This week a somewhat different puzzle from our usual offerings - a puzzle about the shape of the crescent moon.
We are all familiar with the phases of the moon in our sky, as the sun illuminates half of the moon and we observe the various proportions of lit and unlit moon surface.
So, what shape is the crescent moon?
A crescent is formed from the arcs of two circles. The outer edge of a crescent moon is part of a circle; we see the spherical moon as a circle from earth and the outer edge of the crescent moon is that circle.
However, the inner edge of the crescent moon is not the arc of a circle. It is the boundary between the lit and unlit parts of the moon. This is a circle, but not as it is viewed from earth during a crescent moon. [NB we do see it as a circle during a full moon] This circle is rotated from our viewpoint and so the inner edge is not the arc of a circle.
The crescent moon is not crescent shaped - who knew!