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2026-10-04 otherhand.org: "Irvine Ranch measurements (1929-1933)" (T. Mahood)

URL: https://otherhand.org/home-page/physics/historical-speed-of-light-measurements-in-southern-california/irvine-ranch-measurements-1929-1933/ Reached from Google: Michelson Pease Pearson 1935 speed of light variations fluctuations km/s Irvine Ranch vacuum tube. Also found: ADS abstract of Michelson, Pease & Pearson, "Measurement of the Velocity of Light in a Partial Vacuum" (1935), ApJ 82, 26 [ADS: ui.adsabs.harvard.edu]; the otherhand page links a scan of the full 1935 paper.

Purpose: to find what record could lie behind the essay's statement that Michelson-Morley-era records "show variations in the speed of light as large as 3000 meters per second". The essay gives no table or citation. No source giving exactly 3000 m/s was found this run.

Findings [web]

  • Mile-long evacuated pipe (36 in) at the Irvine Ranch, Santa Ana; 233 measurements Feb 1931 - Feb 1933 (Michelson died 9 May 1931; Pease and Pearson continued).
  • "And there were strange, unexplained cyclic drifts in the measurements. It was thought for a while it could be a tidal influence, but correlation was weak. Some of it was put down to the clay like soil ... a drainage ditch".
  • Mean 299,774 km/s against the earlier Mount Wilson-San Antonio value of 299,796 km/s: "a 22 Kilometer/second discrepancy ... never resolved". Survey errors were looked into (Invar tapes from the US Coast and Geodetic Survey).
  • The page lists contemporary press: LA Times and Santa Ana Register, 1929-1931 (Einstein's visit, Feb/Mar 1931). Archives: Huntington Library (Walter S. Adams papers), Mount Wilson Institute.

Bearing on the claims

Claim 4 (light-speed variations "well outside of the margin for error"). This is a documented case from the Michelson group of unexplained cyclic drifts and an unresolved 22 km/s difference between two campaigns. It is a related historical record, not the essay's (uncited) 3000 m/s figure. The reviewers should weigh it against modern measurements and the later redefinition of c. The 1935 paper's data tables are the next primary source to read.