Mark 3 Sextant User’s Guide 00011.220, Rev. E October 2008 Total pages 20 Trim to 5.5 x 8. Black ink only How to Find Your Position with the Mark 3 Sextant STANDARD EDITED BY ROBERT B. KLEID © 2008 Davis Instruments Corp.
INDEX MIRROR ADJUSTMENT SCREWS HORIZON SHADES INDEX SHADES EYE PIECE INDEX ARM HORIZON MIRROR OPTIONAL PROTECTIVE CASE Contact your local dealer or Davis Instruments to order. R014A Sextant Case R014B Foam Set for case Mark 3 Sextant User’s Guide Product #011 © 2008 Davis Instruments Corp. All rights reserved. 00011.220, Rev.
HOW TO FIND YOUR POSITION WITH A SEXTANT This booklet has been written as an introduction to your new Davis sextant. By studying its pages, you will learn how to operate your sextant, how to find the altitude of the sun, and how to use your readings to calculate location. The meridian transit method of navigation described is both easily learned and simply applied. When you finish reading, the mystery surrounding celestial navigation and sextant use should disappear.
MARK 3 SEXTANT ADJUSTMENT Adjusting your Mark 3 Sextant is easy and should be done each time it is used. All adjustments are made with the index mirror, the large movable mirror at the pivot of the index arm (it is not necessary to adjust the small horizon mirror, as the unit construction makes it impossible to be very much in error). On a correctly adjusted sextant, the index mirror is perpendicular to the frame and becomes parallel to the horizon mirror when the sextant reads zero.
Finally, remove the index error. Set the sextant at zero and look at the horizon. With the sextant still held to your eye, turn the screw that is furthest from the frame at the back of the index mirror until the two horizons move together and form one straight line. The index mirror is now parallel to the horizon mirror (Fig. 5). Index mirror not parallel to horizon mirror. Figure 5 Index mirror parallel to horizon mirror.
To measure the sun’s altitude, stand facing the sun with the sextant in your right hand. With your left hand on the index arm, look through the eye piece at the horizon and move the index arm until the sun is visible through the two mirrors and index shades. Rock the entire sextant from side to side so that the sun’s image travels in a half-arc. Now, adjust the index arm to bring the sun’s image down to just touch the horizon (Fig. 7).
LATITUDE, LONGITUDE, and the NAUTICAL MILE A great circle 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 circles, 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 points (Fig. 9).
DECLINATION Every star and planet, including the sun, has a ground position, i.e., the spot on the earth directly beneath it. Standing at the sun’s ground position (G.P.), you would have to look straight up to see the sun; if you were to measure its altitude with a sextant, you would find the altitude was 90°. From the earth, the sun seems to move across the sky in an arc from east to west.
FINDING LOCAL NOON & THE SUN’S ALTITUDE AT MERIDIAN PASSAGE A meridian is an imaginary line drawn on the earth’s surface from pole to pole; a local meridian is one which passes through the position of an observer. When the sun crosses the local meridian, it is at its highest point. It is said to be in meridian passage and the time is local noon.
meridian at 11:43:30 (exactly half the time between 11:23:30 and 12:03:30). Next, you find the Greenwich Mean Time (GMT) of your local noon by listening to the radio time signal, correcting any error your watch may have had. In this example, you tune in the time signal and find that GMT is now 22:10:00. Your watch reads 12:10:00, so it has no error. You know that your local noon occurred at GMT 21:43:30 (26 minutes 30 seconds ago).
FINDING LATITUDE The altitude of the sun at local noon may also be used to calculate latitude. First, the measured altitude must be corrected for index error, height of eye, refraction, and semi-diameter. Refraction correction is negligible for altitudes above 25°, while the semi-diameter correction averages +0° 16' (semi-diameter correction adjusts the sextant reading from an observation of the lower limb of the sun to one of the center of the sun; 16' equals one-half the sun’s diameter).
Presentations shown here are commonly used by navigators to help insure the accuracy of their calculations: Figure 13 Figure 14 Longitude Diagram Latitude Diagram (view of earth looking at the South Pole) (view of earth looking at the Equator) Figure 14 Position plot on a chart. SYSTEMS OF CELESTIAL NAVIGATION The method described above for calculating your position is the oldest method used since the introductions of the chronometer. Please note the following: 1.
A generalized system of position determination which enables you to use observation of the sun and other celestial bodies made at times other than noon requires knowledge of the 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 studies and extensively used by serious navigators throughout the world.
4. Line the two suns up by continuing to move the index arm. For a lower limb observation, bring the bottom of the mirror image into coincidence with the top of the image on the liquid. 5. After the observation has been made, apply the index correction. 6. Halve the remaining angle and apply all other corrections (except for Dip or height of eye correction, which is not applicable) to find the altitude of the sun.
Figure 17 Since the sextant does not have a compass, you don’t need to worry about variation or deviation. However, you must use at least three lines of position. THE SEXTANT AS A HELIOGRAPH You can use the sextant mirror to flash the sun’s rays several miles to attract attention, or to signal another person who is too far away for your voice to reach. If you know Morse code, you could even send a message. 1. Hold the sextant so that the index mirror (the larger of the two mirrors) is just below the eye.
STUDENT NAVIGATION TABLES The tables on the following pages give the approximate declination and equation of time of the sun. Latitude calculated with these values will be accurate to about ±15'. The tables are thus intended for study purposes only, although they may be used for emergency navigation. NAUTICAL ALMANACS, CHARTS and TABLES The Nautical Almanac is published yearly by the U.S. Naval Observatory and the H.M. Nautical Almanac Office, Royal Greenwich Observatory, U.K.
REFERENCE: Approximate Declination & Equation of Time Page 15
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REPLACEMENT PARTS OTHER DAVIS SEXTANTS Contact your local dealer or Davis Instruments to order. Specify that your sextant is the #011 Mark 3. In addition to the Mark 3, Davis offers two models of master marine sextants. R011A Index Shade Assembly R011B 4 Springs, 2 Screws, 3 Nuts R011C Index & Horizon Mirrors w. Springs, Screws, Nuts R011X Factory overhaul ($10.