The precise alignment of temples and pyramids was undoubtedly a result of astronomical observation.  (A feature noticeably absent from Djoser's pyramid at Saqqara).

Extract from (1) - (By John Pappademos):

'Egyptian astronomy has been termed "primitive" or "non-existent" far too many times: it is appropriate to assemble the actual achievements of Egyptian  astronomy and let the facts speak for themselves. The ancients unencumbered by racism, could take a more honest view of the matter: for example, Aristotle wrote: "...the Egyptians and Babylonians have studied these matters thoroughly since time immemorial, and through whom we have many reliable reports about each of the stars."

A clear division can be identified between the orientation of ancient Egyptian temples of upper Egypt when compared with those of Lower Egypt. Lockyer (2), made note of the fact that whereas the early dynastic northern 'Memphite' pyramids, Giza pyramids, and Sphinx were orientated cardinally to face equinoxial phases of the solar cycle, the great temples in the south of Egypt, such as Karnak, the Colossi of Memnon and  Abydoss, were all orientated to capture the suns rays on Solstice days of the year.

The elements of the following compendium have all been separately discussed in the literature, but have not received sufficient emphasis nor have they ever been assembled together. They are listed in chronological order.

The invention of the 365-day calendar, based on astronomical observation - Mankind's first measurement of time. The development probably took place at least as far back as 3,000 B.C. (1)

The development of instruments of quantitative astronomical measurement - These included the sundial, water clocks, and the merkhet (which used a straight-edge and a plumb line, enabled measurements of stellar azimuths). (1)

Knowledge of stellar constellations - At least 43 constellations were familiar to the Egyptians in the 13th century B.C (1).

The writing of astronomical texts - Clement of Alexandria gives the titles of four Egyptian astronomy books (which have not survived): a) On the Disposition of Fixed Stars and Stellar Phenomena, b) On the Disposition of the Sun, Moon and Five Planets, c) On the Syzergies and Phases of the Sun and Moon, d) On Risings. These texts may not have been intended for publication, but were available only to the priesthood, which forbade the general exposure of their philosophy. This may help to explain why so little of Egyptian science has come down to us directly from the Egyptians, rather than indirectly from the Greeks such as Pythagoras, who was initiated into the Egyptian mysteries. (1)

Tables of star culminations and risings. (1)

Knowledge of planetary astronomy - Five planets were known to the Egyptians; the retrograde motion of Mars was known; the revolution of Mercury and Venus around the Sun was known. (1)

Prediction of eclipses.

Discovery of the occulations of the stars and planets by the dark side of the half-moon. (1)

Discovery that the Earth is spherical - The first (Known) measurement of the radius of the earth was made by Erasthenes (b. 275 B.C.), who was the head of the great library of Alexandria. He was born in Cyrene, in what is now Libya. It seems likely that the ancient Egyptians, much before Egypt's conquest by Alexander the great, had already grasped the idea of a spherical Earth, and it was from them that this doctrine was adopted by Pythagoras, who, as we know, spent many years of study in Egypt. (1) The location of certain prominent Egyptian temples suggests a knowledge of longitude and laitude.

Discovery of the obliquity of the ecliptic - Diodorus Siculus (70 B.C.) reports that Egyptian priests claimed it was from Oenopides of Chios learned the sun moved in an inclined orbit and oppositely to the motion of the other stars. In this connection, it should be noted that the priority of Oenopides claim to this discovery is disputed by Pythagoras. In view of the fact that both Pythagoras and Oenopides went to Egypt to study astronomy, it would seem only fair to give their Egyptian teachers at least some of the credit. (1)

Discovery of the precession of the equinoxes - The "procession of the equinoxes" refers to the very slow, cyclic changes in the coordinates of the fixed tars that takes place with a period of some 26,000 years (25,920). The discovery of this phenomena can be divided into three stages. The first stage took place in Egypt, where the successive realignment of the axis of symmetry of various temples noted by Lockyear showed that the Egyptians were aware of the change in positions of the stars over the course of centuries. (The orientations of Egyptian temples were set with extreme precision by astronomical observations in accordance with their worship of the stars or the sun.) The second stage consisted in the measurement, although some have given priority to the Egyptian astronomers. The third stage consisted in the dynamical explanation of the effect, first given by Isaac Newton in the Principia. (1)

Note: It has been noted that the Great pyramid of Giza is a scale model (1:43,200), of the Earth's dimensions:

(A new constellation appears on the horizon each 2,160 years...)

(...and 2,160 x 2 = 4,320 years)

 

First proof that the angular diameters of the sun and moon re unequal - Sosigenes (2nd Cent. A.D), the Egyptian astronomer who gave Europe the Julian calendar, showed that the angular diameters are unequal by advertising to the phenomena of annular eclipses of the sun. (1)

First use of the Clepsydra (water clock), to measure the angular diameter of the sun. (1)

Discovery of the conjunction of the planets with each other as well as the fixed stars - This is on the testimony of Aristotle in his Meterology. (1)

The Heliocentric theory of the rotation of the earth and other planets about the sun. (1)

References:

1). Journal of African Civilisations. Vol.12. Spring, 1994.