If possible, the sextant must be brought to the observation point 20-30 minutes before beginning the observations, so that it will assume the temperature of the external atmosphere. Examine the sextant and be sure it is in good working order; clean the gear rack and tangent screw with a brush. Adjust the telescope and focus it with the eye.

Checking the parallelism of the telescope axis and the plane of the limb (usually performed by the navigator in preparing to get underway). Mount the sextant on a fixed base, place two sights on the limb so that the straight line connecting them is to the eye parallel to the periscope axis. Observe which object (distance to it must be about 10-12 m) is precisely in range of a section of the sights. Observing through the telescope, check to see whether this body is in the center of the telescope reticule. If not, achieve this by alternately tightening and loosening the upper and lower screws of the telescope rack.

Testing the perpendicularity of the index mirror to the plane of the limb. Remove the telescope and hold the sextant so that the plane of the limb is approximately horizontal. Set the index arm at a reading of approximately 40°. Set the sights: the first at the edge of the limb at a reading of 0°, and the second at a reading of 135°. Observe in the index mirror whether the first sight is visible in the right section of the index mirror and the reflected image of the second sight is observed simultaneously (if the second sight is not visible, slightly change the position of the index arm). If the upper sections of the sights are visible, one above or below the other, they can be brought into coincidence by rotating the adjusting screw of the index mirror with a socket wrench.

Testing the perpendicularity of the horizon glass to the plane of the limb. Adjust the periscope, set the index arm at a reading close to 0°, and observe a celestial body (sun, moon or star), aligning the twice-reflected image with the direct image by rotating the micrometer drum (if the sun is observed, light filters must be used). If, in rotating the index arm, the twice-reflected image of the celestial body does not completely coincide with the direct image, passing somewhat to the side of it, then rotate the adjusting screw of the horizon mirror using a socket wrench to match the images.

Determining index correction i. This may be accomplished from the sun, a star or (less accurately) from the visible horizon. In making this determination the index arm must be set at approximately 0°.

If the index correction is determined from the sun, light filters must be selected and adjusted so that both images are not too bright, but also not dull or barely distinguishable. Bring the edges of twice-reflected and direct sun images into contact by rotating the micrometer drum, and record the sextant reading sr 1 · Then achieve tangency of the other edges of the images and record sextant reading sr2. Calculate the mean sextant reading

In order to eliminate error due to backlash of the index arm, the last rotation of the micrometer drum in determining index correction must always occur on the same side where altitudes of celestial bodies are measured.

The result obtained must be checked. For this purpose, subtract the lower reading from the higher and divide the difference by four. Compare the semidiameter of the sun thus obtained with semidiameter R, listed in the Nautical Almanac for the observation day, at the bottom of the "sun" column. If they differ by more than O'.2, the index correction must be repeated.

If the index correction is determined from a star, the telescope must be aimed at a star which is not too bright, and its reflected image brought into coincidence with the direct image. Take a sextant reading and calculate the index correction

this

If the index correction was not made before the altitude measurement, must be done immediately after the observations.

SECTION 43. MEASURING THE ALTITUDES OF CELESTIAL BODIES WITH THE MARINE SEXTANT

1. Measuring the Dip of the Visible Horizon

Adjust so that the eye level is the same as in measuring the altitudes of celestial bodies, and the line of the horizon is observed in two opposite directions. Arranging the inclinometer horizontally with the diaphragm toward the brighter sector of the horizon, focus the images. Rotate the diaphragm ring so that both sectors of the horizon observed in the eyepiece will be of the same brightness. Rocking the instrument, or (if the N-5 inclinometer is used) rotating the serrated ring of the compensator in the eyepiece of the instrument,

match the horizon images so that there is neither a light nor dark interval between them. Take a reading. Rotating 180° (in a circle) turn the inclinometer 180° so that the diaphragm is once again aimed toward the brighter sector of the horizon. Bring the horizon images into coincidence once again and take a reading. Calculate the average instrument reading. Adjust it with a correction taken from the inclinometer log.

Dip measurements are made immediately before and after altitude measurement. The average dip is calculated from the results of these measurements and is taken into account in making altitude corrections.

The observer must:

2. Measuring Sun Altitudes

a) focus the telescope with the eye for daytime observations and mount it on the sextant; select light filters so that the horizon does not give off reflections, but the limbs of the sun are sharply outlined;

b) holding the sextant in a vertical position, point the telescope at the visible horizon under the sun; tighten the index arm handles and, rocking the sextant slightly around the telescope axis, move the index arm until the reflected image of the sun appears in the field of view of the telescope; loosen the index arm handles, engaging the tangent screw with the gear rack;

c) "drop" the limb of the sun below the horizon by rotating the micrometer drum, if the altitude is measured before noon, or raise it above the water if the altitude is measured after noon (Fig. 83); give the command "Stand by" to the assistant;

d) slightly rocking the sextant about the telescope axis and simultaneously slightly turning (rotating) to the right and left so that the lower limb of the sun is constantly in the center of the field of view of the tube, observe when it is tangent to the horizon and give the command "Mark";

e) take a reading from the limb and drum of the sextant and report this to the assistant, who must record the moment from the clock and the measured altitude reading;

f) take a series (3-5) of such measurements and verify the correctness of the entries made by the assistant.

