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This article introduces an understudied source in the history of astronomy, the Astrostereon or the Discourse of the Falling of the Planet (1603). Written by the English astrologer Edward Gresham, this text presents, among other things, the earliest known set of predicted planetary occultations (for 1603–1604) and the use of these phenomena to defend the Copernican cosmology. We analyse those predictions and then briefly survey all known pre-telescopic observations of reported planetary occulations and the motivations for such observations. These data suggest that for early observers, the greater the difference in apparent brightness between the two occulting bodies, the greater the angular separation could be for an occultation nonetheless to be reported. An appendix seeks to explain this finding by considering several factors known from modern experimental analyses of human visual performance.
In first-order approximation, the Moon rotates uniformly in the course of a month, at the same time revolving in her orbit around the Earth with variable angular velocity, according to Kepler's second law. Although the periods of both motions are synchronized, changes in orbital speed cause an observer on Earth to see either the western or the eastern edge of the lunar disk, depending on the conditions. Because of this, the maximum angular shift of surface objects relative to the centre of the Moon is ±7.9 o . This is what we call optical libration in longitude. And, moreover, the Moon's rotational axis is not perpendicular to the orbital plane, but tilted away from the normal to the orbital plane by 6 o 41′, which is the reason why, in the course of the month, we have alternating views of the north and of the south pole of the Moon. This phenomenon, known as optical libration in latitude, leads to the angular shift of surface objects (±6.8 o maximum; see Figure 1), in relation to the centre of Fig. 1. A visual explanation of the phenomenon of optical libration -in longitude at the top, and in latitude at the bottom.JHA, xlii (2011)
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