Davis Mark 3 User Manual

Mark 3 Sextant User’s Guide

00011.220, Rev. G March 2021

How to Find Your Position

with the Mark 3 Sextant

Total pages 20

Trim to 5.5 x 8.5"

Black ink only

EDITED BY ROBERT B. KLEID

Page 1 (front cover)

STANDARD

MARK 3

#011

INDEX SHADES

INDEX MIRROR

ADJUSTMENT

SCREWS

HORIZON

SHADES

EYE

PIECE

HORIZON

MIRROR

OPTIONAL PROTECTIVE CASE

Contact your local dealer or Davis Instruments to order.

R014A Sextant Case

R014B Foam Set for case

INDEX ARM

00011.220, Rev. G March 2021

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. Before becoming an accomplished navigator, however, you will need to study those aspects of navigation which are beyond the

scope of this booklet.

HOW TO READ THE VERNIER

There are two scales on the sextant. The scale on the frame is called the “arc,” while the scale on the index arm is the “vernier.” Each division of the arc equals one degree. Each division of the vernier equals two minutes (2'). To read the number of degrees, find the lines on the arc which are closest to the zero 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 the arc—letʼs say 30°. Now move the index arm very slightly to the left until the first vernier mark to the right of the zero lines up exactly with a mark on the arc. Since the marks on the vernier are 2' apart, you have actually moved the index arm 2' beyond 30°; your sextant reads 30° 02'. Now, move the index arm slightly further to the left so that the next division of the vernier comes into alignment with a division of the arc. Your sextant now reads 30° 04' (Fig. 1).

As you continue moving the index arm, successive divisions of the vernier will come into alignment with a division of the arc. When the last mark on the vernier (60') is in alignment with a division of the arc, the sextant will read 31°. In figure 2 below, the sextant reads 43° 26'.

Figure 1

Figure 2

Page 1

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.

First, adjust the index mirror for “side error” by making it perpendicular to the frame. Holding the sextant in your right hand, raise the instrument to your eye. Look at any horizontal straight edge (the sea horizon, for example, or the roof of a building al least one mile away) and move the index arm back and forth. The 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).

Mirror horizon is not aligned with

the real horizon—index arm is not

in proper position.

Mirror horizon and real horizon

form a single straight line—index

arm is properly positioned.

Figure 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 forth across the vertical line. If the index mirror is not perpendicular to the frame, the line will seem to jump to one side as the mirror

passes it. To correct this, slowly tighten or loosen the screw closest to the frame at

the back of the index mirror until the vertical line no longer appears to jump (fig. 4).

Index mirror screw

too tight.

Index mirror screw correctly adjusted.

Index mirror screw

too loose.

Figure 4

Page 2

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.

On a correctly adjusted sextant, the real and mirror horizons remain in a single line when the instrument is rocked from side to side (Fig. 6).

Figure 5

Index mirror parallel to

horizon mirror.

Figure 6

While you should know how to adjust your sextant for index error, it is not necessary to remove it entirely. It is standard practice to simply note the error and then correct oneʼs reading for this amount each time the sextant is used (as much as 6' index error is allowable). To check for index error, hold the sextant in your right hand and look at the sea horizon. By moving the index arm, line up the real and mirror horizons 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 6ʼ is subtracted. If the reading is below the zero mark, for example –6', the 6' is added (Note: for an index error of –6', the scale actually reads 54').

MEASURING THE SUN’S ALTITUDE

When looking at the sun through the sextant, be sure to use a sufficient number of shades to protect your eyes from the direct rays of the sun. Choose

the combination of index and horizon shades that gives you a clear image of the

sun without glare.

Page 3

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 sunʼs image travels in a half-arc. Now, adjust the index arm to bring the sunʼs image down to just touch the horizon (Fig. 7).

Figure 7

The sun’s image travels

in a short arc which just

touches the horizon.

Being careful not to disturb the setting, read the sunʼs altitude from the scales on

the sextant. Since all calculations in the Navigation Tables use the center of the sun or moon, this lower limb reading must be adjusted for semi-diameter correction, 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 observer is standing. Due to the height of the eye of the observer, however, the visible horizon actually falls below this theoretical plane (Fig. 8).

To correct for the height of the eye, one must apply a “dip correction.” Dip correc-

tion increases as the eye is raised further above the surface of the water (Table 1)

and must always be subtracted from the sextant reading.

Table 1

Height of Eye Correction

Feet Meters Dip

5 1.5 2' 10 3.0 3' 15 4.5 4' 25 7.5 5' 40 12.0 6'

Page 4

Figure 8

Due to the height of the eye of the observer, the

visible horizon (H) falls below the plane (P) tan-

gent to the earth at the point where the observer

is standing.

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