= Clinostat
A <clinostat> rotates a biological specimen so that the direction of <gravity> changes continuously relative to it. A horizontal rotation axis is commonly used in experiments on <plant> <gravitropism>. Rotation changes the history of directional stimulation; it does not switch off the gravitational field.
For an ideal specimen rotating at angular speed $\omega$, a body-fixed representation can be $\mathbf g(t)=g(0,\cos\omega t,\sin\omega t)$. Its average over a full turn is zero, but $|\mathbf g(t)|=g$ throughout. A sufficiently slow biological response might integrate the changing directions, while a faster response can distinguish them. A vertical-axis control does not produce the same change in gravity direction relative to the specimen.
Rotation also introduces an acceleration scale $a=\omega^2r$ at distance $r$ from the axis. At $r=1\,\mathrm{cm}$, one revolution per minute gives approximately $1.1\times10^{-4}\,\mathrm{m/s^2}$, while sixty revolutions per minute gives about $0.39\,\mathrm{m/s^2}$. These are illustrative kinematic calculations, not recommended operating speeds. Moving a specimen farther from the axis also changes this acceleration.
A 1976 experiment combined clinostat rotation with centrifugation. Several developmental outcomes still depended on the applied acceleration, demonstrating incomplete gravity compensation.<Limitation on the Use of the Horizontal Clinostat as a Gravity Compensator by Allan H. Brown and others>[Abstract; centrifuged clinostat-grown plants and morphological endpoints]{ref} This motivates comparing orientation history, rotation effects and genuine reduced-gravity conditions rather than labelling every rotating preparation weightless.
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