Star finder ? How to use it?

Star finder is used to choose suitable stars for celestial observations and for identifying stars. 

Criteria for choosing  the stars should be as follows

1. Angular separation should be as close as possible to 90°. 
2. Chosen stars magnitude should be low i.e. they should be bright. 
3. Altitude should be between 20° – 70°

1. For morning twilight find out LMT Nautical Twilight and for evening twilight find out LMT Civil Twilight 
2. Apply LIT to get GMT and ZT to get SMT. 
3. Now apply run to last known position to get DR Latitude and  
4. Longitude from current SMT to the SMT which obtained. 
5. Now from almanac get GHA Aries 
6. Apply longitude to get LHA Aries. 
7. From DR Latitude  and LHA Aries go to star finder and using the correct template closest to DR Latitude choose 3-4 stars. 

For identifying stars just get the azimuth of star and calculate LHA aries and latitude is known so use suitable template and put the pointer on LHA aries and using the azimuth find the star.

ADMIRALTY STAR FINDER (NP323)

It  consist of the star chart for the northern and southern hemisphere together with eight transparent templates for the latitude 0 to 60 and 75 degree .these charts shoes the total 57 stars tabulated in nautical almanac.  

Star Sight? 

1. First we need to choose suitable stars which can be found using the Star finder.
2. For morning twilight find out LMT Nautical Twilight and for evening twilight find out LMT Civil Twilight
3. Apply LIT to get GMT and ZT to get SMT.
4. Now apply run to last known position to get DR Latitude and Long from current SMT to the SMT obtained.
5. Now from almanac get GHA Aries and apply longitude to get LHA Aries.
6. From DR Latitude and LHA Aries go to star finder and using the correct template closest to DR Latitude choose 3-4 stars.
7. Now for morning sight start with the stars on eastern horizon as they will disappear from horizon first, take the altitude and azimuth note down the time for all the stars, for evening sight we will have to set the sextant at obtained altitude and azimuth from star finder for stars on eastern horizon as the eastern horizon will disappear first. 
8. Use 3 stars for plotting purpose. Apply run to the obtained PL using Intercept to nearest hour. Point of intersection of the PL’s would be your position. 

Morning Sight?

1. First we need to choose suitable stars which can be found using the Star finder. 
2. For morning twilight find out LMT Nautical Twilight apply LIT to get GMT and ZT to get SMT. 
3. Now apply run to last known position to get DR Latitude and Longitude from current SMT to  
4. the SMT obtained. 
5. Now from almanac get GHA Aries and apply longitude to get LHA Aries. 
6. From DR Latitude and LHA Aries go to star finder and using the correct template closest to
7. DR Latitude chooses 3-4 stars. 
8. Criteria for choosing the stars should be as follows 
9. Angular separation should be as close as possible to 90°. 
10. Chosen stars magnitude should be low i.e. they should be bright. 
11. Altitude should be between 20°-70°. 
12. Now for morning sight start with the stars on eastern horizon as they will disappear from horizon first, take the altitude and azimuth note down the time for all the stars, 
13. Use 3 stars for plotting purpose. Apply run to the obtained PL using Intercept to nearest hour. Point of intersection of the PL’s would be your position. 

  WHY WE USE IN MORNING TIME THE LONG BY CHRON METHOD 

Long by Chron is preferred in morning because:

• It is easier to get longitude at noon with “C” correction no need of plotting. 
• Accuracy of fix increased with PL observed with Long by Chron the angle of cut is as close to 90°. 
• In morning the altitude is changing quite fast as compared to Azimuth and in the formula of intercept this fast change of altitude can cause error in calculation of TZD and hence error in intercept whereas in the long by Chron the True Alt is in numerator so a small error does not affect the accuracy of fix a lot. 

NOON SIGHT

1. Get the LMT Meridian pass from daily pages of Almanac.
2.  Calculate DR Position from LMT obtained and apply LIT to it get the GMT and then apply ZT and get SMT
3. Now apply run and get DR Position for the SMT
4. Now take this DR Longitude and apply LIT from this longitude to LMT get GMT apply ZT and get SMT.
5. Repeat this process twice so as to get more accurate LMT Meridian passage, this process is called iteration.
6. Calculate DR for meridian passage from morning Observed longitude and DR latitude.
7. 10 minutes before set the sextant with sun on the horizon, keep bringing the sun down as it goes up.
8. The moment when sun stops going up and starts going down note the time and the highest altitude obtained will be the meridian passage altitude
9. Using this altitude calculate MZD and observed latitude and apply ‘C’ correction and obtain longitude for noon observation

EVENING SIGHT

1. First we need to choose suitable stars which can be found using the star finder.
2. For evening twilight find out LMT Civil Twilight
3. Apply LIT to get GMT and ZT to get SM T.
4. Now apply run to last known position to get DR latitude and longitude  from current SMT to the SMT obtained.
5. Now from almanac get GHA Aries and apply       longitude to get LHA Aries
6. From DR Latitude and LHA Aries go to star finder and using the correct template closest to DR Latitude and choose 3-4 stars.
7. Criteria for choosing the stars should be as follows 
8. Angular separation should be as close as possible to 90°. 
9. Chosen stars magnitude should be low i.e. they should be bright. 
10. Altitude should be between 20°-70°. 
11. Now for evening sight we will have to set the sextant at obtained altitude and azimuth from star finder for stars on eastern horizon as the eastern horizon will disappear first.
12. Use 3 stars for plotting purpose. Apply run to the obtained PL using Intercept to nearest hour. Point of intersection of the PL’s would be your position.

NOON POSITION

During the morning and evening twilights several stars and planets are present, that time we can get the sight by star or by planet and with help of the position lines we can get the fix, During the noon time we will be having sun as the only celestial body which can assist us to get the noon position 

In the noon time we can get the position by 2 ways 

• Latitude by meridian altitude method 
• Ex-meridian method   

1. Latitude by meridian altitude:
2. First of all find out approx. LMT from almanac Daily pages.
3. Apply LIT to get GMT for the time of observation 
4. Now check the sextant for any error (Perpendicularity,side or index)
5. Determine the index error
6. Approximately 15 min before, after doing calculation of meridian passage time clamp the sextant and bring sun to the horizon.
7. Keep the sextant on same position and tilt the sextant till the sun not rise and makes the strait line for the some second. (After that the sun image start dipping) 
8. Note down the time of this observation. 
9. After doing calculation we will get the meridian altitude of sun.
10. The position will be the position of suns meridian passage time of observation 
11. Now transfer the position line of the morning sight to the position line of the latitude of the meridian altitude, where its intersect that is the apparent noon position.
12. After Appling the run to between the time of position obtained and noon time we get the true position.

EX-MERIDIAN METHOD:

1. We use this method when we are not able to take sight at the meridian time due to cloudy weather.
2. When we getting the sight at meridian passage time and could not take the sight that time we will find out the ex-meridian limit by either nourish table or by approximation method by latitude and declination. 
3. After getting the observation we find out the position this is the position of time of observation.
4. After this we run the morning position line and get the fix 
5. After Appling the run to between the time of position
6. obtained and noon time we get the true position.
7. By this way we get the noon position .