Time is an abstract concept created by man to order events in sequences and to orient oneself concerning a certain moment. Since the beginning of humanity, man has used his knowledge of heaven to measure time. The annual solstices and equinoxes provided them with reliable points to build a map of the year, and the constellations provided useful markers for time. The Romans and Greeks considered the time to be one-twelfth of the time from sunrise to sunset. For hundreds of years later, the sun was also used as a marker of the hours of the day employing basic instruments such as the sextant, (an instrument that was also used in navigation) but these instruments and the first clocks that appeared were of doubtful precision. Likewise, astronomers and geographers used different meridians according to their time or country.