Abstract
A short method is given for computing components of differential libration, so that the geocentric values tabulated in the Ephemeris can be converted to those for an observer anywhere, instead of computing the total values ab initio . The method makes slide-rule accuracy amply adequate, where four-figure tables were slightly inadequate before, and it also comprises appreciably fewer steps. The differential correction to the apparent position-angle of the Moon's axis is shown to be necessary, and a formula for it is given. The question of the accuracy actually needed is considered, and it is shown that changes of parallax affect the limb-contour as much as do changes of libration of several units in the last place tabulated at present, so that one might almost speak of a “parallactic libration” in addition to the other two. All three must be specified if the present tabular accuracy in the two standard ones is to be justified.
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