The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured.
Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.
Modern ephemerides are often computed electronically from mathematical models of the motion of astronomical objects and the Earth.
This second view, in the tradition of Gottfried Leibniz Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities.
Nevertheless, there are secular phenomena which cannot adequately be considered by ephemerides.
The greatest uncertainties in the positions of planets are caused by the perturbations of numerous asteroids, most of whose masses and orbits are poorly known, rendering their effect uncertain.
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