Thursday, January 1, 2009

Celestial Navigation Observations

Complete Solution for Celestial Navigation Sights
1. After reading this you will be able to compute a complete celestial observation of the Sun using the Nautical Almanac and the Sight Reduction Tables for Marine Navigation, Pub 229 and plot a celestial line of position.

The December 2008 blogs have dealt with all the aspects needed for determining a line of position from an observation of a celestial body. This will give you the complete solution using the Nautical Almanac and the Sight Reduction Tables for Marine Navigation Pub 229. The steps involved will be covered in the order in which they are to be taken.

Pub. 229 Method
Pub. 229 Sight Reduction Tables for Marine Navigation is a set of six volumes of pre-calculated solutions for the computed altitude (Hc) and the azimuth angle (Z) of the navigational triangle. Entering arguments for the tables are local hour angle (LHA) expressed in whole degrees. This is done by using an assumed longitude, vice a DR longitude, assumed latitude in whole degree, and declination. Values of Hc and Z are tabulated for each whole degree of each of the entering arguments. Tables inside the front and back covers of each volume allow for interpolation of Hc and Z for the exact declination. No interpolation is necessary for LHA or assumed latitude.

Working Sights With Pub. 229
To work a sight with Pub. 229, you enter the tables by selecting the proper volume and turning to the page with the appropriate LHA. Using the assumed latitude and declination extract the tabulated values for Hc and Z. You then determine the exact value of Hc and Z corresponding to the time of observation by interpolation by using the interpolation tables or using the formula that the tables are based upon. For our purposes I will use the formula method.

To find the intercept distance (a), this final Hc is compared to the observed altitude (Ho). If the computed altitude Hc is greater than observed altitude Ho the intercept is AWAY from the direction of the GP of the body. If the Ho is greater than the Hc it is TOWARDS the direction of the GP of the body. When working out a celestial sight a form should be followed so that there will be less chance for leaving out any pertinent information.

Sight Reduction
You will need a Sextant, a Watch, Nautical Almanac the Tables HO 249 or HO 229 marine or air sight reduction tables, I prefer HO 229. For sighting stars, a Star-Finder No. 2101-D, Parallell Rulers or Triangles, Dividers, 0.5 mm Pencil with a good eraser, Scratch Paper, and Universal Plotting Sheets.

With the "intercept" method, you will be comparing the position you think you might be in from dead reckoning on a boat, with what you actually observe. Your observed altitude is compared to a calculated altitude, calculated to be what altitude you would get if you were actually at the position you chose as your assumed position. You have to observe an altitude with the sextant and put your figures on your worksheet and along with the tables what the altitude would be if seen from the assumed position.

The "v" is an extra correction for additional longitude movement of the body, and "d" is an extra correction for additional declination movement. The sun has no "v" correction and the stars have no "v" or "d" correction. The sun needs the "d", and the planets and moon need both "v" and "d".
These steps are for a SUN LINE only.
Step 1: Setup a plotting sheet, DR ahead and enter it in your format.

Step 2: Apply your IE, if it is on the arc, subtract it, if it is off the arc add.

Step 3: Using the DIP Table on the inside cover of the Nautical Almanac, enter with your height of eye. Dip correction is always a minus correction.

Step 4: On page A2 "Altitude Correction Tables 10° - 90° Sun, Stars, Planets" Enter with your Ha and find the Altitude Correction. Remember that Lower limb are always + corrections and upper limbs are - corrections for the sun.

Step 5: Compute your corrected chronometer time.

Step 6: Using GMT, and Greenwich date of observation, enter the Nautical Almanac and record tabulated hourly value of GHA and Tab. Dec. in your format.

Step 7: At the foot of each declination sub-column, get the"d corr". This number is called the "d" correction. This is the average over the three day period that the declination changes per hour of GMT. The "d" is recorded on the "d corr." line off to the side. This is a correction, as with any correction, it is either a + or - . If the declination is increasing (getting larger), then it is a plus ( + ) correction, if the declination is decreasing, then it is a minus ( - ) correction.

Step 8: Turn to the yellow pages of the Nautical Almanac, and find the minute page, enter with your seconds. Then under the Sun and Planets column find the increase in the sun's GHA since the last tabulated (hourly) value, this is your M & S correction and enter this in your format. Always add the GHA and "M & S ''tabulated value together to get the total GHA.

Step 9: While on the minute page under the "v" or "d" correction column, find the "d" on the left hand side. This will be the declination (dec) of the sun at the time of sight.

Step 10: In full sight reduction, LHA has to end in the whole degree and YOUR ASSUMED LONGITUDE has to be within 30' of your DR Longitude.

This is where it's easy to make a mistake.
EXAMPLE for East Longitude
Say your DR Longitude is 93° 58.0 E
GHA 224° 52.9
A Long +94° 07.1 E
LHA 319° 00.0

EXAMPLE for West Longitude
Say your DR Longitude is 93° 58.0 W
GHA 224° 52.9
A Long. - 94° 52.9 W
LHA 130° 00.0

GHA 224° 52.9
A Long. - 93° 52.9 W
LHA 131° 00.0

Enter Pub. 229 with LHA, ALat, and Dec. Then extract the tabulated Hc, base Z, Z for the next whole degree of declination, and the rule for converting Z to Zn and enter this into your format. Make sure to note the sign of the differences. Then using the formula:
Difference x declination increments ÷ 60 = correction, determine your corrections.

Step 12: Add or subtract your corrections to your Tab Hc and base Z to get the Hc and Z.

Step 13: Fill in your Ho. Find the difference between Hc and Ho, this will give your altitude intercept (a). Next you must determine if (a) is; (A-away or T- towards the bearing Zn). You say to yourself, Coast Guard Academy-Computed Greater Away, if not then it is towards, Ho Mo To.

Step 14: Compute the Zn by following the rule.

Step 15: Fill in your assumed latitude (ALat) and assumed longitude (ALong).