Seth Carlo Chandler Jr.

Seth Carlo Chandler Jr. was born in Boston in 1846 and spent his professional life as an actuary in the insurance business. He pursued astronomy as an avocation, with unusual rigour, and for decades kept detailed observations and analyses of astronomical data. He worked particularly on latitude observations — measurements of the local latitude of a fixed observatory, which should be constant if the Earth rotated on a perfectly stable axis but would vary if the rotation pole moved with respect to the Earth's crust.

In 1891, analysing pooled latitude data from observatories worldwide over the previous decades, Chandler discovered a clear periodic variation with a period of about 14 months — substantially longer than the 10-month period Euler had predicted in 1758 for a rigid Earth. Chandler's result was the first direct observational evidence that the Earth is not a perfectly rigid body; its elastic deformation under the stresses of its own wobble lengthens the free-precession period from Euler's 305 days to the observed 433 days. Simon Newcomb worked out the physical explanation within a year of Chandler's discovery.

The 433-day wobble is now called the Chandler wobble, and its amplitude of a few metres at the surface is tracked to sub-millimetre precision by modern space-geodetic techniques. Chandler died in 1913, never having held a formal academic position, but his name is attached to one of the most carefully measured geophysical phenomena of the twentieth and twenty-first centuries.