Retrograde

The planets are fish
    That swim real slow.
They can't beat the current,
    As seen from below.
The stars in this river
    Are jellies that glow.
And planets swim through them,
    Aimed east as they go.

But once every Earth year
    These fish take a break.
They swim a bit downstream
    And loops that they make
Confused ancient thinkers
    On what it could take
To ex-plain this motion.
    It made their heads ache.

So place yourself under
    This scene that is ours,
With planet-fish swimming
    Through jellyfish stars...
Self-luminous jellies,
    Not jelly from jars!
And all so you'll picture
    The motion of Mars.