Nautical Issues
navigational astronomy :: the moon, planets and stars
 
Navigational Astronomy
The Moon, Planets and Stars

Moon

The principle of obtaining the positional line from the Moon is the same like from the Sun. However there are certain differences in details:

  • a v amendment, added to GHA is reaching
  • a HP amendment is reaching (horizontal parallax), needed for correcting the height.
  • you are taking amendments from RA from the different place to the height of the Moon and these amendments are very big essentially.
  • an amendment is reaching (0,5° !), if you are measuring the uphill edge up.

Example: 22.08.1980 20h 42m 15s hour
You measured the bottom edge of the Moon: h = 27°45,0'
Correcting GHA and Dec.:
To RA, table Moon, you are entering with hour 20.

GHA v Dec. d HP
344°21,1' 7,5 S 19°31,7' 1,4 57,9

Correct: GHA about 42m15s +v, and Dec. about d (table MOON in "Increments and corrections")

 
 
344°21,1'
    
   
10°04,9'
  (amend.on 42m15s)
Dec.= S 19°31,7'
   
+ 5,3 
  (amend.on v)
- 1,0 
GHA =
354°31,3'
  
Dec.= S 19°31,7'

 

Attention 1: You always add all amendments to GHA. In this case, amendment on Dec. you are subtract because the declination is getting smaller that day (compare with hour 2100). Sequel as at the Sun.
Calculate LHA according to of pattern LHA = GHA ± λ. Enter LHA, φ and Dec. to HD 605
   
Attention 2: Your latitude is N and declension of the Moon is S, enter: contrary name.

Correcting the height: all amendments are on two last RA pages.

height from sextant =
27°45,0'
    
 
cc  + 0,3'
    
 
27°45,3'
    
 
ci  - 3,0'
    
 
27°42,3'
    
 
k  - 3,3'
   (You are entering the DIP table with the height of the eye =3,5m)
 
27°39,0'
    

You are entering the column with this 25° - 29°, at the vertically enter 27°, at the horizontally enter 40'. You are reading out 59,9'. Go to the bottom with this column far. On crossing of the table the L (Lower limb) and numbers 57,9 off the table HP (horizontal parallax) read the amendment 5,2'.

Add it everything:

27°39,0'
  
59,9'
  
5,2'
  
h = 28°44,1'
  
   

Attention! If you are measuring the uphill edge up, differences are two:

  • finishing touch (HP) you are taking from the U table (Upper limb), not from L (Lower limb)
  • from the final result subtract 30'.

Practical remarks:

1. You can in the month through a few days do two positional lines at the same time to the Sun and the Moon, so you have the position at once then.

2. Always check in the Moon whether his declension is growing whether is decreasing (you are adding the d amendment then or you are taking), because his declension changes less more every two weeks from north on south and the other way round.

3. You can obtain the latitude from the Moon if you catch his culmination. You are taking the time of the culmination from the table Moon, Mer. Pass. Upper. Calculations like by the culmination of the Sun.

 

 

Planets

In practice lines are drawn from Venus and Jupiter, which are very bright. Principle - as at by the Sun. You are taking data on the left RA off Venus and Jupiter tables.

Differences:

1. ...

 

To be continued

 

 

 

Next chapter:
The North Star and Navigation after Great Circle

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