An exercise in precision
The Great Pyramid of Giza is an engineering feat, for its size and for its almost perfect orientation. The structure is aligned to true north with an error of only 3/60th of a degree. This precision is notable for a monument constructed over 4,500 years ago. Theories on how this was achieved include using the fall equinox; an experiment demonstrated that tracking the shadow of a vertical rod (a gnomon) on this day can establish a nearly perfect east-west line. Another theory suggests ancient Egyptian astronomers used two stars, Mizar and Kochab, as they rotated around the North Celestial Pole to find true north.
The pyramid's base is a near-perfect square, with its four sides measuring about 230.4 meters (755.9 feet) and differing in length by only 58 millimeters. The foundation is also horizontally level to within 21 millimeters. Construction involved an estimated 2.3 million blocks of limestone and granite, with a total mass of about 6 million tonnes. While most of the core blocks were quarried locally, the smooth outer casing was made of fine, white Tura limestone, transported from about 15km away. This casing would have made the pyramid's surface smooth and brilliantly reflective in the sun.
Internal structures and cosmic rays
Deep inside the pyramid's mass are several known chambers: the King's Chamber, the Queen's Chamber, and an unfinished subterranean chamber cut into the bedrock. Leading to the King's Chamber is the Grand Gallery, a magnificent ascending corridor. From the King's and Queen's chambers, narrow shafts extend outwards. These are not air shafts but are believed to have had a symbolic, astronomical purpose. Around 2500 BC, the northern shaft from the King's Chamber pointed toward the then-pole star, Thuban, in the constellation Draco. The southern shaft pointed toward Orion's Belt, a constellation the Egyptians associated with the god Osiris.
Modern physics has offered a new way to explore the pyramid's secrets without disturbing its structure. The ScanPyramids project, initiated in 2015, uses muography, a technique that detects cosmic-ray particles called muons to map the density of the structure. In 2017, the project announced a major finding: a large, previously unknown void located above the Grand Gallery. This "ScanPyramids Big Void" is at least 30 meters (98 feet) long and has a cross-section similar to the Grand Gallery. Its function remains unknown, but its discovery shows how modern science reveals new information about this ancient monument.