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1 Altitude Corrections Junior Navigation Chapter 4.

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1 1 Altitude Corrections Junior Navigation Chapter 4

2 2 Learning Objectives Understand how the following factors affect altitude observations: Understand how the following factors affect altitude observations: Dip Dip Refraction Refraction Parallax Parallax Semidiameter Semidiameter Apply altitude corrections from the Nautical Almanac to observations of the sun Apply altitude corrections from the Nautical Almanac to observations of the sun Celestial Tools Celestial Tools

3 3 Comparing hs to Ho and Hc Earth Celestial Horizon Visible Horizon True Horizontal Apparent Position True Position hs – altitude measured with sextant ha – hs corrected for ‘IE’ and ‘dip’ and used to enter Altitude Correction Tables Ho – observed altitude after corrections Hc – altitude calculated using LHA, Dec, and Latitude Ho or HC ha hs

4 4 Sight Reduction Form LAST WEEK NEXT WEEK

5 5 Sequence of Corrections Record height of eye Record height of eye Sextant altitude (hs) Sextant altitude (hs) Index correction (IC) Index correction (IC) Dip Correction (Dip) Dip Correction (Dip) Apparent altitude (ha) (Used to enter altitude correction tables) Apparent altitude (ha) (Used to enter altitude correction tables) Correction from tables (Main) Correction from tables (Main) Observed altitude (Ho) Observed altitude (Ho)

6 6 Nautical Almanac Tables Altitude Correction table for Sun, and Dip correction for a natural horizon - Appendix B Sun table includes corrections for refraction, semidiameter, parallax, and other minor items “Altitude Correction Table” and most of “Dip Table” are critical tables DO NOT require interpolation Extreme right-hand column of “Dip Table” correction (less than 8ft or greater than 70ft) DOES require interpolation

7 7 Nautical Almanac Tables

8 8 Dip on Natural Horizon Correction based on height of eye (HE) above water line where sight taken For HE 70ft <155ft use right column and interpolate For HE between 8ft-70ft use left column and no interpolation required

9 9 Dip on Natural Horizon How to interpolate: HE is 7.5ft HE is 7.5ft From table correction is 2.7 for 8ft and correction is 2.4 for 6ft From table correction is 2.7 for 8ft and correction is 2.4 for 6ft Change in correction is 0.3 in 2ft Change in correction is 0.3 in 2ft Your HE from 8ft is -0.5ft Your HE from 8ft is -0.5ft 0.5ft is 25% of 2ft 0.5ft is 25% of 2ft 25% of 0.3 is % of 0.3 is Correction is = 2.6 Correction is = 2.6

10 10 Dip on Natural Horizon How to select correction: HE is 14.6ft HE is 14.6ft 14.6ft is more than 14.1ft but not more than 14.9ft so correction is ft is more than 14.1ft but not more than 14.9ft so correction is 3.7

11 Sight taken with HE of 10.6 ft, across a distance of 670 yards. (Dip to NH would be -3.2’) Use Almanac tables if: Height of Eye Distance to Horizon at least Feetnmsmyds Distance to shore in Height of Eye in Dip Short (Ds) Formula d = distance & h = height of eye YardsFeetDs = ( x d) + [1146 x (h/d)] Meters Ds = ( x d) + [3438 x (h/d)] Nautical milesFeetDs = ( x d) + [ x (h/d)] Statute milesFeetDs = ( x d) + [ x (h/d)] 11 Dip Short (Appendix A) Ds = ( x d) + [1146 x (h/d)] Ds = ( x 670) + [1146 x (10.6 / 670)] Ds = (1146 x ) Ds = ’, rounded to 18.3’ Dip correction is -18.3’

12 12 Find Ho LL Sun sight is taken on 29 Feb. HE = 9.4 ft NH hs is 17° 26.5´ IE is 1.7’ off the arc – ? ?

13 13 Find Ho UL Sun sight is taken on 25 Jun. UL Sun sight is taken on 25 Jun. HE is 6.9 ft NH HE is 6.9 ft NH IE is 3.1´ on the arc IE is 3.1´ on the arc hs is 56° 31.2´ hs is 56° 31.2´ Use excerpts (Appendix B), pg 212 LL Sun sight is taken on 10 May. LL Sun sight is taken on 10 May. HE is 5.5 ft Ds 345 yds HE is 5.5 ft Ds 345 yds IE is 0.5´ off the arc IE is 0.5´ off the arc hs is 43° 50.0´ hs is 43° 50.0´ READY FOR YOUR ANSWERS?

