

Exercise: Orbit of Halley's comet.

	Write a program to obtain the elliptic orbit of Halley's comet given,
in terms of plane polar coordinates, by

		r = l/(1+e*cos(theta))

where r is the distance from one focus and theta is the angle wrt the 
semi-major axis. a=2820.0 (in units of 10**9 m). The eccentricity e=0.954 
and the constant (semi-latus rectum) l=a*(1-e**2). 

	Send output to a file (fort.20, say) which contains the Cartesian 
coordinates x,y for a series of theta values (0 to 2pi).

	Make use, by imitation, of as many features used in the above 
examples as you can. Credit is given not simply for producing a correct 
program but for one which exhibits good programming practices (of which 
we will talk much later).

Submit your program to me, hcb1, by (to be informed later).
