Physiography for Beginners

Physiography for Beginners

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1898 edition. Excerpt: ...of Greenwich we add to the local time one hour for every 15 of longitude or four minutes for every degree; while, if we are to the east we subtract the same amount. Similarly, if we already have Greenwich time, and we wish to know the local time we can, being aware of the longitude, subtract the same amount for places west of Greenwich and add it for places of east longitude. But these facts are most useful in enabling navigators to determine their longitude. We have seen that clocks keep mean solar time. If, then, the mariner has with him an accurate chronometer keeping Greenwich time, that is, which records twelve o'clock when the sun is on the meridian of Greenwich, he can by noting the time of southing of the sun, which happens at twelve o'clock noon local time, tell the difference between local and Greenwich time. If the local time is slow compared with Greenwich time his longitude is west, and equal to a number of degrees obtained by reckoning i for every four minutes it is slow. If the local time is fast he is in east longitude, and on the meridian which is found by dividing the amount he is fast in the same way. Mean Solar Day.--Equation of Time.--The student has learnt that the length of days measured by the sundial varies in length throughout the year, and will have no difficulty in understanding that no single one of these days will do for a convenient standard of time. But if the lengths of all the days in the year are added together, or the length of a year measured by the sun be divided by the number of days in the year, we obtain an interval of time which is always the same. Such a day, which is of course an imaginary one, is called a mean solar day. Sometimes the mean solar day will be longer than the day measured by the sundial, more

Product details

  • Paperback | 112 pages
  • 189 x 246 x 6mm | 213g
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236605578
  • 9781236605573