Ball, Robert S. (Robert Stawell), Sir, 1840-1913 · Cambridge, University press · leaf 254
of time must vanish at least twice between the vernal equinox and the summer solstice. From S to J. both t and tf are positive, and therefore E is continuously positive from the summer solstice to the apogee. But from A to :0z t' is negative, and as t is zero at d^ and t' is still negative, E must… … Pig. 67. From £h to W both t and t' are continuously negative, and thus the equation of time cannot vanish between these points of the orbit. At P we find E has become positive again, and there- fore it must vanish at least once between W and P- We thus learn that the equation of time vanishes at…
Spherical astronomy
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