The Ancient Egyptians had a limited knowledge of astronomy. Part of the reason for this is that their geometry was limited, and did not allow for complicated mathematical computations. Evidence of Ancient Egyptian disinterest in astronomy is also evident in the number of constellations recognized by Ancient Egyptians. At 1100 BC, Amenhope created a catalogue of the universe in which only five constellations are recognized. They also listed 36 groups of stars called decans. These decans allowed them to tell time at night because the decans will rise 40 minutes later each night. Theoretically, there were 18 decans, however, due to dusk and twilight only twelve were taken into account when reckoning time at night. Since winter is longer than summer the first and last decans were assigned longer hours. Tables to help make these computations have been found on the inside of coffin lids. The columns in the tables cover a year at ten day intervals. The decans are placed in the order in which they arise and in the next column, the second decan becomes the first and so on.
Astronomy was also used in positioning the pyramids. They are aligned very accurately, the eastern and western sides run almost due north and the southern and northern sides run almost due west. The pyramids were probably originally aligned by finding north or south, and then using the midpoint as east or west. This is because it is possible to find north and south by watching stars rise and set. However, the possible processes are all long and complicated. So after north and south were found, the Egyptians could look for a star that rose either due East or due West and then use that as a starting point rather than the North South starting point. This would result in the pyramids being more accurately aligned with the East and West, which they are, and all of the errors in alignment would run clockwise, which they do. This is because of precession of the poles which is very difficult to view, and the Ancient Egyptians did not know about. This theory is further substantiated by the fact that the star B Scorpii’s rising-directions match with the alignment of the pyramids on the dates at which they were built.
Ancient Egyptians also used astronomy in their calendars. There life revolved the annual flooding of the Nile. This resulted in three seasons, the flooding, the subsistence of the river, and harvesting. These seasons were divided into four lunar months. However, lunar months are not long enough to allow twelve to make a full year. This made the addition of a fifth month necessary. This was done by requiring the Sirius rise in the twelfth month because Sirius reappears around the time when the waters of the Nile flood. Whenever Sirius arose late in the twelfth month a thirteenth month was added. This calendar was fine for religious festivities, but when Egypt developed into a highly organized society, the calendar needed to be more precise. Someone realized that there are about 365 days in a year and proposed a calendar of twelve months with 30 days each, with five days added to the end of it. However, since a year is a few hours more than 365 days this new administrative calendar soon did not match the seasonal calendar.
The Cambridge Illustrated History of Astronomy. Michael Hoskin, ed. Cambridge University Press,1997.
Springer-Verlag and Hugh, Thurston. Early Astronomy, New York, Inc. 1994