Astronomy was very important to the ancient Egyptians, who observed the sky periodically. The astronomers named what they saw in the sky and used their observations to create the Egyptian calendar. The beginning of the Egyptian year was declared when there was a flood, as they noticed that the flood begins with the star Sirius, also known as the Dog Star, the brightest star in the sky. This incident represented the beginning of the agricultural year in Egypt. The year had 365 days divided into 12 months and each month had 30 days. They made the remaining five days feast days, called the Epagomenal Days, or the days upon the year, and added them at the end of the year. Months of the year were divided into three seasons, namely: the flood season, the planting season, and the harvest season. The year, the season, the month and the day in which the king assumed power was usually recorded by the Egyptians in their documents.
The ancient Egyptians used instruments or indicators for observing the circumpolar star. They would then draw a north-south axis line on the ground marking its direction, which was required for the proper orientation of important building projects. One of the instruments used was called "Merkhet," which could mean "indicator." It consisted of a horizontal, narrow wooden bar with a hole near one end, through which the astronomer would look to fix the position of the star. The other instrument, called the "bay en imy unut," or palm rib, had a V-shaped slot cut in the wider end through which the priest in charge of the hours looked to fix the star.
In the Greco-Roman era, Claudius Ptolemy, an astronomer, mathematician, and geographer, worked from the data of past astrologers to map over 1,000 stars. He compiled a list of 48 constellations and described the longitude and latitude lines of the earth. He was a believer that the earth was the center of the universe and worked to advance this theory. He developed the Ptolemaic system to explain why some planets seemed to move backwards for periods of time in their orbit around earth. He theorized that each planet also revolved in a smaller circle as well as a larger one. This was called the "epicycle." This theory would survive for 1400 years, until it was finally accepted that the earth was itself another planet in orbit around the sun.
The Greco-Romans used a calendar based on the Julian calendar calculations that used the leap year. Egyptian Christians, called Coptics, adopted this calendar to follow the sun and calculate the days, seasons, and solar years. The lunar year was also important to the Coptics, whose lunar calendar was used to determine the date of Easter and other important religious holidays. The Greco-Roman period also saw the invention of the astrolabe, a navigational tool that was perfected during the Islamic era. The astrolabe played an important role in guiding ships, whether for military or commercial purposes.