When I run the code below I get an output of 6378136.5 instead of 6378136.3
PROGRAM test
implicit none
real*8 radius
radius = 6378136.3
print*,radius
END
I have read this other link (Precision problems of real numbers in Fortran) but it doesn't explain how to fix this problem.
The reason this is happening is not because the variable you are using lacks precision, but because you initialized the value using a single precision number.
Take a look at this answer for a good explanation, and an elegant solution to your problem for any larger programs.
If you just want to solve it quickly, then you only have to change one line:
radius = 6378136.3d0
Though this will still give you a value of 6378136.2999999998 because of floating point precision.
