The `NBODY1` code by Sverre Aarseth was, as mentioned in Subsection
5.2.2, made to do the calculations in physical coordinates, while the
`AVP` code does it in comoving coordinates. To be able to compare the orbits produced
by these two codes, the physical coordinates of the `NBODY1` code need to be
transformed into comoving coordinates. The following subroutine do just that by reading
the output data from `NBODY1` and calculating the comoving coordinates by using the
Runge-Kutta method to determine the scale factor at a given time using equation
(3.33). The code is written in the *Fortran 77* programming language,
so that it can be included in the `NBODY1` code.

Mon Jul 5 02:59:28 MET DST 1999