It is the illusion that the Moon appears larger when it is on
or near the horizon than it does when it is high in the sky.
I just saw this illusion tonight in the setting crescent Moon
seen from just south of the border (with Washington). We were
treated to a beautiful double rainbow display that lasted for
the better part of an hour. The horizon was clear up to about
ten degrees altitude, and a light rain was falling fifteen
miles south of Bellingham. We could see very clearly the
relatively darker sky between the bows, and I could barely
glimpse the supernumeraries within the primary bow, only
because I knew they were there. I made sure my four-year old
granddaughter saw the principal feaures, and I'm eager for
her to grow up so I can tell her *why* they are the way she
sees them.
Yes, I played hooky on my last day before retirement, but I
will be back in my office bright and early tomorrow morning,
as a Professor Emeritus, but with a much smaller pay packet.