Line of apsides. The line joining the perigee and apogee through the center of the earth.
Can I rewrite the above sentence as follows?
Line of apsides. The line joins the perigee and apogee through the center of the earth.
The line "that joins," not just "joins."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.