24. Polar distance. An arc of a meridian or circle of declination between the object and that pole which is nearest to the observer. PS in Fig. 5 is the Polar Distance. 25. Right Ascension. An arc of the Equinoctial between the first point of Aries and a meridian or circle of declination passing through the centre of an object. A P M and A P T are R. A. See Fig. 9. 26. Dip or Depression of the Horizon. A correction to be subtracted from the observed altitude because the visible horizon is depressed below the sensible, horizon. The angle H A V in Fig. 2 is the Dip. 27. Refraction. A correction to be subtracted from tbe altitude, because the rays of light coming from an object are bent towards the vertical in passing through the air, thus making the object appear higher than it is. The dark circle is the earth, A the position of the observer, s the true position of the Sun, the three fine curves are successive layers of the atmosphere. A ray of light coming from S is bent at each new layer, and finally reaches A in the direction CA; then A sees the Sun in the . direction A CS, and the angle S A S, is the refraction. 28. Parallax. The error caused by looking at the object from the surface instead of from the centre of the earth. It is added to the altitude. The angle CS A in Fig. 2 is the Parallax. 29. Semi-diameter. Is half the diameter of the object, and is applied to the altitude of the object because we observe its edge instead of its centre. The angle S A E in Fig. 2 is the Semidiameter. 30. Augmentation of the Moon's semi-diameter. A correction to be applied to her semi-diameter because it increases with her altitude. C is the centre of the earth, A the place of the observer, and M the Moon. The semidiameter, as viewed from C, is given in the Nautical Almanac; but A being nearer to M than C is, the semidiameter as seen from A will be larger in the ratio that C M is larger than A M. The difference between these semidiameters is the augmentation. 31. Observed Altitude. The altitude as taken by a sextant without error. In Fig. 2. the angle S, A V is the Observed Altitude. 32. Apparent Altitude. An observed altitude after it is corrected for Dip and semi-diameter. Fig. 2, the angle S, A H is the Apparent Altitude. 33. True Altitude. The apparent altitude after it is corrected for Refraction and Parallax. Fig. 2, SC R is the True Altitude. 34. Zenith Distance. An arc of a vertical circle between an object, and the zenith or the true altitude subtracted from 90°. Fig. 5, Z S is the Zenith distance. 35. Vertical Circles. Great circles perpendicular to the horizon, and passing through the Zenith and Nadir. 36. Prime Vertical. That vertical circle that passes through the East and West points of the horizon. Fig. 6, NZD, W ZE are Vertical Circles; A Z and S Z are also parts of Vertical circles. W ZE is the Prime Vertical. 37. Civil Time. When the day begins at midnight and ends the next midnight ; the first 12 hours being called a. M., and the remaining 12 hours P. M. 38. Astronomical Time. When the day begins at the noon following the beginning of the corresponding civil day. It is reckoned from Oh. up to 24h. 39. Sidereal Time. The Westerly hour angle of the first point of Aries. 40. Mean Time. The Westerly hour angle of the mean Sun. 41. Apparent Time. The Westerly hour angle of the real Sun. 42. Equation of time. The difference between mean and apparent time at any instant. 43. Hour angle of a celestial object. The time before an object will be on the meridian, or since it has passed the meridian. P Let P be the pole of the heavens, 'X Y the equinoctial, A the first point of Aries, P A the hour circle passing through the first point of Aries, P M that passing through the Mean Sun, PT that passing through the True Sun, and PS that through the Ship : Then the angle A P M is the Right Ascension of the Mean Sun, APT the R. A. of the True Sun; APS the R. A. of the Meridian or the Sidereal Time ; MPS Mean Time; T P S Apparent Time; and M P T the Equation of Time. TPS is also the Hour Angle. 44. Complement of an arc or angle. The remainder after the arc or angle has been subtracted from 90°. 45. Supplement of an arc or angle. The remainder after the arc or angle has been subtracted from 180°. COMMERCIAL CODE OF SIGNALS. 1. How many Flags are special to the Code ? Nineteen. 2. Describe them. One code pennant, one burgee, fonr pennants, and thirteen square flags. 3. What is the object of the Code pennant ? To distinguish the Commercial Code from all the other codes in use. 4. How is it used ? Hoisted under the ensign, it means that the ship so hoisting it wishes to communicate by the Commercial Code of Siynals. Hoisted by itself, it is the answering pennant. 5. How many Flags are there in an Attention Signal ? Two. 6. How many in a Compass Signal ? Two. 7. And how many in an Urgent Signal ? Two. 8. Ho do you distinguish one from another ? The Attention Signal has a Burgee uppermost; the Compass Signal a Pennant uppermost ; and the Urgent Signal a square flag uppermost. 9. What do Signals of three Flags in a hoist principally relate to ? To general subjects of inquiry or communication, including latitude, longitude, and time signals. 10. How many Flags are there in a hoist for a Geographical Signal ? and how do you distinguish this Signal from others made with the same number of Flags ? Four. The Burgee is uppermost. 11. How do you know when a Man-of-War is signalling her name ? When I see Four Flags, Pennant G uppermost. 12. How do you know when a Merchant Ship of any nation is signalling her name ? When I see Four Flags, a Square Flag uppermost. 13. What kind of a hoist would it be if you saw Four Flags, the Pennant C, D, or F being uppermost ? National Vocabulary. 14. What is the “ Distress Signal ?” NC, or a Square Flag having either above or below it a Ball. 15. What is the “Pilot Signal ?" The Jack with a white border, or PT. For Distress and Pilot Signals, see Appendix H at the end. |