The vowels

The five vowels

These correspond to the equinoxes and solstices of the year. Yet, unlike the months, they are points in time rather than periods of time. The Spring and Autumn Equinoxes (O and E) and the Summer Solstice (U) form three of these points, whilst the Winter Solstice contains two letters which might seem curious until it is appreciated that they define A as the start and I for the close of the year.