. It is the same as the path of a projectile 
under gravity. 
This deduction from the law of gravitation was verified by 
observation and the astronomer Halley calculated the paths ot 
all the comets then known. He discovered that certain comets 
B 2 
