pale rhombus

On returning from a late-night visit to the loo, flicking the light off, I noticed a pale blue white rhombus floating on the wall.

A moon beam!

Stuck my head around the corner. Luna was hovering outside, slightly more than last-quarter, somewhere in the south-east sky, lined up so the shaft of reflected light passed through the window on the upper floor and continued unimpeded down the stairwell.