Commonsense Celestial Navigation by Hewitt Schlereth

(New York: Regnery Publishing, 1975), 240

My fascination while home for Christmas this year was Celestial Navigation. We took a tour of the USS Midway, including the bridge and navigation room where they showed the charts and navigation instruments. I found this book at home when we got back and it was a helpful overview of how celestial navigation works. I also read this article about Lindberg and the Weems navigation system, and re-read Longitude, which tells the story of the race between developing the lunar distance method and a seaworthy chronometer in the mid-eighteenth century. Reading these, you have to marvel at the ingenuity of the people who figured out how to map out the stars and use them for navigation and create the H-4 mechanical chronometer that lost only five seconds after 81 days at sea! You also can't help feeling grateful for all the technology we enjoy today: cheap quartz watches that receive automatic daily radio signals from an atomic clock, and GPS which you can put on your wrist for $200.

Notes


Contents


Chapter 1: Broadly Speaking

  • Celestial navigation is the application of two sciences—astronomy and mathematics (spherical trigonometry)—to the problem of determining location.
  • The geographical position (GP) is the point on earth directly underneath a celestial body at a given time. GPs are tabulated in the Nautical Almanac

Chapter 2: The Sextant

  • The sextant measures the angle between the horizon and a celestial body and allows you to compute the zenith distance between your position and the GP of that body.

$$\boxed{\text{zenith distance [nm]}=(90º - \text{sextant reading}) \cdot 60 \text{ [nm/deg]}}$$

Chapter 3: The Eternal Triangle

  • The principles of celestial navigation are based on constructing a triangle between: your assumed position, the GP of the celestial body you are observing, and the north pole.
  • The process for this is approximately:
    1. Find a known GP
    2. Determine your distance from that GP using a sextant
    3. Assume a position and construct a navigational triangle
    4. Solve the triangle for your theoretical zenith distance from the known point
    5. Compare your actual distance from the known point as measured by the sextant wiht the theoretical or computed distance
    6. Lay out the appropriate line of position relative to your assumed position based on whtether your measured distance is greater or less than computed

Chapter 4: Shoot! Don't Compute!

  • Data is tabulated in the Nautical Almanac, so your main task is to take readings with your sextant rather than making computations

Chapter 5: Celestial Navigation in Your Backyard

  • How to use a sextant, he recommends EBBCO sextants
  • “Dip”: you need to subtract an adjustment from your reading to account for how far your eyes are above sea level

Chapter 6: Time

  • Use GMT/UTC in almanacs
  • Use a radio receiver to update the time

Chapter 7: Almanacs

  • Reference the Nautical Almanac and the Air Almanac

Chapter 8: A Day with the Sun

  • Gives a method of taking two sun sights several hours apart and plotting a position from them (ref ch 3)

Chapter 9: Sun-Moon Fix

  • Instead of multiple sun positions as in the previous chapter, you can take simultaneous sights of the sun and moon and establish a fix by where the lines of position cross

Chapter 10: Celestial Navigation without Sigh Reduction Tables

  • Longitude from almanac by time of noon (96)
  • "Before the advent of chronometers and reliable time, the noon sight was celestial navigation." (99)

Chapter 11: Planets and Stars

  • Use civil twilight for sighting stars and planets when it is dark enough to see them but light enough to see the horizon (101)

Chapter 12: Celestial Navigation without a Sextant

  • If not sextant, you can take readings as usual when the sun or moon is at the horizon so the angle is 0º

Chapter 13: Celestial Navigation without a Sextant or Tables

  • For a long voyage, find a star over your destination and navigate toward that star

Chapter 14: Celestial Navigation without a Chronometer

  • Lunar distance method: measure the angle between the moon and a star or planet (requires tedious arithmetic and was dropped from the almanac in 1914)
  • This chapter presents a method from Sir Francis Chichester from his book Along teh Clipper Way

Chapter 15: Solving the Navigation Triangle Directly

  • Presents ways of solving the navigation triangle directly to escape the bulk and weight of sight reduction tables
  • Makes this book a complete celestial navigation system (Long-Term Almanac, Sin-Cos tables, sextant, watch)
  • Trade offs of methods of celestial navigation: sight reduction tables are bulky, but fast; solving by formulas and slide rule is slower but requires less; there are also calculators available (159)

Chapter 16: Tools of the Trade

  • Davis, EBBCO sextants $60
  • short wave radio to get time signals
  • “It is hard to find a quartz-crystal chronometer for less than $200.” (213)
  • The Air Almanac (215)

Topic: Navigation

Source

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Created: 2024-12-19-Thu
Updated: 2024-12-27-Fri