The older DOS version was sold on a 720K 3.5" disk (may have also been on 5.25" disk?) and can be used on most IBM-compatible computers. It will not function in DOS 2.11, but works fine in DOS 3.2. Unlike many programs from this brand name, it can be run directly from the disk instead of needing to use an installation program.
When testing this program on a 7MHz 8088 computer from 1987, it worked, but ran too slowly. A 386 or faster computer would probably be preferable (for DOS; might be higher for Windows).
"Expert Astronomer" for Windows is on CD-ROM, although it may have been offered on disk at some time. The Macintosh version appears to have been offered both on CD-ROM and disk.
This program is most likely to be found for purchase on eBay.com or at websites which sell older/used software. Other programs from this brand include Expert Landscape, Home Design, and 3D Font Creator.
UPDATE: The DOS version of this software will not work under Windows XP or Vista. It may be possible to use it with a DOS emulator (like DOSBox) or by booting the computer with a MS-DOS (or DR-DOS) boot disk.