The thing about space is, there aren't any sunsets.
Well, that's one of the things. There are others, of course, but it's something that Leonard finds unexpectedly that he misses. No violet and orange and rose-colored clouds, no light that's so golden that the Greeks would've attributed it to a goddess, no air cooling imperceptibly as the sun vanishes over the horizon.
There are spectacular views in space, nebulae and binary stars and hell, even most planets are beautiful, really, but there's nothing like a real Earth sunset. Even the sunsets on other planets aren't quite the same. The sky is a different shade, more green than blue, or the light is orange instead of gold, or something. It's never what he remembers.
So when they have leave, and Jim asks where he wants to go, Leonard replies without hesitation that he wants to go to Earth, to the Pacific coast, somewhere without too many people, and walk along the beach and watch the sunset there.
Jim laughs at him a little, the way he does, but is willing to do what Leonard wants as long as they spend some time in San Francisco or another, more interesting place as well.
They go, and drive along the narrow coast highway to a town whose name Leonard will never remember, and stop at a shabby old motel for the night.
And after they've put their suitcases in the room, and eaten at a diner whose vintage is probably older than the motel, they find the footpath to the beach, and follow it.
Standing on the sand with Jim, hands clasped, no words needed between them, the sunset is everything that Leonard has remembered all these years.