Or,

18.

The Error of the Compass is the combined effect of the variation and deviation.

19. Lee Way is the angle between the ship's wake and the line of her keel.

20. Meridian Altitude of a Celestial Object is its angular distance above the horizon, when the object is on the meridian.

21. Azimuth is the angular distance of a celestial object from the observer's meridian; it is measured on the horizon.

22. Amplitude is the angular distance of a celestial object on the horizon, from the prime vertical.

23. Declination is the angular distance of a celestial object from the equinoctial; it is on a meridian.

24. Polar Distance is the angular distance of a celestial object from the elevated pole; it is measured on a meridian.

25.

Right Ascension is the angle between two meridians, one passing through the celestial object, the other through the first point of Aries; it is measured eastward on the equinoctial.

26. Dip or Depression of the Horizon is the angular distance of the visible below the sensible horizon.

Figure 5.

C

Def. 19. Fig. 5. Let ABC be the line of the ship's keel, and BD the direction of her wake, then CBD is the leeway.

Def. 20. Fig. 6. If M be the object on the meridian, HAM is its Meridian Altitude.

Def. 21. Fig. 6. If X be the object, the angle HZB, or arc HB, is the Azimuth.

Def. 22. Fig. 6. If B be the object, the angle AZB, or arc AB is the Amplitude.

Def. 23. Fig. 6. If X be the object, the arc XD is the measure of the Declination.

Def. 24. Fig. 6. The arc PX is the measure of the Polar Distance of X.

Def. 25. Fig. 6. The arc AQED is the measure of the Right Ascension of X.

Def. 26. Fig. 2. The angle HOB is the Dip.

27. Refraction is the bending of rays of light caused by the atmosphere.

28. Parallax is the angle at a celestial object subtended by that radius of the earth which is drawn from the observer's position,

29. Semi-diameter is the angle ta the earth's centre subtended by the radius of the celestial object.

30. Augmentation of Moon's Semi-diameter is the increase of the semi-diameter, owing to the increase of the moon's altitude.

81. Observed Altitude is the angular distance of an object above the horizon as shewn by a sextant.

32. Apparent Altitude is the observed altitude corrected for index error, dip, and semi-diameter.

33. True Altitude is the apparent altitude corrected for refraction and parallax.

Def. 27. Fig. 7. The angle SOS' is the Refraction.

Def. 28. Fig. 8. The angle CSO is the Parallax.

Def, 29. Fig. 9. The angle ACM, or BOM is the Semi-diameter.

Defs. 31, 32, and 33. Fig. 10. Let E be the centre of the celestial object, then the angle HCE is the True Altitude of the centre. This is found by applying the Index Error and Dip to the Observed Altitude of the Lower Limb, the result is the Apparent Altitude of the Lower Limb, to which the Refraction, Parallax, and semi-diameter are applied.

34. Zenith Distance is the angular distance of a celestial object from the zenith; or it is=90°—altitude.

35. Vertical Circles are great circles passing through the zenith, and therefore perpendicular to the horizon.

36. Prime Vertical is the vertical circle which passes through the true East and West points of the horizon.

37. Civil time begins at Midnight and ends at .the following Midnight. The first 12 hours is called A.M., the second P.M.

38. Astronomical Time begins at noon and ends at the following

noon.

39. Sidereal Time is the hour angle of the first point of Aries.

40. Mean Time is the hour angle of the mean sun.

41. Apparent Time is the hour angle of the true sun.

42. Equation of Time is the difference between mean and apparent time.

43. Hour Angle of a Celestial Object is its angular distance from the observer's meridian; it is measured on the equinoctial.

44. Complement of an Arc or angle is the difference between the arc or angle and 90°. ·

45. Supplement of Ditto is the difference between the arc or angle and 180°.