DEFINITIONS. NAVIGATION, ASTRONOMY, FOR SECOND AND ONLY MATES. & C. Arranged by Mr. W. H. BOLT, of 10, King Street, Tower Hill. Note. This arrangement of the Definitions has been specially recommended by the Board of Trade to the Local Marine Boards throughout the United Kingdom, and are here produced by Mr. Bolt's special permission. N.B.-The Candidate is to write a short definition against so many of the following terms as may be marked with a cross by the Examiner. The Examiner will not mark less than 10. The writing should be clear, and the spelling should not be disregarded. 1. THE EQUATOR is a great circle, supposed to be drawn round the earth 90° from each pole. 2. 4. THE POLES of the earth are the ends of the axis around which it revolves. 3. A MERIDIAN is a great circle passing through the poles, and cutting the equator at right angles. THE ECLIPTIC is the great circle in which the sun appears to move annually among the fixed stars. 5. THE TROPICS are small circles parallel to the equator, and 231° on each side of it. 6. LATITUDE of any place is its distance North or South of the equator, measured on a meridian. 7. PARALLELS OF LATITUDE are small circles, parallel to the equator. 8. LONGITUDE of a place is the arc of the equator, between the first meridian and the meridian of the place. 9. THE VISIBLE HORIZON is the circle limiting the observer's view at sea. 10. THE SENSIBLE HORIZON is a plane to which the plumb line at the position of the observer is perpendicular. B II. 12. THE RATIONAL HORIZON is a plane passing through the centre of the earth, every point of which is 90° from the observer's zenith. ARTIFICIAL HORIZON.-The surface of any liquid in a state of rest is an artificial horizon. It is used for taking altitudes on shore. 13. TRUE COURSE OF A SHIP is the angle a ship's track makes with a true meridian. 14. MAGNETIC COURSE is the angle a ship's track makes with the magnetic meridian. 15. COMPASS COURSE is the angle a ship's track makes with the compass needle. 16. VARIATION OF THE COMPASS is the angle the magnetic meridian makes with the true meridian. 17. DEVIATION OF THE COMPASS is the angle the compass needle makes with the magnetic meridian. 18. THE ERROR of the COMPASS is the angle the compass needle makes with the true meridian. 19. LEEWAY is the angle the ship's track makes with her fore and aft line. 20. 21. 22. MERIDIAN ALTITUDE OF A CELESTIAL OBJECT is the angular height of that object above the horizon, when it is on the meridian. THE AZIMUTH of a celestial object is the arc of the horizon between the N. or S. points, and a vertical circle drawn through the object. THE AMPLITUDE of a celestial object is the arc of the horizon between the E. or W. points, and the object when rising or setting. 23. THE DECLINATION of a celestial object is the arc of a celestial meridian between the equator and the object. 24. 25. POLAR DISTANCE is the arc of a celestial meridian 26. DIP OR DEPRESSION OF THE HORIZON is the angle at the observer's eye between the visible and sensible horizons. 27. REFRACTION is the bending of a ray of light toward the perpendicular, when passing through the atmosphere. 28. PARALLAX is the angle at the centre of a celestial object, subtended by the earth's radius at the position of the observer. 29. SEMIDIAMETER of a celestial object is the angle at the eye of the observer, subtended by the radius of its circular disc. 30. AUGMENTATION OF THE MOON'S SEMIDIAMETER is the apparent increase in the moon's semidiameter, arising from the decreased distance of the observer from the moon, as her altitude increases. 31. OBSERVED ALTITUDE is the angular height of an object read from the arc of the instrument by which the altitude has been measured. 32. APPARENT ALTITUDE is the apparent angular height of an object's centre above the sensible horizon. 33. TRUE ALTITUDE is the true angular height of an object's centre above the rational horizon. ZENITH DISTANCE of a celestial object is the arc of a vertical circle between the object and the zenith of the observer. 34. 35. 36. 37. 38. VERTICAL CIRCLES are great circles passing through ASTRONOMICAL TIME is the time used in all astro- 39. SIDEREAL TIME is the time reckoned from the passing of the first point of Aries across the meridian. 40. MEAN TIME is the time which would be shown by the sun if he revolved in the plane of the equator, with the mean angular velocity with which he revolvès in the ecliptic. 41. APPARENT TIME is the angle at the pole between the meridian passing through the sun and the meridian of the observer, reckoned Westerly. 42. EQUATION OF TIME is the difference between apparent and mean time. 43. THE HOUR ANGLE OF A CELESTIAL OBJECT is the angle at the pole between the meridian passing through the object and the meridian of the observer. 44. THE COMPLEMENT OF AN ARC is what that arc differs from 90o. 45. THE SUPPLEMENT OF AN ARC is what that arc differs from 180°. THE ADJUSTMENTS OF THE SEXTANT: The Applicant will answer, in writing, on a sheet of paper which will be given him by the Examiner, all the following questions, numbering his answers with numbers corresponding to the questions. QUESTIONS. What is the first adjustment of the sextant? Describe how you make that adjustment. I. 2. 3. What is the second adjustment? 4. 5. 6. 7. 8. 9. IO. Place the index at error of How do you make the third adjustment? In the absence of a screw, how would you proceed? How is it applied? clamp it, and leave it. minutes to be added, Note.-The Examiner will see that it is correct. |