That's a problem with the Sharp - programs are stored in battery-backed RAM instead of more modern FLASH memory.
Why not buy a cheap Palm Pilot and download a copy of
CelestNav from Mobile Geographics? You can try it out, and if you like it, purchase a license key from them. I first used it several years ago, but I'm just starting to use it again to brush up my celestial navigation skills prior to taking off next year.
You could load up the Palm Pilot with other software, like TideTool, and use the standard PDA functions as well.