The .f90 files in this directory make up a simple code for 
simulating the earth - moon system. The code is heavily 
annotated. Any questions at all then mailto:mijp1@york.ac.uk and 
I'll be happy to help.

constants.f90  - simple module to store pi and a double
                 precision type.
		 
gravity.f90    - a utility module for simulations of bodies
               - interacting under gravity.
	      
earthmoon.f90  - uses the above modules to integrate the equations
               - of motion for the earth moon system using Runge-Kutta.
	       - (Hopefull you have all seen Runge-Kutta before in MAPLE)	      


To compile on Linux ....

gfortran -o earthmoon constants.f90 gravity.f90 earthmoon.f90

(the order matters)

then you can execute the program

./earthmoon

To plot the path of the moon, first filter the output to just 
contain the lines that contain "Moon" and then strip out the 
(x,z) coordinates:

./earthmoon | grep Moon | awk '{print $6,$8}' > Moon.dat

You can then plot the output, e.g.

xmgrace Moon.dat

Try tweaking the initial conditions in earthmoon.f90 to change the orbit.
You will need to re-compile each time you change the code. If you are feeling
really brave, try adding in another planet!