Celestia 1.6.1 via pkg:
# Celestia's package can be installed one of multiple ways on FreeBSD systems via pkg(8)
$ pkg install celestia
# or
$ pkg install astro/celestia
# or
$ pkg install celestia-glut
# Or with the gtk front end
$ pkg install celestia-gtk
# or
$ pkg install astro/celestia-gtk
Celestia 1.6.1 via ports:
# Celestia 1.6.1 can be installed via ports with
$ cd /usr.ports/astro/celestia/ && make install clean
# or for gtk
$ cd /usr.ports/astro/celestia-gtk/ && make install clean