Latest Debian based distros don't include Celestia in their repositories and you need to compile the software to enjoy it.
Just need to type the following commands in your console (Ubuntu and derivates distros):
sudo apt-get install libglew-dev libjpeg-dev libpng-dev libtheora-dev libgl1-mesa-dev libeigen3-dev libfmt-dev qtbase5-dev qtbase5-dev-tools and libqt5opengl5-dev freeglut3-dev libgtkglext1-dev liblua5.2-dev git
git clone https://github.com/CelestiaProject/Celestia
cmake .. -DENABLE_QT=ON
sudo make install
If you use Debian type first su - and then type the same commands as Ubuntu case but don't type sudo