Page:History of the Royal Society.djvu/204



Clipses of the Moon are observ'd for two principal Ends; one Astronomical, that by comparing Observations with Calculations, the Theory of the Moon's motion may be perfected, and the Tables thereof reform'd: The other Geographical, that by comparing among themselves Observations of the same Ecliptical Phases, made in diverse Places, the difference of Meridians, or Longitudes of those places may be discover'd.

The Knowledge of the Eclipse's Quantity and Duration, the Shadows, Curvity and Inclination, &c. conduce only to the former of these Ends: The exact time of the Beginning, Middle, and End of the Eclipses, as also in total ones, the Beginning and End of total Darkness, is useful for both of them.

But because these Times considerably differ in Observations made by the bare Eye, from those with a Telescope, and because the Beginning of Eclipses and the End of total Darkness are scarce to be observed exactly, even with Glasses; (one not being able clearly to distinguish between the true shadow and Penumbra, unless one have seen, for some time before, the Line, separating them, pass along upon Rh