LEGEND:
Blue = information provided
Black = calculated values
Red = table values, etc.
Green = ANSWER
Celestial Observations
Amplitude ( Celestial Horizon ) - Sun
USCG 1654-NP-5
STEPS
On 28 September in DR position LAT 27° 16.7' S, LONG 113° 27.2' W, you observe an
amplitude of the Sun. The Sun's center is on the celestial horizon and bears 273° psc. The
chronometer reads 01h 17m 26s and is 01m 49s slow. Variation in the area is 6° W. What is the
deviation of the standard magnetic compass?
SOLUTION:
Bearing 273° psc = SUNSET
The est. GMT is just that
so one can figure out the real GMT time.
Assume 1800 ZT + 8 ZD = est. GMT 0200 (actual 0100)
(date change)
CT 01h 17m 26s
GMT
01h 19m 15s
CE +
01m 49s slow ZD
-8 (113° W = 8)
GMT 01h 19m 15s
ZT
17h 19m 15s
SEE PAGE 4 for
excerpts from the
Nautical Almanac
September 29, 1991
Dec
d (+1.0)
Dec
01h 19m 15s GMT
S 2° 18.2'
+ 0.3'
S 2° 18.5'
GMT is determined to be AM or PM
based upon your ZD and bearing of
the Sun. If ZT is not provided, then
for easterly bearings assume a 0600
sunrise and for westerly bearings
assume an 1800 sunset. Watch for a
change in GMT date. Lookup the
Sun's Declination for the GMT in the
Almanac. The d correction is applied
based upon how Declination is
increasing or decreasing with time on
the date used. Apply corr d from the
minutes table appropriately (+/-).
A bearing (psc or pgc) of a body can be observed when it is on either the Celestial or the
Visible Horizon. The Sun is on the Celestial Horizon when its lower limb is approximately
1/2 to 2/3's of a diameter or 21' of arc above the Visible Horizon. When observing a body
on the Visible Horizon, an additional correction from Bowditch Table 28 (New Table 23)
must be applied. This correction accounts for the change in bearing as the body moves between
the Visible and Celestial Horizons. It reduces the bearing on the Visible Horizon to the
Celestial Horizon, from which the table is computed.
1654-np-5.pdf
Page 1 of 4
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LEGEND:
Blue = information provided
Black = calculated values
No additional corrections needed!
Sun
Red = table values, etc.
Green = ANSWER
Correction from Bowditch Table 28 (New
Table 23) needs to be applied in this case!
Sun
Celestial Horizon
Visible Horizon
Visible Horizon
The best time to observe an amplitude of a celestial body is on the Celestial Horizon. There
are fewer corrections that have to be applied.
xx*xxThe Sun is on the Celestial Horizon when its lower limb is 1/2 to 2/3's of a diameter,
xxxxxor 21' of arc, above the Visible Horizon.
xx*xxThe Moon is on the Celestial Horizon when its upper limb is on the Visible Horizon.
xx*xxStars and Planets are on the Celestial Horizon when they are approximately one Sun
xxxxxdiameter above the Visible Horizon.
Observed bearing: 273° psc
Dec S 2° 18.5'
LAT 27° 16.7' S
xxxxTable 27 Excerpt
Declination
xxLat… ...2.0°xxx2.5°
2° 18.5'
xx25°… ..2.2xxxx2.8
27°16.7'
xx30°… ..2.3xxxx2.9
1654-np-5.pdf
The true computed amplitude (a) is obtained from Bowditch
Table 27 (New Table 22) “Amplitudes” or by formulas used
to derive the table. Both methods for solution will be
shown. Choose the one that is easiest for you. The 2nd
method is for verification.
Interpolate 4 ways for
the exact Amplitude.
Page 2 of 4
Amplitude = W 2.3° S
(Setting and Declination)
© Copyright 2004 - LAPWARE, LLC
LEGEND:
Blue = information provided
Black = calculated values
BY FORMULA:
Sin Amp = Sec Lat x Sin Dec
Use the formula in the “Explanation of
tables” for Bowditch Table 27 (New Table
22) “Amplitudes.”
Sin Amp = Sin Dec / Cos Lat =
Sin Amp =
Red = table values, etc.
Green = ANSWER
Sin 2.308
Cos 27.278
0.04028
0.88879
=
Convert the Dec and LAT into
degrees and tenths of degrees.
Insert into the formula and
solve for Amplitude (Amp).
0.0453 =
2.60°
W 2.6° S
(Setting and Declination)
This angle is prefixed E if the body is rising and W if it is setting. This is the only
angle in celestial navigation referenced FROM East or West, i.e. from the prime
vertical. The angle is suffixed N or S to agree with the name of the body’s
declination. A body, whose declination is zero, rises and sets exactly on the prime
vertical. In this example, the angle is labeled West and South.
Amplitude
True Bearing
270.0°
W 2.6° S
267.4°
TRUE
Variation
Magnetic
Deviation
Compass
The bearing is in the SW quadrant. Subtract
(a) from 270° to determine the Amplitude.
This is the TRUE bearing of the Sun.
T
V
M
D
C
Bearing
267.4°
6.0° W
273.4°
0.4° E
273.0°
+W
psc
Mnemonic - "TVMDC"
East is least, West is best when uncorrecting.
Uncorrecting is going away from T to C or T to G.
Compass is best, error is WestxxxxGyro is best, error is West
Compass is least, error is EastxxxxGyro is least, error is East
1654-np-5.pdf
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LEGEND:
Blue = information provided
Black = calculated values
Red = table values, etc.
Green = ANSWER
Select the closest answer.
A) 0.2° W
B) 0.4° E
C) 0.6° W
D) 0.8° E
1654-np-5.pdf
ANSWER
Page 4 of 4
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