# JN Chapter 4 - St Paul Sail & Power Squadron

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```Altitude
Junior Navigation
Corrections Chapter 4
1
Learning Objectives



Understand how the following factors affect
altitude observations:
 Dip
 Refraction
 Parallax
 Semidiameter
Apply altitude corrections from the Nautical
Almanac to observations of the sun
Celestial Tools
2
Comparing hs to Ho and Hc
hs – altitude measured with sextant
ha – hs corrected for ‘IE’ and ‘dip’ and used to enter
Altitude Correction Tables
Apparent
Position
Ho – observed altitude after corrections
Hc – altitude calculated using LHA, Dec, and Latitude
True
Position
ha
hs
Earth
True Horizontal
Celestial
Horizon
3
Sight Reduction Form
4
Sequence of Corrections




Record height of eye
Sextant altitude (hs)
Index correction (IC)
Dip Correction (Dip)

Apparent altitude (ha) (Used to
enter altitude correction tables)
Correction from tables (Main)

Observed altitude (Ho)

5
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
6
Nautical Almanac Tables
7
Dip on Natural Horizon



Correction based on
height of eye (HE) above
water line where sight
taken
For HE <8ft or >70ft
<155ft use right column
and interpolate
For HE between 8ft-70ft
use left column and no
interpolation required
8
Dip on Natural Horizon

How to interpolate:


HE is 7.5ft
From table correction is 2.7 for
8ft and correction is 2.4 for 6ft

Change in correction is 0.3 in 2ft

Your HE from 8ft is -0.5ft

0.5ft is 25% of 2ft

25% of 0.3 is 0.077

Correction is 2.7 - 0.08 = 2.6
9
Dip on Natural Horizon

How to select correction:


HE is 14.6ft
14.6ft is more than 14.1ft but
not more than 14.9ft so
correction is 3.7
10
Dip Short (Appendix A)
Sight taken with HE of 10.6 ft,
across a distance of 670 yards.
(Dip to NH
would
-3.2’)
Use Almanac
tablesbe
if:
Distance to
shore in
Yards
Meters
Ds
Height Height
of
of Eye
Eye in
Feet
Feet
3
Distance to Horizon
Dip Short (Ds)
at least
Formula
d = distance & h = height of eye
nm
sm
yds
Ds 2.0
= (0.0002052
x d) + [1146 x (h/d)]
2.3
4103
Meters 5 Ds x
= (0.0002244
x d) x
+ (h/d)]
[3438 x (h/d)]
= (0.0002052
+ [1146
2.6d)3.0
5296
Nautical miles
Feet 7 Ds 3.1
= (0.4156
x6267
d) + [0.5658 x (h/d)]
3.6
Ds
= (0.0002052
x 670)
+ [1146 x (10.6 / 670)]
Statute miles
Feet
Ds = (0.3611 x d) + [0.6511 x (h/d)]
3.5
4.0
Ds = 0.1374849 + (1146
x 7106
0.015820896)
10
3.7
4.3
7490
Ds = 18.268’, rounded to 18.3’
15
4.5
5.2
9174
Dip correction
-18.3’
20
5.2
6.0is 10593
11
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
9 4
17 26 5
?
1 7
?
3 0
1 7 3 0
– 1 3
17 25 2
13 2
17 25.2
13 2 0 0
+ 13 2
17 38 4
12
Find Ho
15 13 11FOR
9 YOUR
7
5 ANSWERS?
4
3
2
1
READY


UL Sun sight is taken on 25 Jun.
 HE is 6.9 ft NH
 IE is 3.1´ on the arc
 hs is 56° 31.2´
LL Sun sight is taken on 10 May.
 HE is 5.5 ft Ds 345 yds
 IE is 0.5´ off the arc
 hs is 43° 50.0´
Use excerpts (Appendix B), pg 212
13
Find Ho

UL Sun sight is taken on 25 Jun.
 HE is 6.9 ft NH
 IE is 3.1´ on the arc
 hs is 56° 31.2´
Use excerpts (Appendix B), pg 212


6 9
56 31 2
3 1
2 5
0 0 5 6
– 5 6
56 25 6
HE is 6.9ft
From table correction is 2.7 for
8ft and correction is 2.4 for 6ft

Change in correction is 0.3 in 2ft

Your HE from 8ft is -1.1ft

1.1ft is 55% of 2ft

55% of 0.3 is 0.165

Correction is 2.7 - 0.2 = 2.5
16 5
0 0 16 5
– 16 5
56 09 1
14
Find Ho

LL Sun sight is taken on 10 May.
 HE is 5.5 ft Ds 345 yds
 IE is 0.5´ off the arc
 hs is 43° 50.0´
Use excerpts (Appendix B), pg 212
Ds = (0.0002052 x d) + [1146 x (h/d)]
Ds = (0.0002052 x 345) + [1146 x (5.5 / 345)]
Ds = 0.070794 + (1146 x 0.015942029)
Ds = 18.34036’, rounded to 18.3’
Dip correction is -18.3’
5 5
43 50 0
0 5
18 3
0 5 18 3
- 17 8
43 32 2
15 0
15 0 0 0
+ 15 0
43 47 2
15
‘Celestial Tools’
16
Using ‘Celestial Tools’
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.
The Sailings – calculate, using mid-latitude or Mercator sailing, or the method
Sight
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17
Using ‘Celestial Tools’
18
Using ‘Celestial Tools’
30 June 2012
12-58-29
–
+
1
Sun LL
0-14
12-58-15
1 0
65 21 2
4 0
4 0
3
30 June 2012
12-58-29
–
71
39 50 6
15-58-15
30 June
0-14
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1 0
–
3 0
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71
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17 0
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15 6
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+ 15 6
71 56 8
19
Using ‘Celestial Tools’
71
17 0
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– June
30
3 0 2012
12-58-29
41
2
0-14
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12-58-15
71
15
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15-58-15
30 June
15 6
00
+ 15 6
71 56 8
20
Quiz
1. Dip correction is the angular difference between
the visible horizon and a true horizontal.
a. True
b. False
21
Quiz
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
22
Quiz
3. Find the dip correction:
HE
Correction
18 ft
- 4.1'
____
22 ft
- 4.5'
____
72 ft
- 8.2'
____
23
Quiz
4. For the following data, determine main
sextant altitude corrections:
Body
Date
ha
Main
Sun UL
21 June
17° 32.1'
-18.8'
Sun LL
29 Sept
43° 46.0'
+15.0'
Sun UL
29 Sept
43° 46.0'
-16.8'
Sun LL
10 Mar
61° 41.6'
+15.7'
24
Next class
BRING INFORMATION
ON ANY SIGHTS YOU
HAVE TAKEN.
25
Altitude Corrections
End of
Junior Navigation
Chapter 4
26
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