Most people know that the Great Pyramid has four sides. Many assume that they are flat. What is not well known is that the pyramid’s sides are not perfectly flat. And they never have been. At the time they were completed, the sides were covered with casing-stones made of white Tura limestone, which gave the finished Pyramid a smooth surface. Most of the original casing stones have been removed. The process of removing them started about 700 years ago. Originally, they were removed so that they could be used in the construction of other buildings in the Cairo vicinity. The remaining core masonry blocks are set in a stair-step fashion.

Flinders Petrie 1903
William Matthew Flinders Petrie, 
source: Wikipedia 

The fact that the sides of the Great Pyramid are not perfectly flat and straight was discovered 130 years ago by Sir William Matthew Flinders Petrie. Flinders Petrie is known as the father of Egyptology. Petrie, a skillful surveyor, decided to do a precise survey of the Great Pyramid’s base and structure and also make thorough measurements of the pyramid’s interior chambers, passageways, and features. While doing his survey, Petrie noticed that the middle of each base-side of the pyramid’s core masonry was “distinctly hollowed in” (some 30 inches) instead of straight from corner to corner, and that “each side has a sort of groove” that ran from the top down the middle of the sides. 

From the outer edges, the surface slants slightly inward toward the center, making a vertical center-line from the apex down the middle of each triangular side. Near the bottom, the center-line branches off left and right to form a triangle inset at the bottom center of each base-side.


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Each side of the Great Pyramid was built with these angles in it, and some photographs of the pyramid show them. They are most visible when the sun makes shadows on the angles of the sides.

Why did the ancients build the pyramid’s sides angled in towards the center-line instead of making the sides perfectly flat? I believe there was a logical reason, which I will explain.

When Petrie began to do his survey of the Great Pyramid, he cleared away the sand and rubble where the four cornerstones had once sat. He found the rectangular flat depressions or sockets that were carved in the bedrock at the four corners; the casing-cornerstones sat in these corner-sockets until they were removed hundreds of years ago with most of the other casing-stones. Petrie measured the distance from the end of one corner-socket to the end of the next corner-socket at the other end of the pyramid’s base-side. He measured all four sides in this manner.

The four corner-sockets defined the pyramid’s perimeter around the base. When Petrie’s lengths of the four sides (defined by the sockets) are added together it equals 36,521 inches. That is significant because 36,525 is the number of days in the year x 100 (there are 365.25 days in the year). The ancient builders made the pyramid’s base perimeter around the four corner-sockets to equal the number of days in the year (x 100).


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The distance of 36,525 inches around the base perimeter is also significant because a half-minute of longitude at the earth’s equator is equal to 36,525 inches. The earth is divided with longitude and latitude lines. The modern calculation of one-half minute longitude at the equator is 36,523 inches. It is a reasonable deduction, as I see it, that the builders deliberately made the base perimeter equal to one-half minute longitude. 

pyramid casing stonesPetrie found the remaining casing-stones of the Great Pyramid sitting on the base-platform. Casing-stones remained at the bottom of the pyramid in the middle of the four base-sides. Petrie projected the casing base-line down each side of the pyramid to the corners. Thus he obtained a square base-line around the pyramid, which seemed to show the original base-line of the pyramid. But his projected square base-line did not go to the far ends of the corner-sockets as one would have expected.

 Petrie assumed that the original casing base-line was straight from one pyramid corner to the next corner. But with his survey, he discovered that the core-masonry blocks in the middle of the base-sides were set back some 30 inches more than the corners of the Pyramid’s core-masonry; which made the middle of the base-sides’ core-masonry slightly “hollowed in.” As for the casing-stones that covered the core blocks, Petrie assumed that the casing-stones in the middle of the base-sides had been thicker than the casing-stones near the corners (those near the Pyramid’s corners were no longer there), thereby making the casing base-line straight from corner to corner.

 He was wrong. Petrie (along with everyone else) did not realize that the original casing base-sides were not perfectly straight to the corners. Actually, the casing-stones followed the angles of the core-masonry. Near the corners of the Great Pyramid the original casing began to slightly turn outward to the outside corners of the corner-sockets where the casing-cornerstones sat.

Petrie’s measurement of the four projected sides added together equals 36,275 inches around the base perimeter. Petrie’s base-perimeter distance is the same as the distance of one-half minute of latitude at the equator. The modern calculation of one-half minute of latitude at the equator is 36,278 inches.

 The Great Pyramid’s constructors built in the smaller projected square base to show one-half minute of equatorial latitude. So the builders showed in the pyramid’s casing-base perimeter both one-half minute longitude and latitude at the equator. In effect the pyramid was a smaller square base inside the larger perimeter—a smaller pyramid inside a larger pyramid. 

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 Article text and drawings Copyright ©2012 Timothy Milby