"Muse on" vs. "muse about"

I came across the verb muse, which roughly means to be absorbed in thought about any given thing. I wondered, when specifying what someone was musing about, would it be better to say:

I muse about what happened that day.

Or:

I muse on what happened that day.

Which is correct, or better?

Muse can be used with about, on, over or upon and they all mean essentially the same.

Both are possible; muse on is more common than muse about. Also, muse on is more focused and specific, and muse about is more general.

Muse on this.

Also, Merriam-Webster's Dictionary of English Usage says this about the matter:

Still, I think it might come down to what you're musing on or about.

For example muse on the day returned only one hit in Google books, while muse about the day returns at least four. I might be more inclined to say muse about when the object of my musing is less focused – I mused on my predicament, yet I mused about my day – but you would be foolish to regard that as a hard and fast rule.

Incidentally, if you restrict the Ngram to more recent sources, it seems to contradict the usage note in M-W, meaning about might be becoming more like upon and over, with on remaining the frontrunner. (That said, it's a silly notion to think that you should always follow what Ngram shows is most common. Muse on that advice for awhile.)