Site Reduction Form

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Because Computers Can Do This Faster... Wait... What?

This is the basic format for reducing sights. It is a bit tedious but makes sense after enough practice.

Name of Body
Full name of body (Sun, Moon, star) with Greek symbol for stars and whether it is an upper or lower limb observation for the sun and moon.

DR Lat

Latitude of dead-reckoning position for time of observation

DR Long

Longitude for dead-reckoning position at time of observation

CT
Time according to ship's chronometer when the observation was taken
CE
Known chronometer error. CE is given to you in Coast Guard problems, or can be found in tabular or graphical format on the bridge.
(+/-) ZD
Zone Description ([+W/-E Longitude]/15)
GMT/Date
Sum of the above (CT,CE, and ZD). All of the times listed in the Nautical Almanac are in GMT, so you need to get this right.

Hs
Raw sextant altitude (what you observed and noted from the sextant alone)
I
Instrument Error (Always published somewhere. Only applies to that particular sextant)
IC
Index Error (If you find the error to be "on" the arc, subtract the value, i.e. take it "off". If the error is "off" the arc, add the value, i.e. put it "on". "If it's on take it off, if it's off put it on.")
-Dip
Correction for the difference between your height of eye and the center of the earth. Value is always subtracted and is found on the first page in the NA, or by the equation .97√(Height of eye)
Ha
Apparent Altitude, equal to the sum of Hs, I, IC, and -Dip.
Note: This is the value you enter the NA with for the remaining sextant corrections.
A2
Altitude correction due to refraction. Found on the first yellow page of NA
A4
Altitude correction for Non-Standard Conditions. Found on the third yellow page of the NA. Accounts for variations in temperature and barometric pressure.
HP
Horizontal Parallax, only applied when observing the moon. Found in the Daily Pages of the NA.
Ho
Observed Altitude, Sum of Ha, A2, A4, and HP corrections, used in determining intercept.

aLat
Assumed Latitude. Found by rounding your DR latitude to the nearest whole degree.
GHA
GHA for whole hour increment of GMT, found in the NA under the daily pages.
+ m/s
Minutes and seconds correction for remainder of time after whole hour (used to find GHA) until time of observation. Conversion of arc to time can be done with the table on the first yellow page in the back of the NA.
GHA
Corrected GHA. Sum of initial GHA and m/s correction.
+/- aλ
Assumed Longitude. For west longitude, minutes value must be the same as minutes value of GHA, and total value must be within 30 minutes of DR Long. For east longitude, the minutes value is equal to 60 - the minutes value of GHA (or the sum of the GHA and aλ is equal to 60), and total value must be within 30 minutes of DR Long. The idea here is to get a whole number value for LHA (1, not 1.1) for entering into Pub 229.
LHA
Local Hour Angle of body, equal to the sum of GHA and aλ, and must be a whole number value (integer) in order to enter into Pub 229. LHA determines which page to use.

Dec
Declination: found in daily pages of NA for whole hour value of GMT
+/- d corr ( )
Correction for additional minutes. Note the "d" value at the bottom of the column, place value in ( ) and turn to Increments and Corrections (yellow pages) in the back of the NA. For the correct minutes, find the correction for the corresponding "d" value.
Dec
Sum of initial Dec and "d" correction

Ht
Initial Predicted Altitude of body at the given time (found in the NA)
Corr
Altitude Corrections (found through interpolation in Pub 229)
DSD
Double-second-difference correction
Hc
Computed Altitude (Ht+Corr+DSD)
Ho
Observed Altitude (as found above)
"a" (T/A)
Intercept (If Ho is more, intercept is toward the GP of the body)

Z
Azimuth Angle (found in NA)
Z corr
Calculated Correction
Z
Corrected Azimuth Angle
Zn
Actual Azimuth to the body