Longitude of perihelion; climatic precession কী?

The longitude of perihelion $\varpi$ is the angle, measured in the plane of the orbit from the direction of the vernal equinox, between that direction and the direction of the perihelion seen from the Sun; it is counted from the moving equinox, so it changes with both the wobble of the axis and the turning of the orbit. The climatic precession is the quantity $e\sin\varpi$, also called the precession index.