Once the grahā: are in a line between rāhu and kētu, they appear to move like a snake.. slight side to side (north – south) and slightly like an earthworm due to direct and retrograde movements in the east-west direction.
In a geo-centric view you can’t really “see” that saturn is much further than mars. You can just see the chain of grahā: kind of wiggling about over a long period of time. Of course a real snake has no earthworm movement, neither do some vertebrae move past others ( eg., sun and moon move past other grahā: ).
kāla can mean time as well as black. sarpa can mean a creeping, tortured motion. It takes some poetic imagination!!