A number of years back, while on Linux Mint 11, in the Software Store(not the actual name but you get the idea) I happened across Celestia, a fun little Astronomy program. It seems as though it has been wiped out since then. Is there somewhere I can get this program for installation in Ubuntu 18.04? I really enjoyed this program, and would like to have it again.
celestia package was removed from Ubuntu before 16.04 due to lack of interest by the community. The previous community volunteers who had packaged the software from upstream found new interests, and no community volunteer has stepped to resume packaging in either deb or snap format.
In the meantime, here is one way to build Celestia. Since it's not packaged for Ubuntu, installation questions or problems are likely to get faster answers if directed to the Celestia support forum.
Alternately, there are several other overlapping applications like Stellarium and Kstars which are packaged for Ubuntu and are in the Software Center.