Where does nutation collide?

James Bradley's 1748 report to the Royal Society, A letter concerning an apparent motion observed in some of the fixed stars, described a small periodic nodding of Earth's axis that he traced to the changing pull of the Moon on the planet's equatorial bulge. Nutation is this slight rocking of a spinning body's axis, laid on top of the much slower coning motion called precession, so the pole traces tiny loops as it drifts. The referent here is the wobble of a rotating body's axis in space, timed over years and reported in arc seconds of tilt, not the circling of a plant's growing tip or the rocking of a bone inside a joint.[1]

Unit

One nodding cycle of a rotating body's axis, scaled in arc seconds of tilt

Recovery means

Not applicable

Measured by

Tracked by precise astrometry of star and pole positions and modelled as periodic terms in the axis orientation over time

Charles Darwin's 1880 book The Power of Movement in Plants, written with his son Francis, named circumnutation: the helical, nodding path that the growing tip of a stem, root, or tendril traces as its zone of elongation bends in a slowly turning direction. Stolarz's 2009 review Circumnutation as a visible plant action and reaction gathered the physiological, cellular, and molecular accounts of how this oscillating growth is driven and how its rhythm varies across species. The referent is the circling movement of a living, elongating plant organ, followed over minutes to hours, and it shares only the root word with a planet's wobbling axis or a joint's bony rotation.[2][3]

Unit

One circling cycle of a growing plant tip, scaled by its period and the width of its arc

Recovery means

Not applicable

Measured by

Recorded by time-lapse imaging of the tip's path and quantified by the period, amplitude, and shape of the traced loops