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A Cel: URL is a bookmark that saves the time, location, rendering options and other settings of a specific Celestia event or place. These URLs can be exported and published as regular HTML, so that any Celestia user can share a Celestia scene with any other.
The Windows version of Celestia includes a Bookmark menu. Those bookmarks are not the same as Cel: URLs.
To record a Cel: URL to your computer's "clipboard", type a Ctrl-C, or the usual "copy selected region" keyboard shortcut. On a Windows system, you can type a Ctrl-Insert.
To paste a Cel: URL into a document, type a Ctrl-V, or the usual "paste clipboard".
Potential incompatibility warning
Unfortunately URLs recorded by one version of Celestia often are not compatible with other versions. The origin of Celestia's coordinate system changed in V1.5. As a result, URLs recorded by previous versions of Celestia place the viewpoint hundreds of AU from the intended viewpoint. Similarly, the rotation of the Earth was greatly improved in Celestia v1.6. As a result, URLs recorded by V1.5 for viewpoints on the Earth's surface will place the viewpoint tens of kilometers from the intended viewpoint. Also, although URLs recorded by previous versions of Celestia can be read by V1.6, the URLs generated by V1.6 cannot be used by previous versions of Celestia.
Fix for Linux if clicking on a Cel: URL does not work
Depending on the specifics of your installation, Celestia might not have been registered to handle the Cel: URLs. If that is the case, you can remedy as follows (this should work on any system compliant with the XDG specifications).
xdg-mime default celestia.desktop x-scheme-handler/celin a terminal. (This registers Celestia as the program that handles cel: links.)
celestia.desktop, replacing the line
Exec=celestia-gnome %u, so that the system knows that it has to pass the URL argument to Celestia, when launching it. The desktop file is usually found in
/usr/share/applications/and if you don't want to edit files with root, you can instead copy it to
$HOME/.local/share/applications/and edit it there (or more generally copy it from
$XDG_DATA_HOME/applications/). Optionally, you can also add
MimeType=x-scheme-handler/cel;to the desktop file. (See the XDG desktop entry spec.)