The American Ephemeris and Nautical Almanac

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Page 467 - ... time. Mean Solar Time, which is perfectly equable in its increase, is measured by the motion of this mean sun. The clocks in ordinary use and the chronometers used by navigators are regulated to mean solar time. True, or Apparent Solar Time is measured by the motion of the real sun. The difference between apparent and mean time is called the Equation of Time.
Page 467 - A Solar Day is the interval of time between two successive transits of the sun over the same meridian ; and the hour.angle of the sun is called Solar Time.
Page 468 - PM The Astronomical Day commences at noon on the civil day of the same date. It also comprises twenty-four hours, but they are reckoned from 0 to 24, and from the noon of one day to that of the next following.
Page 467 - Sidereal Time. — Sidereal time is measured by the daily motion of the stars; or, as it is used by astronomers, by the daily motion of that point in the equator from which the true right ascension of the stars is counted. This point is the vernal equinox, and its hour-angle is called Sidereal Time.
Page 467 - To avoid the irregularity which would arise from using the true sun as the measure of time, a fictitious sun, called the Mean Sun, is supposed to move in the equator with a uniform velocity. This mean sun is supposed to keep, on the average, as near the real sun as is consistent with perfect uniformity of motion; it is sometimes in advance of it, and sometimes behind it, the greatest deviation being about 16 minutes of time.
Page 468 - ... with the instant of the passage of the true vernal equinox over the upper meridian, and ending with its return to the same meridian. About March...
Page 468 - The rule, then, for the transfor mation of civil time into astronomical time is this: @ If the civil time is marked AM, take one from the day and add twelve to the hours, and the result is the astronomical time wanted...
Page 467 - ... the ephemerides of the planets Mercury, Venus, Mars, Jupiter, and Saturn, is designed for the special use of navigators. The remainder contains the ephemerides of Uranus and Neptune, the heliocentric co-ordinates of the seven major planets, the rectangular equatorial co-ordinates of the sun, the moon's longitude and latitude, data for the libration of the moon, the obliquity of the ecliptic, the equation of the equinoxes, etc.
Page 468 - Tile astronomical as well as time civil time may be either apparent or mean, according as it is reckoned from apparent noon or from mean noon. The civil day begins twelve hours before the astronomical day ; therefore the first period of the civil day answers to the last part of the preceding astronomical day, and the last period of' the civil day corresponds to the first part of the same astronomical day.
Page 489 - Since we must know the value of т, approximately, before we can take £'i from the table, this equation can be solved only by successive approximations. The approximations converge so rapidly as to offer no difficulty. It will be best to begin by comparing values of r for the two extremes of x', namely, x' = 0.48 and x' = 0.60, because the approximate values of т can then be interpolated for all the intermediate values of x'.

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