Page 371 equipment and instruments 4525 



When celestial bodies that have an appreciable diameter are observed, one of the 

 limbs, usually the lower, is brought into coincidence with the horizon, the observed 

 angle being corrected for the semidiameter of the body. 



For observations of the sun, shade glasses are nearly always required to reduce 

 the intensity of the sun's rays and under certain conditions may also be required to 

 diminish the glare of the horizon. The errors which may be introduced through the 

 use of imperfect shade glasses are described in 4515. 



There is no difficulty in finding the reflected image of the sun in a sextant. 

 Because of its brightness, its reflected image is usually easily brought into view in the 

 horizon mirror by pointing the sextant at the horizon in the general direction of the 

 sun and moving the index arm until the reflected sky begins to appear brighter, after 

 which the sun's image may soon be found. When a star or planet is to be observed 

 there is greater difficulty in bringing the reflected image into view. Sometimes this 

 may be done by the same method used for the sun, but if this method is used, the pattern 

 of stars in the vicinity of the one selected must be compared with those seen in the 

 mirror in order to ensure that the observation is being made on the selected star. 



When the star cannot be found by the above method, there are several other 

 methods of finding it and bringing it into approximate coincidence with the horizon: 



(a) With the index arm set approximately at zero the telescope may be pointed directly at the 

 star, when both the direct and reflected images of the star should be visible. Holding the sextant 

 steady and always keeping the reflected image of the star in the horizon mirror, the index arm is 

 moved slowly along the arc at the same time moving the sextant slowly in a vertical plane until the 

 star is brought down to the horizon. 



(b) The horizon may be brought up to the star by holding the sextant, set approximately at 

 zero, in an inverted position with the telescope pointed at the star. With the star maintained in 

 direct view the index arm is moved until the reflected image of the horizon is in approximate coin- 

 cidence. This is a somewhat awkward procedure since the sextant must be held in the left hand, 

 for which it was not constructed, and care must be taken to avoid intercepting the line of sight 

 with the right hand, which is moving the index arm. After approximate coincidence has been obtained, 

 the position of the sextant is changed to normal for the actual observation. 



(c) The approximate altitude of a celestial body may be determined for any given instant and 

 position on the earth by calculation, from tables, or graphically (see 3387B). Once this approxi- 

 mate altitude is known, it may be set on the sextant and should be close enough to the actual altitude 

 so that the selected body will appear in the reflected field of view when the telescope is pointed in 

 the proper direction at the horizon below. A slight rotation of the sextant to the right or left of where 

 it is first pointed is sometimes necessary to bring the star or planet into view. 



In all altitude measurements of celestial bodies the telescope must be accurately 

 focused to the end that both the horizon and the celestial object or its limb appear 

 as sharp and definite as possible. Only then will accurate observations result. 



4525. Sextant Angles at Night 



When altitudes of celestial bodies are measured at night, provision should be made 

 to illuminate the arc of the sextant while it is being read. If ordinary illumination ia 

 used the sudden brightness of the light contracts the pupil of the observer's eye to inter- 

 fere with the observations. 



Some sextants are specially equipped for this purpose with a small electric bulb, 

 carried on the index arm, located in a position to illuminate the arc. The bulb is lighted 

 from a small battery concealed within the handle. A sextant not so equipped can be 

 altered on shipboard for this purpose. The wooden handle should be removed and 

 replaced by one of larger cross section within which one or two longitudinal holes are 



