4523 HYDROGRAPHIC MANUAL PaGE 370 



be seen only through the telescope and, because the horizon is devoid of reference 

 points, there is great difficulty in finding even the direct object unless it is watched 

 constantly. In survey operations of this nature, while the same or approximately 

 the same course is followed, it is advantageous to note the bearmg of the direct object 

 with reference to some part of the ship's bridge or by pelorus, if necessary. When 

 frequent or large changes in course are made, the only recourse is to note the approxi- 

 mate bearing of the direct object by compass. If the direct object can be maintained 

 in view or found when desu-ed, the reflected object may be found by noting the rate 

 of change of the angle, or by bringing the two objects together in the sextant a sufficient 

 number of times between positions to keep the angle always approximately on. 



In using any of the above methods for picking up faint objects, observers, and 

 especially beginners, are warned against getting an approximate angle and letting their 

 imaginations do the rest. Such methods can, of course, be used only to aid in getting 

 the objects within the field of view. If one of the objects cannot actually be seen or if 

 they are not in coincidence at the mark, the angle must be reported to the officer-in- 

 charge as a miss. 



4523. The Measurement of Large Angles 



Angles greater than 145° or 150° cannot be measured with the ordinary sextant. 

 Occasionally in hydrographic surveying such angles must be measured. This is some- 

 times done by splitting the angle between two observers. To do this it is essential that 

 there be a well-defined object somewhere near the horizon to serve as a middle object 

 to which the two parts of the sum angle may be observed. In three-point fix hydrog- 

 raphy where the control stations are buoys out of sight of land it is sometimes practi- 

 cable, especially in the tropics, to use well-defined points of cloud formations near 

 the horizon for this purpose. 



A sextant has been designed in Europe embodying a prism in such a way that 

 angles up to 180° can be read on it. This sextant has not been used by the Coast 

 and Geodetic survey. 



4524. Vertical Sextant Angles 



The sextant was designed primarily for use at sea by mariners in observing the 

 altitudes of celestial bodies and it is also used for this purpose by hydrographic surveyors. 

 To make such an observation the instrument is held so that its plane is vertical, with 

 the telescope directed toward the horizon. Observing the horizon in direct view, the 

 index arm is moved along the arc until the image of the celestial object is seen reflected 

 in the horizon mirror. When both the celestial object and the horizon are seen in the 

 telescope, the arm is clamped and by means of the tangent screw an exact coincidence 

 is obtained when the plane of the sextant is truly vertical. 



The principal difficulty in measuring such an angle is that it must be marked so 

 as to measure the angle in a vertical plane. To ensure this, the observer must swing 

 the instrument slightly to the right and left of the vertical. The reflected object will 

 appear to describe the arc of a circle, the lowest point of which marks the true vertical, 

 and when coincidence is obtained at this point the angle should be marked. Altitudes 

 of celestial bodies should not be measured where the sextant is pointed through the 

 rigging or where other nonvertical parts of the vessel are in view to distort the observer's 

 sense of perpendicularity. 



