Bear with me, this is a very specific question for a scifi book I'm writing. If we had a ring in place of the moon, and you lived somewhere in the United States, how often would the ring intersect with the sun when it's setting (or rising, if it makes it easier). I know that it can take up to 40 years for a total eclipse of the sun in the US. But if we're talking about the whole moon orbit, it's more frequent than that. But adding the restriction of the 30 minutes when it's setting may make it harder.