The expression “Once in a Blue Moon” is colloquial for “The thing hardly ever happens.” But If you meant it literally, then that thing will occur every two or three years.
So, like most cosmic happenings, Blue Moons aren't entirely rare, even if we want to think they are.
@neiltyson Always raining on someone’s parade. Feel free to ruin that saying, too
@neiltyson Considering the modern average attention span, every two or three years is like eons, so... 🤷♂️
@neiltyson We're so high we have 4 full moon view Halloween.
@neiltyson How about once for every observable blazar? I hear those are rare.
@neiltyson That’s what happens when you apply human timelines to the cosmic machine.
@neiltyson Neil this was my party fun fact and now you’ve ruined it! I hate 2020.