INARA: Sihnon isn't that different from this planet. More crowded, obviously, and I guess more complicated. The great city itself is... pictures can't capture it. It's like an ocean of light.
The jewel of the Core worlds, Sihnon is described in such a way as to imply that it far surpasses the sophistication and technology level of any of the planets we see during the show, with the possible exception of Ariel (and Inara seems to imply that Ariel is 'lovely', but still a far cry from her home world).
One of the two main core worlds (the other being Londinium). According to Joss Whedon, Sihnon is based on Chinese culture.
During the classroom scene in the Serenity Movie, Sihnon is the first planet listed upon the screen to the left of the teacher.