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From JOHN GRANDAGE

Q: I am confused by the appearance of a gibbous Moon. On Monday 31 January,
I was driving eastwards at dawn towards the rising Sun. Through the side
window of my car – about 120 degrees to the right – I could see the Moon
with its fuller surface directed towards the sun.

Well, not quite. The Moon was tilted a few degrees upwards and did not
present its illuminated face slightly downwards or horizontally as you might
expect.

I confirmed the tilt of the Moon against a nearby vertical pole, as
did my wife who was sitting in the passenger seat. She dismissed my protestations
with a throwaway: ‘I expect that it is an optical illusion’.

Maybe it is, but how . . . and why?

John Grandage Cambridge