The American Ephemeris and Nautical Almanac

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Page 485 - ... of the sun is called Solar Time. This is the most natural and direct measure of time. But the intervals between the successive returns of the sun to the same meridian are not exactly equal, owing to the varying motion of the earth around the sun, and to the obliquity of the ecliptic.
Page 492 - Greenwich noon indicated on the left hand side of the page, and the other for the noon which is midway between that date and the date next below it. In the case of Mercury, this intermediate date is mean noon of the day immediately following ; in the case of Venus, Mars, Jupiter, and Saturn, it is mean noon of the second day following ; and in the case of Uranus and Neptune, mean noon of the fourth day following. Pages 264 — 271 contain the rectangular co-ordinates of the centre of the sun, referred...
Page 485 - ... 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 486 - It is about 3 minutes and 56 seconds shorter than the mean solar day; 365U8olar days, or a year, being divided into S66'/4 sidereal days. It is divided into 24 hours. The sidereal hours are counted from 0 to 24, commencing with the instant of the passage of the true vernal equinox over the upper meridian, and...
Page 495 - In case they would have differed, the minute which would have been numerically larger is diminished by one, and the seconds increased by sixty, so that there is always a correspondence between the two numbers. The hourly motions in right ascension and declination are given for the moment of mean noon, but may be regarded as having the same values for apparent noon. The Equation of Time for Apparent Noon is the correction to be applied to apparent time in order to obtain mean time. It is, therefore,...
Page 490 - ... stars, as they would appear to an observer at the centre of the earth. They are given for every third hour of Greenwich mean time, beginning at noon; the dates are therefore astronomical. All the distances that can be observed on the same day, are grouped together under that date; and the columns are read from left to right, across both pages of the same opening. The letter W. or E. is affixed to the name of the sun, planet or star, to indicate that it is on the west, or east side of the moon.
Page 486 - 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. The astronomical as well as the 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...
Page 486 - ... the last part of the preceding astronomical day, and the last part of the civil day corresponds to the first part of the astronomical day. Thus, January 9, 2 o'clock pm, civil time, is also January 9, 2h astronomical time; and January 9, 2 o'clock am, civil time, is January 8, 14h, astronomical time.

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