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Basic Principles of Celestial Navigation James A
Basic principles of celestial navigation James A. Van Allena) Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa 52242 ͑Received 16 January 2004; accepted 10 June 2004͒ Celestial navigation is a technique for determining one’s geographic position by the observation of identified stars, identified planets, the Sun, and the Moon. This subject has a multitude of refinements which, although valuable to a professional navigator, tend to obscure the basic principles. I describe these principles, give an analytical solution of the classical two-star-sight problem without any dependence on prior knowledge of position, and include several examples. Some approximations and simplifications are made in the interest of clarity. © 2004 American Association of Physics Teachers. ͓DOI: 10.1119/1.1778391͔ I. INTRODUCTION longitude ⌳ is between 0° and 360°, although often it is convenient to take the longitude westward of the prime me- Celestial navigation is a technique for determining one’s ridian to be between 0° and Ϫ180°. The longitude of P also geographic position by the observation of identified stars, can be specified by the plane angle in the equatorial plane identified planets, the Sun, and the Moon. Its basic principles whose vertex is at O with one radial line through the point at are a combination of rudimentary astronomical knowledge 1–3 which the meridian through P intersects the equatorial plane and spherical trigonometry. and the other radial line through the point G at which the Anyone who has been on a ship that is remote from any prime meridian intersects the equatorial plane ͑see Fig. -
Celestial Navigation Tutorial
NavSoft’s CELESTIAL NAVIGATION TUTORIAL Contents Using a Sextant Altitude 2 The Concept Celestial Navigation Position Lines 3 Sight Calculations and Obtaining a Position 6 Correcting a Sextant Altitude Calculating the Bearing and Distance ABC and Sight Reduction Tables Obtaining a Position Line Combining Position Lines Corrections 10 Index Error Dip Refraction Temperature and Pressure Corrections to Refraction Semi Diameter Augmentation of the Moon’s Semi-Diameter Parallax Reduction of the Moon’s Horizontal Parallax Examples Nautical Almanac Information 14 GHA & LHA Declination Examples Simplifications and Accuracy Methods for Calculating a Position 17 Plane Sailing Mercator Sailing Celestial Navigation and Spherical Trigonometry 19 The PZX Triangle Spherical Formulae Napier’s Rules The Concept of Using a Sextant Altitude Using the altitude of a celestial body is similar to using the altitude of a lighthouse or similar object of known height, to obtain a distance. One object or body provides a distance but the observer can be anywhere on a circle of that radius away from the object. At least two distances/ circles are necessary for a position. (Three avoids ambiguity.) In practice, only that part of the circle near an assumed position would be drawn. Using a Sextant for Celestial Navigation After a few corrections, a sextant gives the true distance of a body if measured on an imaginary sphere surrounding the earth. Using a Nautical Almanac to find the position of the body, the body’s position could be plotted on an appropriate chart and then a circle of the correct radius drawn around it. In practice the circles are usually thousands of miles in radius therefore distances are calculated and compared with an estimate. -
Printable Celestial Navigation Work Forms
S T A R P A T H ® S c h o o l o f N a v i g a t i o n PRINTABLE CELESTIAL NAVIGATION WORK FORMS For detailed instructions and numerical examples, see the companion booklet listed below. FORM 104 — All bodies, using Pub 249 or Pub 229 FORM 106 — All Bodies, Using NAO Tables FORM 108 — All Bodies, Almanac, and NAO Tables FORM 109 — Solar Index Correction FORM 107 — Latitude at LAN FORM 110 — Latitude by Polaris FORM 117 — Lat, Lon at LAN plus Polaris FORM 111 — Pub 249, Vol. 1 Selected Stars Other Starpath publications on Celestial Navigation Celestial Navigation Starpath Celestial Navigation Work Forms Hawaii by Sextant How to Use Plastic Sextants The Star Finder Book GPS Backup with a Mark 3 Sextant Emergency Navigation Stark Tables for Clearing the Lunar Distance Long Term Almanac 2000 to 2050 Celestial Navigation Work Form Form 104, All Sights, Pub. 249 or Pub. 229 WT h m s date body Hs ° ´ WE DR log index corr. 1 +S -F Lat + off - on ZD DR HE DIP +W -E Lon ft - UTC h m s UTC date / LOP label Ha ° ´ GHA v Dec d HP ° ´ moon ° ´ + 2 hr. planets hr - moon GHA + d additional ° ´ + ´ altitude corr. m.s. corr. - moon, mars, venus 3 SHA + stars Dec Dec altitude corr. or ° ´ or ° ´ all sights v corr. moon, planets min GHA upper limb moon ° ´ tens d subtract 30’ d upper Ho units d ° ´ a-Lon ° ´ d lower -W+E dsd dsd T LHA corr. + Hc 00´ W / 60´ E ° d. -
6.- Methods for Latitude and Longitude Measurement
