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Mark 3 Sextant User’s Guide

00011.220, Rev. E

October 2008

Total pages 20

Trim to 5.5 x 8.

Black ink only

Page 1 (front cover)

EDITED BY ROBERT B. KLEID

© 2008 Davis Instruments Corp.

How to Find Your Position

with the Mark 3 Sextant

STANDARD

MARK 3

#011

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Summary of Contents for Mark 3

Page 1: ...Total p Trim to Black EDITED BY ROBERT B KLEID 2008 Davis Instruments Corp How to Find Your Position with the Mark 3 Sextant STANDARD MARK 3 011 ...

Page 2: ...p All rights reserved 00011 220 Rev E October 2008 INDEX SHADES HORIZON MIRROR INDEX ARM INDEX MIRROR ADJUSTMENT SCREWS HORIZON SHADES EYE PIECE OPTIONAL PROTECTIVE CASE Contact your local dealer or Davis Instruments to order R014A Sextant Case R014B Foam Set for case ...

Page 3: ...o mark on the vernier The zero mark is usually somewhere between two lines The correct arc reading is always that of the lower value i e the line to the right of the zero mark To read fractions of a degree find the division of the vernier which is in alignment with a division of the arc To get a clear picture of how this works set the zero on the vernier exactly beneath any whole degree mark on th...

Page 4: ...he real horizon will remain still while the mirror horizon will appear only when the scales read close to zero Line up the mirror horizon and the real horizon so that both appear as a single straight line fig 3 Now do a vertical adjustment Without changing the setting look through the sextant at any vertical line a flag pole for example or the edge of a building and swing the instrument back and f...

Page 5: ...s so that both appear as a single straight line Now look at the scale If it reads zero there is no index error If the scale reads anything but zero there is an index error which must be added to or subtracted from each reading For example if the scale reads 6 when the horizons are aligned the 6 is sub tracted If the reading is below the zero mark for example 6 the 6 is added Note for an index erro...

Page 6: ...on shown later HEIGHT OF EYE When measuring the altitude of the sun we want to measure the angle formed by a ray from the sun and a plane tangent to the earth at the point where the observ er is standing Due to the height of the eye of the observer however the visible horizon actually falls below this theoretical plane Fig 8 Figure 7 Figure 8 The sun s image travels in a short arc which just touch...

Page 7: ...e is a circle on the surface of the earth the plane of which passes through the center of the earth A small circle is a circle whose plane does NOT pass through the center of the earth The equator and the meridians are great cir cles while parallels of latitude are small circles which become progressively smaller as the distance form the equator increases At the poles 90 N or S they are but single...

Page 8: ...like manner each star has a ground position and a declination The decli nation of Polaris is 89 05 N it is nearly directly above the North Pole In the northern hemisphere you can find your approximate position by taking a sight on Polaris The reading will vary depending upon the time of night but will never be more than 55 miles off This is a useful check each evening the altitude of Polaris will ...

Page 9: ...alf way between the times of the two sights Record the local time and the sextant reading when the sun was at the highest point These two readings will serve to locate your position The time is used to determine longitude and the sextant reading to determine latitude AN EXAMPLE OF A COMPLETE SIGHT Let us assume for this example that your ship is sailing from San Francisco to Hawaii and you have be...

Page 10: ... always due south or 180 true at noon by the clock The difference in time between the true sun and the mean sun is call the equation of time The equation of time for any given day may be found in a Nautical Almanac its approximate value may be found in the student tables at the end of this booklet See figure 13 for a diagram based on this example meridian at 11 43 30 exactly half the time between ...

Page 11: ...un 16 equals one half the sun s diameter After the cor rections are made determine the declination of the sun from the Nautical Almanac or from the approximate declination values at the end of this booklet Finally calculate latitude by combining the altitude of the sun at local noon with the declination of the sun from the navigation tables Assuming you are north of the sun the following formula i...

Page 12: ...since the introductions of the chronometer Please note the following 1 Latitude may be determined at noon if you know the corrected altitude of the sun and its declination You need not know the time The accuracy of your cal culation is limited only by the accuracy of measurement of the sun s altitude and by the accuracy of the declination tables 2 To determine longitude you must know both the time...

Page 13: ...he navigation triangle circles of equal altitude assumed position and associated navigation tables such as the Nautical Almanac and Sight Reduction Tables These systems of celestial navigation are thoroughly stud ies and extensively used by serious navigators throughout the world Sets of work forms for the Sight Reduction Tables are used by nearly all naviga tors to help prevent errors and omissio...

Page 14: ...the three features The point of intersection of the three angles is your position Fig 17 Note Since the sextant reading made with an artificial horizon must be halved the maximum altitude that you can observe with the artificial horizon is equal to one half the maximum arc graduation on your sextant There may be several hours around noon during which the sun is too high to take a sextant reading w...

Page 15: ...t below the eye 2 With your other arm extended and the thumb held upright look at the person you wish to signal 3 Hold your thumb to a position just below the person so that your eye with the mirror under it your thumb and the person are in a straight line Fig 18 4 Using the mirror flash the sun on your thumb The sun flashes simultaneously on the distant person Figure 17 Figure 18 Since the sextan...

Page 16: ...on are available in many maine stores and larger bookstores Most often recommended are The American Practical Navigator U S Navy Hydrographic Office and Dutton s Navigation and Plotting U S Naval Institute Other popular and regularly available titles are published by Cornell Maritime Press and the International Marine Publishing Co Videos are available from larger marine dealers video mail order h...

Page 17: ...Page 15 REFERENCE Approximate Declination Equation of Time ...

Page 18: ...Page 16 ...

Page 19: ...Page 17 ...

Page 20: ...4 0M V V M I4 0M V4 0Y I4 0M I4 0M O K e O2 6 K O 26K O2 6 2 6 26K O K O2 6Kg O KgO26KeO26K O KO Ke O K O K O KO2 g g O K O2 6K O26KO2 6KO2 6KO2 6KO KO2 6 2 g g O2 6K e O2 6K O26K e O2 6K O26KO26K O KfO26 2 f J g f W2 g 0M I 0M f f g 5 3 f 7 g 0M I 0MhfI4 M g h M g Mg I MhN M g h f N g H N f g 0M h I4 0Mg M g M I4 0MhN 0M I4 Hh h Hh H hN H h Hh 5 h Lg g 5 3 3 f g 0M I he Yg J H I4 Mhe M I h 5hJ 5 ...

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