The word ‘month’ takes its root from the Moon. A month was originally defined to be either 29 or 30 days, roughly equal to the 29.5-day cycle of the lunar phases. Some of our calendar months were later padded out with extra days so that 12 months would make up one complete 365-day solar year.
With a diameter of about 2,159 miles (3,475 kilometers), is bigger than Pluto. (Four other moons in our solar system are even bigger.) The moon is a bit more than one-fourth (27 percent) the size of Earth.