Chapter 6 Copyright © 1997-2004 Henning Umland All Rights Reserved Methods for Latitude and Longitude Measurement Latitude by Polaris The observed altitude of a star being vertically above the geographic north pole would be numerically equal to the latitude of the observer ( Fig. 6-1 ). This is nearly the case with the pole star (Polaris). However, since there is a measurable angular distance between Polaris and the polar axis of the earth (presently ca. 1°), the altitude of Polaris is a function of the local hour angle. The altitude of Polaris is also affected by nutation. To obtain the accurate latitude, several corrections have to be applied: = − ° + + + Lat Ho 1 a0 a1 a2 The corrections a0, a1, and a2 depend on LHA Aries , the observer's estimated latitude, and the number of the month. They are given in the Polaris Tables of the Nautical Almanac [12]. To extract the data, the observer has to know his approximate position and the approximate time. When using a computer almanac instead of the N. A., we can calculate Lat with the following simple procedure. Lat E is our estimated latitude, Dec is the declination of Polaris, and t is the meridian angle of Polaris (calculated from GHA and our estimated longitude). Hc is the computed altitude, Ho is the observed altitude (see chapter 4). = ( ⋅ + ⋅ ⋅ ) Hc arcsin sin Lat E sin Dec cos Lat E cos Dec cos t ∆ H = Ho − Hc Adding the altitude difference, ∆H, to the estimated latitude, we obtain the improved latitude: ≈ + ∆ Lat Lat E H The error of Lat is smaller than 0.1' when Lat E is smaller than 70° and when the error of Lat E is smaller than 2°, provided the exact longitude is known. -
Celestial Navigation At
Celestial Navigation at Sea Agenda • Moments in History • LOP (Bearing “Line of Position”) -- in piloting and celestial navigation • DR Navigation: Cornerstone of Navigation at Sea • Ocean Navigation: Combining DR Navigation with a fix of celestial body • Tools of the Celestial Navigator (a Selection, including Sextant) • Sextant Basics • Celestial Geometry • Time Categories and Time Zones (West and East) • From Measured Altitude Angles (the Sun) to LOP • Plotting a Sun Fix • Landfall Strategies: From NGA-Ocean Plotting Sheet to Coastal Chart Disclaimer! M0MENTS IN HISTORY 1731 John Hadley (English) and Thomas Godfrey (Am. Colonies) invent the Sextant 1736 John Harrison (English) invents the Marine Chronometer. Longitude can now be calculated (Time/Speed/Distance) 1766 First Nautical Almanac by Nevil Maskelyne (English) 1830 U.S. Naval Observatory founded (Nautical Almanac) An Ancient Practice, again Alive Today! Celestial Navigation Today • To no-one’s surprise, for most boaters today, navigation = electronics to navigate. • The Navy has long relied on it’s GPS-based Voyage Management System. (GPS had first been developed as a U.S. military “tool”.) • If celestial navigation comes to mind, it may bring up romantic notions or longing: Sailing or navigating “by the stars” • Yet, some study, teach and practice Celestial Navigation to keep the skill alive—and, once again, to keep our nation safe Celestial Navigation comes up in literature and film to this day: • Master and Commander with Russell Crowe and Paul Bettany. Film based on: • The “Aubrey and Maturin” novels by Patrick O’Brian • Horatio Hornblower novels by C. S. Forester • The Horatio Hornblower TV series, etc. • Airborne by William F. -
Lunar Distances Final
A (NOT SO) BRIEF HISTORY OF LUNAR DISTANCES: LUNAR LONGITUDE DETERMINATION AT SEA BEFORE THE CHRONOMETER Richard de Grijs Department of Physics and Astronomy, Macquarie University, Balaclava Road, Sydney, NSW 2109, Australia Email: [email protected] Abstract: Longitude determination at sea gained increasing commercial importance in the late Middle Ages, spawned by a commensurate increase in long-distance merchant shipping activity. Prior to the successful development of an accurate marine timepiece in the late-eighteenth century, marine navigators relied predominantly on the Moon for their time and longitude determinations. Lunar eclipses had been used for relative position determinations since Antiquity, but their rare occurrences precludes their routine use as reliable way markers. Measuring lunar distances, using the projected positions on the sky of the Moon and bright reference objects—the Sun or one or more bright stars—became the method of choice. It gained in profile and importance through the British Board of Longitude’s endorsement in 1765 of the establishment of a Nautical Almanac. Numerous ‘projectors’ jumped onto the bandwagon, leading to a proliferation of lunar ephemeris tables. Chronometers became both more affordable and more commonplace by the mid-nineteenth century, signaling the beginning of the end for the lunar distance method as a means to determine one’s longitude at sea. Keywords: lunar eclipses, lunar distance method, longitude determination, almanacs, ephemeris tables 1 THE MOON AS A RELIABLE GUIDE FOR NAVIGATION As European nations increasingly ventured beyond their home waters from the late Middle Ages onwards, developing the means to determine one’s position at sea, out of view of familiar shorelines, became an increasingly pressing problem. -
