نظير مسّ السماء
A parallel example is touching the sky.
If someone swears to touch the sky, the ordinary literal performance is beyond normal human ability. This raises the question whether usage establishes an intelligible related sense and how the oath is legally classified.
It is not enough simply to say “the literal act is impossible.” The existence of a recognized figurative sense, intended usage, and legal restrictions on oaths are also relevant. Whether an oath is binding and how expiation is discussed depends on those distinctions.