The observer must:

3. Measuring the Altitudes of Stars

a) focus the telescope with the eye for night observations and mount it on the sextant;

b) begin measurements: in the evening-with the stars located in the darker (eastern) sector of the sky; in the morning-with the stars located in the brighter (also eastern) sector of the sky; cloudiness and cloud movement must be taken into account in the process;

c) set the azimuth circle of the compass at the compass bearing reading calculated with the aid of a celestial sphere, and the index arm of the compass at a reading equal to the star altitude taken from the celestial sphere; pointing the sextant in the direction of the horizon which the azimuth circle is facing, find the star selected for observations;

d) if the bearing and altitude of the star were not calculated earlier, set the index arm at a reading of about 0°, point the telescope at the star; tighten the index arm handles and, bringing the telescope down to the horizon, move the index arm, constantly maintaining the image of the star in the field of view of the telescope, until the line of the horizon appears there; loosen the index arm handles; observe and record the star bearing;

e) rocking the sextant about the telescope axis, rotate the index arm so that the arc described by the star sets into the water with easterly azimuths or is in the background of stars with westerly azimuths; order the assistant to “Stand by";

f) without touching the micrometer drum, continue rocking the sextant and, when the star touches (is in contact with) the horizon, give the order to "Mark";

g) take a reading from the limb and drum of the sextant and report this to the assistant, who records the time from the clock and the measured altitude reading;

h) after measuring 3-5 altitudes of each star, check the entries made by the assistant.

4. Recording Measurement Results

The observer's assistant must:

a) with the command to "Stand by", begin observing the second hand of the clock, counting the seconds for himself;

b) with the command to "Mark", stop counting, record the number of seconds, then the number of hours and minutes at that moment in time and the sextant reading reported by the observer.

In making subsequent altitude measurements in the same series, the number of hours and minutes at that moment in time and the number of degrees in the sextant reading do not have to be recorded if they have not changed. But it must be verified that they were correctly recorded earlier.

SECTION 44. CALCULATIONS AND PLOTTING THEIR RESULTS

1. Correcting Altitudes of Stars

After completing measurements, the mean time from the clock and the average sextant reading must be calculated for each series of altitudes. Calculate the DR coordinates or obtain them from the chart: in sun altitude measurements at the mean moment of each series of altitudes; in celestial altitude measurements at the mean moment of the last series. In determining position from the sun, the DR coordinates for determination of the second line of position are calculated using graphic or analytical DR methods, based on the DR coordinates used in calculating the first line. Record all initial data on an Sh-8, Sh-8a or Sh-8b astronomical calculation form: date, DR coordinates, eye level, course and speed of the ship, air temperature and pressure, names of the stars, mean moments of the series, clock correction, average sextant readings, sum of index corrections i, and sextant instrument correction s, then begin calculating the elements of the lines of position (see pages 244 a, b, c).

Correction for altitudes of celestial bodies is made taking into account the corrections selected from the back of the cover of the VAS-58 tables or tables 11-14 of MT-63, as follows:

a) attributing to the average sextant reading the value (i + s) with its sign, find the measured altitude "Meas h";

b) if the altitude was measured above the visible horizon with a marine sextant, subtract from the measured altitude the dip of the visible horizon (measured with an inclinometer) or introduce an always negative correction Aha, selected according to the eye level of the observer e from the Dip Correction Table, thereby obtaining the visible altitude of the celestial body "Vis. h" (if the altitude is measured with a sextant with an artificial horizon, the dip correction is not taken into account);

c) select and record corrections:

for the sun for refraction and parallax Ahp+p; for semidiameter Ro (positive if the altitude of the lower limb was measured, negative if the upper limb was measured; it is not taken into account if the measurement was made using a sextant with an artificial horizon);

for a star or planet: for refraction Ah,;

for a planet; for parralax Ahp;

for all celestial bodies (if the apparent altitude is less than 30°): for air temperature Ah, and for atmospheric pressure Ah;

d) after adjusting the apparent altitude with the sum of these corrections, calculate the true altitude "True h";

e) in determining from the stars: for each of the observed stars, except for the last, after subtracting the DR altitude and azimuth, calculate the azimuth on a circle basis Ac and the difference A-CMG or CMG-A (if the current and drift are negligible, the true course of the vessel can be used instead of CMG); from