Since 28-Aug-13 the IDL Astronomy Library has required IDL V6.4 or later.   
This directory contains frozen versions of the Astronomy Library compatible with
earlier versions of IDL.    Note, however, that these procedures do not
include enhancements or bug fixes since the frozen date

IDL Version          Freeze date         Tar file name

V4                   May 2002            astron_v4.tar.gz
V5.1                 Jan 2003            astron.dir_v51.tar.gz
V5.2                 Feb 2004            astron.dir_v52.tar.gz
V5.3                 Apr 2006            astron_v53.tar.gz
V5.4                 Sep 2006            astron_v54.tar.gz
V5.5                 Sep 2007            astron_v55.tar.gz
V5.6                 Jul 2009            astron_v56.tar.gz
V6.1                 Aug 2013            astron_V61.tar.gz

Also stored as .zip files are snapshots of the Astronomy library at certain
times.