The planets Mercury, Venus, Mars, Jupiter, and Saturn are visible to the naked eye in the night sky and appear brighter than most fixed stars. They were known in antiquity and were identified with the chief gods by many cultures. The names still in use today come from Roman mythology. The Moon is, after the Sun, the brightest object in the sky; at the same time, its unique brightness and phase changes between full moon and new moon can also be easily observed with the naked eye. The first appearance of the crescent moon in the evening sky (as a new light) marks the beginning of the respective month in some cultures. The lunar phases and solar or lunar eclipses were certainly observed by humans early on. The exact length of the sidereal and synodic month was known as early as the 5th millennium BC.