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Updated: May 3, 2025
The astronomical part of our ephemeris, therefore, gives the positions of the principal fixed stars, the sun, moon, and all the larger planets at the moment of transit over our own meridian. The third class of data in the ephemeris comprises phenomena to be predicted and observed. Such are eclipses of the sun and moon, occultations of fixed stars by the moon, and eclipses of Jupiter's satellites.
The entire circuit of the heavens along the equator is divided into twenty-four hours of right ascension, each hour covering 15° of space. With these explanations we may proceed to find a planet by the aid of the Nautical Almanac and our charts. I take, for example, the ephemeris for the year 1901, and I look under the heading "Jupiter" on page 239, for the month of July.
Since taking charge of the American Ephemeris I have endeavored to ascertain what nautical almanacs are actually used by the principal maritime nations of Europe. I have been able to obtain none except those above mentioned. As a general rule I think the British Nautical Almanac is used by all the northern nations, as already indicated.
Greenwich noon, being that necessarily used by the navigator, is adopted as the standard, but we must not conclude that the ephemeris for Greenwich noon is referred to the meridian of Greenwich in the sense that we refer a longitude to that meridian.
The fact is that what started more than a century ago as a nautical almanac has since grown into an astronomical ephemeris for the publication of everything pertaining to times, seasons, eclipses, and the motions of the heavenly bodies. It is the work in which astronomical observations made in all the great observatories of the world are ultimately utilized for scientific and public purposes.
Whether the smaller thoroughfares took the same lines can be determined only by excavation. It would be a gentle guess to think so. Tafrali, Topographie de Thess. pp. 121 foll. and plan. E. Sachau, Reise in Syrien , p. 76; Mommsen, Ephemeris epigr. iv, p. 514, and Mon. Ancyr. Further south, on the edge of the Haurân, stood the town of Gerasa.
Gauss published an approximate ephemeris of probable positions when the planet should emerge from the sun's light. The mean distance from the sun was found to be 2.767, agreeing with the 2.8 given by Bode's law. Its orbit was found to be inclined over 10 degrees to the ecliptic, and its diameter was only 161 miles.
Since it is noon at some point of the earth all the time, it follows that such an ephemeris will always be referred to noon at some meridian. What meridian this shall be is purely a practical question, to be determined by convenience and custom.
The mechanism is as clear, as unquestionable, as absolutely settled and universally accepted, as the order of movement of the heavenly bodies, which we compute backward to the days of the observatories on the plains of Shinar, and on the faith of which we regulate the movements of war and trade by the predictions of our ephemeris. The mechanism, and that is all.
The men who have done it are therefore in intellect the select few of the human race, an aristocracy ranking above all others in the scale of being. The astronomical ephemeris is the last practical outcome of their productive genius.
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