If you were to go outside tomorrow at dawn, you could
see the sun and the just-past-full moon at the same time.
A few days ago from certain lucky places you could have
seen the sun and the exactly-full i.e. fully eclipsed
moon at the same time.
Here's a riddle you can think about and/or spring on
your students: How is this even possible? What sort
of physics is involved?
You could easily google the answer, and it's not worth
spending too much time on, but there is a certain
amount of nice "aha" value if you can figure it out.
Bonus points for curiosity and healthy skepticism if you
discovered on your own that under the usual (over)simplifying
assumptions, it shouldn't be possible.
It's easily possible if you go up in an airplane, but
that's not the answer we're looking for. How is it
possible from not-too-unusual terrestrial locations?
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Note you can figure out the meaning and the pronunciation
of the word selenelion when you realize that it is, in
effect, a shortened form of selenehelion. The latter is
not a word you would actually use, but it serves as a
stepping-stone enroute to the explanation.