לוח שנה משולב

The isotopic similarity of the Earth and Moon has motivated a recent
investigation (Cuk and Stewart, 2012) of the formation of the Moon
with a fast-spinning Earth. Angular momentum was found to be drained
from the system through the evection resonance, a resonance between
the pericenter of the Moon and motion of the Earth about the Sun.
However, tidal heating within the Moon was neglected. Here we explore
the coupled thermal-orbital evolution of the early Earth-Moon system,
taking account of tidal heating within the Moon. Large tidal heating
in the Moon significantly changes the tidal parameters in the Moon, with
consequent early escape from the evection resonance. Insufficient
angular momentum is withdrawn from the system to be consistent with
the current configuration of the Earth-Moon system.

Mechanical cues from the extracellular microenvironment play a central role in regulating the structure, function and fate of living cells. Nevertheless, the precise nature of the mechanisms and processes underlying this crucial cellular mechanosensitivity remains a fundamental open problem. Here we provide a novel framework for addressing cellular sensitivity and response to external forces by experimentally and theoretically studying one of its most striking manifestations – cell reorientation to a uniform angle in response to cyclic stretching. We first report on extensive new experiments of adherent cell reorientation under cyclic stretching and show that they cannot be quantitatively explained by existing models. We then propose a new theory which focuses on the cell’s stored elastic energy, corresponding to a 2D anisotropic linear elastic material. The variation of this energy with the cell’s orientation is shown to drive the dissipative reorientation process. Our theory is in excellent quantitative agreement with the complete temporal reorientation dynamics of individual cells, measured over a wide range of experimental conditions, thus elucidating a basic aspect of mechanosensitivity. It, moreover offers new venues for predicting and controlling cell behavior in response to mechanical cues from the microenvironment.