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DEFINITIONS.
NAVIGATION, ASTRONOMY, &c.

FOR SECOND AND ONLY MATES.

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. 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. 4. 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. 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

7.

12.

II. 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 com

pass needle makes with the true meridian. 19. LEEWAY is the angle the ship's track makes with her

fore and aft line. 20. MERIDIAN ALTITUDE OF A CELESTIAL OBJECT is the

angular height of that object above the horizon,

when it is on the meridian. 21. 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. POLAR DISTANCE is the arc of a celestial meridian

between the elevated pole and the object. 25. RIGHT ASCENSION of a celestial object is the arc of

the celestial equator between the first point of Aries, and the meridian passing through the object.

22.

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. 34. ZENITH DISTANCE of a celestial object is the arc of

a vertical circle between the object and the

zenith of the observer. 35. VERTICAL CIRCLES are great circles passing through

the zenith perpendicular to the horizon. 36. THE PRIME VERTICAL is the vertical circle which

meets the horizon in the E. and W. points. Civil Time is the time used in ordinary life 'to

record events. The day commences at midnight

and terminates at the following midnight. 38. ASTRONOMICAL TIME is the time used in all astro

nomical calculations, and is reckoned from noon to noon.

40.

39. SIDEREAL TIME is the time reckoned from the

passing of the first point of Aries across the

meridian. 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 revolves 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 90°. 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 ? 2. How do you make that adjustment ? 3

What is the second adjustment? 4

Describe how you make that adjustment. 5.

What is the third adjustment ? 6. How do you make the third adjustment? 7 In the absence of a screw, how would you proceed ? 8. How would you find the index error by the horizon? 9.

How is it applied ? IO. Place the index at error of minutes to be added,

clamp it, and leave it.

Note.--The Examiner will see that it is correct.

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