DAY (5 letters) The term 'Period of a Revolution' can be broken down into two crossword solutions: 'HOUR' (4 letters) and 'DAY' (5 letters). A period of a revolution is a unit of time that is used to measure the length of a revolution. It is typically measured in terms of the amount of time it takes for an object to make one complete rotation around its axis. For example, the Earth's period of revolution is 24 hours, or one DAY.