H.O. Publication No. 229 is one of many types of sight reduction tables, I like 229 just because I am more familiar with it. This PUB relates the position of the sighted celestial body to our ship's real position by means of an intermediate position, an assumed position or AP. This AP is close enough to our DR so that it can be plotted on small position plotting sheets in the vicinity of our DR. From the AP we will plot our lines of position to establish our fix.
H.O. 229 is printed for bands of 15° of latitude. Volume 1 covers 0° through 15° (north or south), Volume 2 covers 15° through 30° etc. Numbers used to enter the volume are local hour angle (LHA) found in the four corners of the book opening, latitude heading each column, and declination on the left and right columns of each page, used to establish the line in the column we want. To find out which page of the opening we need (since LHA for both pages can be the same), we have to find out whether our latitude is the same name or contrary name to the declination. If latitude and declination were both north or both south, name would be SAME. If one value were north and the other south, the names would be CONTRARY. LHA in three of the four corners is the same, in the lower right corner is different than in the other three. The horizontal stepped line on the right page divides the page in half, the lower half belonging to the lower right LHA, the upper half belonging to the upper right LHA. All the entry values to H.O. 229 are whole degrees, with no additional minutes.
To obtain the entry values, first you have to get GHA and Dec of the celestial body and know a rough dead reckoning position near the time of our sight. Assume the position of our DR and of the AP latitude would be the closest whole degree to your DR latitude. The AP longitude needs to be close to our DR longitude, within 30', that when subtracted from GHA (if west longitude) or added to GHA (if east longitude), the resulting number is a whole degree with no additional minutes. Local hour angle is defined as the angle measured westward from our meridian to the hour circle going through the celestial body. The limits of LHA are 0° to 360°. To get the values from PUB 229 we need 5 pieces of information LHA, AP LAT, TAB DECLINATION (in whole degree of declination),DECLINATION INCREMENT (the additional minutes of declination) and is it SAME or CONTARY. Once you have this you should beable to get your Hc (altitude of the celestial body) d (the change in Hc for one degree of declination and its sign + or -,and Z azimuth (bearing of celestial body).
Remember that these numbers are obtained assuming a whole degree of declination.
The Hc will be corrected for the additional minutes of declination. The Z we interpolate by eye in the tables. If our declination increment (Dec Inc) is 20.0, 1/3 of a degree, we interpolate 1/3 of the way between Z in our line of Tabular Values and the Z in the next line down (the line for the declination one degree greater than ours.)
After getting Hc, we then enter Ho from the altitude correction section and subtract the smaller of the two altitudes from the larger. This gives us the intercept, labelled "a". Assuming our DR and sextant sight are accurate and we have reduced the sight properly, the intercept is usually less than 30.0. When you enter H.O. 229, a quick check to see if we have made a error up to that point is to compare the tabular Hc with our Ho. They should be within a DEGREE. Our net correction will reduce this difference.
After obtaining our intercept, you then check to see which value, Hc or Ho, is greater. If Ho is more we circle T which stands for towards, remembered by Ho Mo To. If Hc is greater than Ho, we circle A which stands for away.
Our plotting will be done from the assumed position. The Zn establishes the azimuth of the GP from the AP or RP, assuming they are reasonably close to each other. Our question now is whether we should plot on the bearing of Zn toward the GP from our AP or away from the GP from our AP. In this case we need to plot from the AP away from GP. If Hc is greater than Ho, we plot away from the GP, in the opposite direction of Zn. The intercept is the distance between the two circles of position.
In preparation for plotting we need three pieces of information from our sight reduction form
1) Coordinates of the assumed position
2) Intercept, toward or away
3) Zn
For labelling the line of position:
1) Zone Time of the sight
2) Name of celestial body observed
Once I figure out how to make up my forms on this blog I will type them out so you can see this better. Its not hard and basic math is all you need.
