Complex sentence: One of the suitable co-ordinates system used is so called "synodic" or co-rotating system, placed at the barycentre, with the line connecting the two masses μ 1, μ 2 chosen as X axis and the length unit equal to their distance.
Split into simpler sentences: One of the suitable co-ordinates system used is so called synodic or co-rotating system. The line connecting the two masses μ 1, μ 2 is chosen as X axis, with the length unit equal to their distance.