Odd what you forget to remember
I didn't recall this little moment until I read this just now, a memory from a far-off time (mid-July of 2003) in a far-off place (Corvallis, OR, USA).
My father and I gathered up the telescope-stuff, tossed it in the Xterra, and took off to find a dark, light-pollutionless hillside from which to watch the Moon occult Mars, only to get out there and discover pretty quickly that the angles were wrong and the Moon was most likely going to pass under Mars - which it did, by a considerable number of degrees.
So we repacked everything and came home.
Nobody said we were geniuses.
Actually, joke aside, this would not have been terribly difficult to have figured out at the time, but, with smartphones and apps, it would be an absolute breeze to figure out now.