The Carters, a father-daughter geodesist-astronomer team, tell the story of how an actuary at a life-insurance company, in 1891, came to understand latitudinal variation and nail down its law.
Among the most vexing problems for 19th-century astronomers was the unpredictable scatter of their celestial observations due to unexplained problems with the angle of aberration and parallax. Although variation in latitude—that is, the mysterious movement of a place relative to the fixed line of latitude—had been suggested as the culprit for some time, it was not until Seth Chandler, an American amateur astronomer working in the life-insurance industry, devised his own tools for measurement. Chandler’s measurements were then reconciled with dynamic theory by Simon Newcomb, and the variation was understood. The reasons for it came to be known as the Chandler Wobble, after the movement of the Earth’s pole, but that doesn’t come close to explaining the phenomenon, and the Carters suffer no fools in their story. Their story sits in that funny land between popular study and learned essay. The Carters have decided not to eschew the language of science (“the centrifugal force acting on any point in a rotating body increases linearly with the distance from the axis of rotation, and the square of the angular velocity”). Despite the slices of historical narrative and biographical material, this is destined mainly for an enlightened amateur audience comfortable with the language. At the same time, there are windows of opportunity for the non-specialist to become acquainted with physical properties of Earth, including the revolution of Earth’s pole, fluidity, elasticity, centrifugal force, and periodicity, which in turn may offer readers a glimmer as to why their personal GPS devices may need minute recalibrations every so often.
Wading through the physics of it all, the Carters manage to convey a sense of Earth’s dynamic nature—the swiftness of its transformations—and the impermanence of all things measured.