See George A. Smith, Jerusalem, From the Earliest Times to A.D. 70 (London: Hodder and Stoughton, 1908), vol. 2, p. 588, n. 3: 20.67 inches = sacred cubit. See also Arye Ben David, “The Hebrew-Phoenician Cubit,” Palestine Exploration Quarterly (PEQ) (1970), pp. 27–28: the Philaeterian-Ptolemaic cubit = 525 mm (20.67 inches). Joachim Jeremias (Jerusalem in the Time of Jesus [London: SCM, 1969], p. 11) defines the Philaeterian Cubit as equal to 525 mm, or almost 21 inches. He quotes Didymus (end of first century A.D.) “who calculates the Egyptian cubit of Roman times as 1 ½ Ptolemaic feet.” As this foot was 350 mm long, the cubit was 1.5 × 350 = 525 mm. David Ussishkin, “The Original Length of the Siloam Tunnel,” Levant 8 (1976), pp. 82–95: cubit = 52.5 cm (= 20.67 inches). Cf. Asher S. Kaufman, “Determining the Length of the Medium Cubit,” PEQ 116 (1984), p. 131. Kaufman is close, but his measurement is not exact (20.319 inches instead of 20.66925). My own research has shown that 500 cubits of 20.67 inches equals exactly the distance from the step to the eastern wall, and therefore I believe that this was the cubit used for laying our the 500-cubit-square Temple Mount.

This cubit originated in Egypt and is also called the Egyptian long cubit; the short cubit was only 450 mm long. The long cubit is also known as the royal cubit, and had been in use since the 15th century B.C. This cubit rarer became known as the Philaeterian cubit, after the family name of the kings of Pergamum.