Practical Astronavigation

Practice - algorithm of acting in seven points



1. To measure the height with the sextant CN (ho) and to catch the time.
2. To correct the measured height for all amendments.
3. To make an assumption that you are in the moment of the measurement
in the point X (φ, λ).
4. To calculate what height would be (hc) and azimuth CN if you were really in the point X.
5. From the X point to cross the azimuth off and to put the distance back on the ruler of the azimuth (in nautical miles) Δh=hc- hs
6. To draw in the perpendicular from this point to the azimuth
7. To be glad that you have the positional line.

Today is 14 August 1980
Position summed up of the yacht: φ=46°37'N ; λ=10°11'W (a bit to the west of the English Channel).

1. You measured the height of the bottom edge of the Sun at the hour 10h 06m 29s (time GMT). Wysokość = 43°36,4'=ho

2. You are calculating:


  ho = 43°36,4'    
+ cc = +0,2    (permanent error, take from the certificate for 43°)
+ ci = 3,0    (mistake of the index, measure before or after the measurement with method on 'Sun' or 'to the horizon')
+ k = - 3,3    (amendment to the height of the eye by the water; RA, p.2 or stiff insert. Table DIP. You are entering to Ht of Eye number for example 3,5m, you are reading out off the table Corrn number -3,3)
+ 0 = + 15,0    (amendment to the bottom edge; like high, table SUN: APR-SEPT. You are entering with the number 43°30' to the table App. Alt. You are reading out off the table Lower Limb number +15,0)
  hs = 43°45,3'    

You received in this way hs that is true height of the Sun above the horizon.

3. Assume that you are on the closest parallel of your position summed up, in this case 47°N. Don't establish the longitude at the moment, he will leave for you alone. It is a well-known fact at the moment, that approximately 10° W.
4. Establish what height and azimuth would be, if you were really there. Open RA on the side '14 AUGUST'. Enter the GMT table hour 10. Copy what is there in table SUN, in columns GHA and Dec.

You will receive:
GHA 10 = 328° 51,2'  ;   Dec = N 14°15,4'

On the bottom of the column Dec. there is an amendment d=0,8. Bequeath it to yourself aside. You received GHA and Dec. for the 1000 hour. It is necessary to mend it for minutes (06m) and seconds (29s). Open RA on 'yellow pages', that is 'Increments and corrections'. Find the table '6m', enter her with the number '29s', read out off the table 'Sun, planets' number 1°37,3'

To the neighbouring table /v or d/ enter number d=0,8 (amendment of declension). Read out off the table Corrn number 0,1.


GHA 10 = 328° 51,2'
6m29s     1° 37,3'
GHA = 330° 28,5'

Attention 1: you always add amendments to GHA, according to with definition, GHA always grows in the 360° system with the passage of time.
Attention 2: amendment to Dec. to add or to take, depending on the season, because declension of the Sun he is growing from 22 December to 22 June and he is decreasing then. To test it straight, looking at the Dec. number from the next hour. If is bigger - add the amendment, if smaller - take

They are entering the table HD 605 with data: LHA, Dec., φ, rounded off to full grades.

You are accepting:
φ=47° (closest for your φ summed up)

Needed for you still LHA in full grades..
You are calculating:

GHA = 330°28,5'
- λ = 10°28,5'
LHA = 320°00,0'


Attention next time:
if you are to the east of Greenwich, the pattern will be then GHA + λ = LHA. Accept such an end to your longitude then so that she gives the neat grade after adding to the GHA end.
For the example:

GHA = 330°28,5'
+ λ = 010°31,5'
LHA = 341°00,0'

Open now HD 605, the 4th volume, on the page 40°, 320° LHA (alone name).

You are entering the table 47° with number Dec. = 14°
Prescribe yourself:


You have the azimuth ready, because LHA > 180. If LHA < 180, it is an azimuth = 360 - z, (look attention to the right upper margin of the HD 605 page)
The height is given for declension 14°. It is necessary to correct her about 15,3', well the number really 14°15,3'.
Open the left hand of the volume. Title: "Interpolation table".
Enter the table Dec. inc. with number 15,3. Amendment d=47,8 divide in decades (tens)=40, unities (units)=7 and the tenth parts (decimals)=8. Off the table 40 tens read out 10,2. On crossing tables units 7 i decimals 8 read out 2,0.

Add:   43°08,7'
Hc = 43°20,9'

Summary: if you were really in the point about geographic coordinates φ=47° ; λ=10°28,5', he azimuth of the Sun would take it out 121°, height whereas 43°20,9'.

Calculate the difference height for the measured with the sextant and calculated:

Hs = 43°45,3'
- Hs = 43°20,9'
Δh = 24,4'

Remember the rule next time: if hs > hc, to Δh put back to the Sun, if hs < hc - from the Sun.
Practical drawing:


5 i 6. We are after a map now





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Admission, theory, notions, amendments
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Good pointers, the culmination of the Sun



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