14 14 Find Ho UL Sun sight is taken on 25 Jun. UL Sun sight is taken on 25 Jun. HE is 6.9 ft NH HE is 6.9 ft NH IE is 3.1´ on the arc IE is 3.1´ on the arc hs is 56° 31.2´ hs is 56° 31.2´ – – HE is 6.9ft HE is 6.9ft From table correction is 2.7 for 8ft and correction is 2.4 for 6ft From table correction is 2.7 for 8ft and correction is 2.4 for 6ft Change in correction is 0.3 in 2ft Change in correction is 0.3 in 2ft Your HE from 8ft is -1.1ft Your HE from 8ft is -1.1ft 1.1ft is 55% of 2ft 1.1ft is 55% of 2ft 55% of 0.3 is % of 0.3 is Correction is = 2.5 Correction is = 2.5 Use excerpts (Appendix B), pg 212

15 15 LL Sun sight is taken on 10 May. LL Sun sight is taken on 10 May. HE is 5.5 ft Ds 345 yds HE is 5.5 ft Ds 345 yds IE is 0.5´ off the arc IE is 0.5´ off the arc hs is 43° 50.0´ hs is 43° 50.0´ Find Ho Use excerpts (Appendix B), pg 212 Ds = ( x d) + [1146 x (h/d)] Ds = ( x 345) + [1146 x (5.5 / 345)] Ds = (1146 x ) Ds = ’, rounded to 18.3’ Dip correction is -18.3’

16 16 ‘Celestial Tools’

17 Sight Planner – calculate the twilight times for a selected date and location and find the azimuth, altitude, and magnitude of the visible navigational bodies at a selected time, as a list or a star chart. It will also find the times of moonrise and moonset, the phase of the Moon, and the availability of acceptable Sun-Moon fixes. Sight Averaging – analyze a run of sights and calculate the average watch time and sextant altitude. CTS/SOA – calculate the “Course to Steer” and “Speed of Advance” for a course and speed affected by current. Arc Time – convert an angular value to its equivalent time and vice versa. Distances – a distance to the natural horizon calculator, a geographical range of visibility of an object calculator, and three distance by vertical angle calculators. Interpolation – will do single or double interpolation, and includes a sexagesimal-to-decimal converter. 60D=ST – calculate speed, time, or distance when two of the values are known. Yellow Pages – will produce the values of the Nautical Almanac Increments and Corrections table. Dist. by 2 Brngs – calculate the distance to a fixed object (and related quantities) by taking two bearings on the object from a moving vessel. 17 Using ‘Celestial Tools’ Sight Reduction and Fix – completely reduce a sight to intercept and azimuth by the Law of Cosines method, allowing students to check the quality of their sights while still “at the beach”. After a minimum of two sights are reduced and saved, you have the option of establishing a fix using sights selected from the list of up to ten. Noon Sight – calculate the time of Local Apparent Noon (LAN) for a selected date and longitude, and then calculates latitude from a noon sight. It will also calculate longitude from the observed zone time for LAN. LoC/NASR and Fix – allows the user to enter data from the upper part of a sight reduction form (derived from data extracted from the Nautical Almanac) and reduce the sight using the Law of Cosines and Nautical Almanac Sight Reduction methods, giving the same intermediate and final values as would be obtained with a manual reduction. The Sailings – calculate, using mid-latitude or Mercator sailing, or the method used in VPP2, course and distance from initial and final positions, final position from initial position, course, and distance, and set and drift from DR position, fix position, and elapsed time. It will also calculate, using great circle sailing, initial course and distance or final position, plus maximum latitude, final course, and points on the great circle route. TVMDC – will update the values of the compass variables (True, Variation, Magnetic, Deviation, and Compass) when any one is changed. It will also calculate the value of variation for a selected year based on the data provided on a chart. MoBoard – perform several calculations typically done on a maneuvering board, e.g. Closest Point of Approach between two moving vessels (or between a moving vessel and a stationary object), course and speed of contact vessel, true wind from apparent wind. Favorite Places – save the latitudes, longitudes, range of useful azimuths, type of horizon, and dip short distance for locations that will be reused for sight-taking sessions. Automatically loads this date into the Sight Planner. Length of Degree – calculate the length of a degree of latitude and longitude in various linear units for both a spherical earth for a specified latitude, north or south. The WGS84 spheroid output duplicates Bowditch 2002 Table 7. Although the data produced by Celestial Tools is fairly accurate, the program should not be used for navigation. Its primary purpose is instructional: to aid a student in planning sights; to allow a student to quickly check the quality of sights; and to help a student find errors in his/her work. It is not intended to do the work for the student.

18 18 Using ‘Celestial Tools’

19 30 June Sun LL June – – G g o 19 Using ‘Celestial Tools’ 30 June Sun LL – G g

20 20 Using ‘Celestial Tools’ o – June June + –

21 21 1. Dip correction is the angular difference between the visible horizon and a true horizontal. a. True b. False Quiz

22 22 2. A ship's captain takes a sight from the ship's rigging with a HE of 54 feet. His mate takes a sight from the deck of the ship with a HE of 18 feet. For which HE will the dip correction be greatest? HE of 54 feet Quiz

23 23 3. Find the dip correction: HE Correction 18 ft ____ 22 ft____ 72 ft____ - 4.1' - 4.5' - 8.2' Quiz

24 24 4. For the following data, determine main sextant altitude corrections : Body Date ha Sun UL 21 June 17° 32.1' Sun LL 29 Sept 43° 46.0' Sun UL 29 Sept 43° 46.0' Sun LL 10 Mar 61° 41.6' Main ' ' ' ' Quiz

25 25 BRING INFORMATION ON ANY SIGHTS YOU HAVE TAKEN. Next class

26 26 Altitude Corrections End of Junior Navigation Chapter 4


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