Calculating the setting to put on the sextant... Computing altitudes is related to the altitude of a celestial body at the time meridian passage It is the process of calculating the alt
Trang 1Computing Altitudes
i.e.
Calculating the setting to put on the
sextant
Trang 2Computing altitudes is related to the altitude of a celestial body at the time meridian passage
It is the process of calculating the altitude to ‘set’ on the sextant in order to observe a body at the time of
meridian passage
The process of computing the altitude of a body at
meridian passage is therefore the reverse of the
procedure in “Latitude by Meridian Altitude” problems
Trang 3• As you will remember from Latitude by
Meridian Altitude problems, there is a
relationship between Latitude, Declination, Zenith Distance (ZX) and True Altitude
Trang 4Applying Declination to ZX
Observers Meridian
Let us now imagine the 2/O moves to
10 0 North and observes a body with Declination of 10 0 S
Pn
Z Q
Lat ZX
E W
Ps
X
Z
Equinoctial
Q
TA
TA
Dec
Trang 5This was how we solved Lat x Mer Alt problems
Z
P
• Sextant Altitude : 450 25.5’ S
• Index Error (I.E.) : 00.2’
• Observed Altitude : 450 25.7’
• Dip : 8.7’
• Apparent Altitude : 450 17.0’
• Total Correction : 000 15.3’
X
Dec
Lat
• Total Correction : 000 15.3’
• True Altitude : 450 32.3’ S
• + or – Dec : 340 27.7’ S
• Latitude : 100 00.0’ N
Trang 6Computing Altitudes is the complete reverse
Z
P
• Sextant Altitude : 450 25.5’ S
• Index Error (I.E.) : 00.2’
• Observed Altitude : 450 25.7’
• Dip : 8.7’
• Apparent Altitude : 450 17.0’
• Total Correction : 000 15.3’
X
Dec
Lat
• Total Correction : 000 15.3’
• True Altitude : 450 32.3’ S
• + or – Dec : 340 27.7’ S
• Latitude : 100 00.0’ N
Trang 7Begin by obtaining the LMT of Meridian Passage
Apply longitude in time to obtain the GMT/UTC of
Meridian Passage
Obtain the Declination of the celestial body from the
Nautical Almanac
Then proceed as
Trang 8follows:-Computing Altitudes is the complete reverse
S A 450 25.5’S
O.A 450 25.7’
A.A 450 17.0’
T.C 000 15.3’
• Apply Latitude to Declination to obtain the ZX
• Difference the ZX from 900 00’ to obtain the TA
• Apply the T.C using the TA as the argument in the Altitude Correction Tables, A2/A3
• As you are in fact doing a reverse procedure
T.C 000 15.3’
T.A 450 32.3’S
~ 900 00.0’
ZX 440 27.7’N
Dec 340 27.7’S
Lat 100 00.0’N
• As you are in fact doing a reverse procedure please remember the correction MUST be applied in reverse!
• This applies to all corrections i.e T.C., Dip & I.E.
• Then apply ‘Dip’ to obtain the O.A followed by Index Error to obtain the S.A or conversely, the setting to put on the sextant.
Trang 9Be Careful!
I When combining Latitude and
Declination
II Also when naming ZX
Z
X
Q
Lat
ZX
Pn
II Also when naming ZX
III A simple sketch will always help
IV All corrections MUST be applied
with the OPPOSITE sign
Dec
TA
Trang 10Be Careful!
• Questions will also require the true
bearing of the body at meridian
passage (either 0000 or 1800)
X
Z Q
Lat
ZX
Pn
• This will be named opposite to the
ZX and can be checked by using the
Latitude, Declination and a small
sketch.
TA
Dec
Trang 11LMT Mer Pass 29d 12h 06m 00s
UTC Mer Pass 29d 14h 10m 00s
Lat 40 0 30’S
Proforma Layout
Find the UTC and LMT of meridian passage of the Sun (LL) and also the setting to put on the sextant Date is July 29 th 2000 in DR position 40 0 30’S, 031 0 00’W Index Error 2.0’ off the arc and Height of Eye
of 28m
State the bearing of the body at meridian passage
ZX
Dec 18 0 35.2’N
“d”(0.6) 0.1 – Dec 18 0 35.1’N
Dec 18 0 35.1’N
ZX 59 0 05.1’S
~ 90 0 00.0’
TA 30 0 54.9’N TC(+) 14.4’ (-)
AA 30 0 40.5’
Dip (-) 9.3 (+)
OA 30 0 49.8’
IE (+) 2.0’ (-)
SA 30 0 47.8’N
TA
X
Z
Q
Lat
ZX
Dec