Celestial Navigation Practical Theory and Application of Principles
Celestial Navigation Practical Theory and Application of Principles By Ron Davidson 1 Contents Preface .................................................................................................................................................................................. 3 The Essence of Celestial Navigation ...................................................................................................................................... 4 Altitudes and Co-Altitudes .................................................................................................................................................... 6 The Concepts at Work ........................................................................................................................................................ 12 A Bit of History .................................................................................................................................................................... 12 The Mariner’s Angle ........................................................................................................................................................ 13 The Equal-Altitude Line of Position (Circle of Position) ................................................................................................... 14 Using the Nautical Almanac ............................................................................................................................................ 15 The Limitations of Mechanical Methods ........................................................................................................................ -
Concise Sight Reductin Tables From
11-36 Starpath Celestial Navigation Course Special Topics IN DEPTH... 11-37 11.11 Sight Reduction with the NAO Tables money and complexity in the long run by not having to bother with various sources of tables. With this in Starting in 1989, there was a significant change in the Location on table pages N S Declination mind, we have developed a work form that makes the Top Sides available tables for celestial navigation. As we have same + D° D' use of these tables considerably easier than just following sign B = sign Z learned so far, the tables required are an almanac Latitude N S LHA contrary - 1 and a set of sight reduction tables. In the past sight the instructions given in the almanac. With the use + if LHA = 0 to 90 SR Table A° A' + B° B' + Z1 - if LHA = 91 to 269 of our workform, the NAO tables do not take much 1 ------------------> - - reduction (SR) tables were usually chosen from Pub + if LHA = 270 to 360 249 (most popular with yachtsmen) and Pub 229 longer than Pub 249 does for this step of the work. F° F' Naturally, the first few times go slowly, but after a few which is required on USCG license exams. The latter bar top means rounded examples it becomes automatic and easy. The form value 30' or 0.5° rounds up have more precision, but this extra precision would guides you through the steps. + if F = 0 to 90 rarely affect the final accuracy of a celestial fix. Pub A F SR Table H° H' P° P' + Z2 2 ------------------> - - if F = 90 to 180 229 is much heavier, more expensive, and slightly We have included here a few of the earlier examples, more difficult to use. -
Chapter 19 Sight Reduction
CHAPTER 19 SIGHT REDUCTION BASIC PROCEDURES 1900. Computer Sight Reduction 1901. Tabular Sight Reduction The purely mathematical process of sight reduction is The process of deriving from celestial observations the an ideal candidate for computerization, and a number of information needed for establishing a line of position, different hand-held calculators, apps, and computer LOP, is called sight reduction. The observation itself con- programs have been developed to relieve the tedium of sists of measuring the altitude of the celestial body above working out sights by tabular or mathematical methods. the visible horizon and noting the time. The civilian navigator can choose from a wide variety of This chapter concentrates on sight reduction using the hand-held calculators and computer programs that require Nautical Almanac and Pub. No. 229: Sight Reduction Ta- only the entry of the DR position, measured altitude of the bles for Marine Navigation. Pub 229 is available on the body, and the time of observation. Even knowing the name NGA website. The method described here is one of many of the body is unnecessary because the computer can methods of reducing a sight. Use of the Nautical Almanac identify it based on the entered data. Calculators, apps, and and Pub. 229 provide the most precise sight reduction prac- computers can provide more accurate solutions than tabular tical, 0.'1 (or about 180 meters). and mathematical methods because they can be based on The Nautical Almanac contains a set of concise sight precise analytical computations rather than rounded values reduction tables and instruction on their use. It also contains inherent in tabular data. -
