"Set" is, in most ordinary uses, a transitive verb needing an object. It means to put or place something in a certain position, or to arrange. Its principal parts are: set, setting, set, set. Examples: 1) She set the table. 2) He set the watch.
"Sit" is, in ordinary usage, an intransitive verb. It means to rest somewhere (like a chair) in an upright position. Its principal parts are: sit, sitting, sat, sat. Examples: He sits down. "Sit" can also be used to talk about where an object is located. Example: The clock sits on the shelf. In a few instances "sit" is used as a transitive verb, such as in: He sat himself down.
- When heavenly bodies sink below the horizon, they are referred to as "setting," not "sitting." Examples: 1) the sun set at 6:00 p.m. 2) let's watch the setting sun.