arc
Explanation
This comic takes the concept of "the arc of history" and applies it literally to astronomical and physical phenomena with darkly comic results.
One character asks: "Do you think the arc of history bends toward justice?" The other responds with "Of course" -- but then offers examples that undermine any optimistic interpretation. The moon is "moving away from Earth constantly and will eventually get farther away every year." Comets go "towards the sun before being flung out of the solar system, never to be seen again." Dwarf planets have "elongated eccentric orbits" that bring them closer to the sun before "flinging off for eons."
The final panel delivers the punchline: the second character says "You're saying electromagnetic radiation ought to be more careful," and the first responds "No, I'm saying it's accurate" -- implying that the "arc" of everything in the universe bends toward isolation, separation, and oblivion, not justice.
The humor is classic SMBC philosophical pessimism: it takes a famous optimistic quote (Martin Luther King Jr.'s "The arc of the moral universe is long, but it bends toward justice") and systematically dismantles it by pointing out that actual arcs in nature tend toward entropy, separation, and destruction. The comic finds dark comedy in the gap between human moral aspirations and the indifferent mechanics of the physical universe.