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Ephemerides - BALMER Print E-mail


Since antiquity the ephemerides were the predicting tables applied to the visibility of planets and stars. They were veritable scientific models of such phenomena. Based upon cyclical hypotheses, they showed first the fundamental harmonic, and after, with increasingly accuracy in the agreement with observed data, the combination of more harmonics. Ptolemy collected data from Chaldean astronomy, with the knowledge of the eclipses and planetary stationary points and synodical periods, realizing the great synthesis of History of Astronomy through the Almagest (150 AD). In the Almagest the harmonics for solar, and planetary motions are two, while for the Moon are three. The motions of the fixed stars was a combination of two rotations, the diurnal one and the precession. To find more components it is necessary to wait until Tycho Brahe Galileo and Newton for the Moon motion; ancient Arabic astronomers and James Bradley for the Earth's axis motion and stellar aberration. At the present time in the ephemerides are included all the gravitational and gravitodynamical effects predicted by the General Relativity theory, along with non-gravitational corrections for comets and asteroids, available after the observations, more and more accurate. Also variable stars have their own ephemerides, as their physical models become reliable with the observations. "Saving the Phenomena" was the Ptolemy's motto, and it is valid also in the modern astrophysics: a model is good until newer and better observations falsify it. Hevelius proposed an epicycle to explain Mira Ceti variations of luminosity in the Historiola Mirae of 1662, while intrinsically wrong the model works slightly well to simulate the lightcurve as a Fourier first harmonic. Nowadays there are still variable stars classified as irregular, which have multiple harmonics and a relatively short timespan of available observations. It is the noteworthy case of Antares. A section dedicated to some selected ephemerides reflects the study of astrophysical models along History of Astronomy, that we conducted since 25 years. There is another application of the ephemerides, which is related with some special instruments devoted to the solar astrometry: the pinhole giant meridian lines. They were developed in Italy at the turn of the modern science revolution, and highlighted by J. Heilbron (1934-2023) with the famous book the Sun in the Church. The ephemerides for such instruments allow to use them for their original purpose, but also permit to enlighten the theoretical and technological advancement at the time of realization of each of them.
For these reasons we have also inaugurated a Journal, called BALMER, where the contribution on these studies are collected. The Astronomical Bulletin for the Meridian Lines and Ephemerides of Rome, in the Italian order of the words Bollettino Astronomico per le Linee Meridiane ed Effemeridi di Roma = BALMER, acronym which also recalls Johann Jacob Balmer (1825-1898) known by all astrophysicists for the Hydrogen lines.

Costantino Sigismondi




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Fig. 1: Padua, Palazzo della Ragione, pinhole of the solar meridian line (1761). Fig. 2: Rome, Vatican, beginning of meridian line (1817) at the Capricorn's marble disk.
 
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