The Motion of the Observer in Celestial Navigation
The Motion of the Observer in Celestial Navigation George H. Kaplan U.S. Naval Observatory Abstract Conventional approaches to celestial navigation are based on the geometry of a sta- tionary observer. Any motion of the observer during the time observations are taken must be compensated for before a fix can be determined. Several methods have been developed that ac- count for the observer's motion and allow a fix to be determined. These methods are summarized and reviewed. Key Words celestial navigation, celestial fix, motion of observer Introduction The object of celestial navigation is the determination of the latitude and longitude of a vessel at a specific time, through the use of observations of the altitudes of celestial bodies. Each observation defines a circle of position on the surface of the Earth, and the small segment of the circle that passes near the observer's estimated position is represented as a line of position (LOP). A position fix is located at the intersection of two or more LOPs. This construction works for a fixed observer or simultaneous observations. However, if the observer is moving, the LOPs from two consecutive observations do not necessarily intersect at a point corresponding to the observer's position at any time; if three or more observations are involved, there may be no common intersection. Since celestial navigation normally involves a single observer on a moving ship, something has to be done to account for the change in the observer's position during the time required to take a series of observations. This report reviews the methods used to deal with the observer's motion. -
Navigation: the Mariner's Quadrant
Navigation: The Mariner's Quadrant The quadrant is a very simple tool that allows the user to determine his or her latitude by measuring the altitude of a heavenly body. When used in celestial navigation or astronomy, altitude means the angle of elevation between the horizon and celestial bodies like the sun, planets, moon, or stars. The quadrant takes it name from its shape, which is a quarter of a circle. Invented by the Greeks around 240 B.C., several different types of quadrants have been used over the past 2500 years. Early quadrants used a 90 degree arc and a string Quadrants were often designed for bob to determine the angle of elevation to the sun, Polaris, and other celestial bodies. a specific use. Artillery officers used a simplified quadrant known as a gunner’s quadrant for aiming their guns and measuring distance. Astronomers used a far more complicated version of the quadrant that was engraved with geometrical and square lines showing the sun's path through the signs of the zodiac. Although average seamen would not have had much use for the complicated quadrants used by astronomers, they found that a simplified version was very useful for determining their latitude at sea. Cut from a piece of metal plate or wood, the mariner’s quadrant had a radius of about 6-8 inches. It had a pair of rectangular plates with pinhole sights on one of the straight edges for sighting. The only scale needed was the degree marks on the curved edge. It was light and easy to handle. -
Chapter 16 Instruments for Celestial Navigation
CHAPTER 16 INSTRUMENTS FOR CELESTIAL NAVIGATION THE MARINE SEXTANT 1600. Description and Use In Figure 1601, AB is a ray of light from a celestial body. The index mirror of the sextant is at B, the horizon glass at C, The marine sextant measures the angle between two and the eye of the observer at D. Construction lines EF and points by bringing the direct image from one point and a CF are perpendicular to the index mirror and horizon glass, double-reflected image from the other into coincidence. Its respectively. Lines BG and CG are parallel to these mirrors. principal use is to measure the altitudes of celestial bodies Therefore, angles BFC and BGC are equal because their above the visible sea horizon. It may also be used to measure sides are mutually perpendicular. Angle BGC is the vertical angles to find the range from an object of known inclination of the two reflecting surfaces. The ray of light AB height. Sometimes it is turned on its side and used for is reflected at mirror B, proceeds to mirror C, where it is measuring the angular distance between two terrestrial again reflected, and then continues on to the eye of the objects. observer at D. Since the angle of reflection is equal to the angle of incidence, A marine sextant can measure angles up to approxi- ABE = EBC, and ABC = 2EBC. ° mately 120 . Originally, the term “sextant” was applied to BCF = FCD, and BCD = 2BCF. the navigator’s double-reflecting, altitude-measuring ° instrument only if its arc was 60 in length, or 1/6 of a Since an exterior angle of a triangle equals the sum of ° ° circle, permitting measurement of angles from 0 to 120 .