Building a Merkhet — The Egyptian Star Transit Instrument for Finding True North

The merkhet (Egyptian: mrḫt, meaning 'instrument of knowing') is the oldest known astronomical observation instrument, used in ancient Egypt from at least 2500 BC. It consists of two simple parts: a bay — a straight bar with a V-notch from which a plumb line hangs, establishing a precise vertical reference — and the merkhet itself — a straight sighting bar with a narrow slot. By aligning the sighting slot with the plumb line string and a target star, the astronomer could determine the exact moment a star crossed the meridian (the north-south line through the zenith). This technique, called transit observation, served two critical purposes: finding true north for aligning temple and pyramid foundations with extraordinary precision (the Great Pyramid of Giza is aligned to within 3 arcminutes of true north), and telling time at night by tracking when specific 'clock stars' (decans) crossed the meridian. The merkhet is depicted on the ceiling of the tomb of Ramesses VI and was used by hour-watching priests who kept the official time for temple rituals throughout the night. This blueprint builds a functional merkhet and bay from hardwood and a plumb bob, capable of finding true north and performing star transit observations exactly as Egyptian astronomers did over four thousand years ago.