Egyptian astronomy in the context of "Upper Egypt"

Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry to explain their origin and their overall evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is the branch of astronomy that studies the universe as a whole.

Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Egyptians, Babylonians, Greeks, Indians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars.

Ancient Greek astronomy is the astronomy written in the Greek language during classical antiquity. Greek astronomy is understood to include the Ancient Greek, Hellenistic, Greco-Roman, and late antique eras. Ancient Greek astronomy can be divided into three phases, with Classical Greek astronomy being practiced during the 5th and 4th centuries BC, Hellenistic astronomy from the 3rd century BC until the formation of the Roman Empire in the late 1st century BC, and Greco-Roman astronomy continuing the tradition in the Roman world. During the Hellenistic era and onwards, Greek astronomy expanded beyond the geographic region of Greece as the Greek language had become the language of scholarship throughout the Hellenistic world, in large part delimited by the boundaries of the Macedonian Empire established by Alexander the Great. The most prominent and influential practitioner of Greek astronomy was Ptolemy, whose Almagest shaped astronomical thinking until the modern era. Most of the most prominent constellations known today are taken from Greek astronomy, albeit via the terminology they took on in Latin.

Greek astronomy was influenced heavily by Babylonian astronomy, as well as Egyptian astronomy to a lesser degree. In later centuries, Greek-language astronomical works were translated into other languages, enabling their further spread. Most notably, Arabic translations of these works benefitted astronomers and mathematicians throughout the Muslim world during the Middle Ages.

The ancient Egyptian calendar – a civil calendar – was a solar calendar with a 365-day year. The year consisted of three seasons of 120 days each, plus an intercalary month of five epagomenal days treated as outside of the year proper. Each season was divided into four months of 30 days. These twelve months were initially numbered within each season but came to also be known by the names of their principal festivals. Each month was divided into three 10-day periods known as decans or decades. It has been suggested that during the Nineteenth Dynasty and the Twentieth Dynasty the last two days of each decan were usually treated as a kind of weekend for the royal craftsmen, with royal artisans free from work.

Because this calendrical year was nearly a quarter of a day shorter than the solar year, the Egyptian calendar lost about one day every four years relative to the Julian calendar. It is therefore sometimes referred to as the wandering year (Latin: annus vagus), as its months rotated about one day through the solar year every four years. Ptolemy III's Canopus Decree attempted to correct this through the introduction of a sixth epagomenal day every four years but the proposal was resisted by the Egyptian priests and people and abandoned until the decree by Augustus in 25 BC that established the Alexandrian or Coptic calendar. The introduction of a leap day to the Egyptian calendar made it equivalent to the Julian calendar, although (like the latter) it continues to diverge from the Gregorian calendar at the turn of most centuries.

The heliacal rising (/hɪˈlaɪ.əkəl/ hih-LY-ə-kəl) of a star or a planet occurs annually, when it becomes visible above the eastern horizon at dawn in the brief moment just before sunrise (thus becoming "the morning star"). A heliacal rising marks the time when a star or planet becomes visible for the first time again in the night sky after having set with the Sun at the western horizon in a previous sunset (its heliacal setting), having since been in the sky only during daytime, obscured by sunlight.

Historically, the most important such rising is that of Sirius, which was an important feature of the Egyptian calendar and astronomical development. The rising of the Pleiades heralded the start of the Ancient Greek sailing season, using celestial navigation, as well as the farming season (attested by Hesiod in his Works and Days). Heliacal rising is only one of several types of alignment for stars' risings and settings; mostly the risings and settings of celestial objects are organized into lists of morning and evening risings and settings. Culmination in the evening and the culmination in the morning are separated by half a year, while on the other hand risings and settings in the evenings and the mornings are only separated by a half-year at the equator, and at other latitudes set apart by different fractions of the year.

Spanning over three thousand years, ancient Egypt was not one stable civilization but in constant change and upheaval, commonly split into periods by historians. Likewise, ancient Egyptian architecture is not one style, but a set of styles differing over time but with some commonalities.

The best known example of ancient Egyptian architecture are the Egyptian pyramids and Sphinx, while excavated temples, palaces, tombs, and fortresses have also been studied. Most buildings were built of locally available mud brick and limestone by paid laborers and craftsmen. Monumental buildings were built using the post and lintel method of construction. Many buildings were aligned astronomically. Columns were typically adorned with capitals decorated to resemble plants important to Egyptian civilization, such as the papyrus plant.

The decans (/ˈdɛkənz/; Ancient Egyptian: 𓅡𓎡𓏏𓁐𓅱𓏼, romanized: bꜣkt.w, lit. '[those] connected with work') are 36 groups of stars (small constellations) used in ancient Egyptian astronomy to conveniently divide the 360 degree ecliptic into 36 parts of 10 degrees each, both for theurgical and heliacal chronometrical purposes. The decans each appeared, geocentrically, to rise consecutively on the horizon throughout each daily Earth rotation. The rising of each decan marked the beginning of a new decanal "hour" (Greek hōra) of the night for the ancient Egyptians, and they were used as a nocturnal (sidereal clock) beginning by at least the Ninth or Tenth Dynasty of Egypt in the 21st century BC.

Because a new decan also appears heliacally every ten days (that is, every ten days, a new decanic star group reappears in the eastern sky at dawn right before the Sun rises, after a period of being obscured by the Sun's light), the Greeks called them dekanói (Koine Greek: δεκανοί; pl. of δεκανός dekanós) or "tenths".