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| 1 Introduction | 9 Altitude Intercept | 17 Latitude by Meridian Altitude |
| 2 Corrections to Sextant | 10 Using Position Plotting Sheets | 18 Latitude and Azimuth by Polaris |
| 3 Altitude Corrections | 11 Plotting Lines of Position | 19 Running Fixes |
| 4 Time | 12 Summary of 1 thru 11 | 20 Time of Sunrise Sunset |
| 5 Finding GHA and Declination | 13 Finding Deviation or Gyro Error | 21 Star Identification/Selecting for Fix |
| 6 Assumed Position and Local Hour Angle | 14 Finding Azimuth by 229 | 22 Time Tick Problems |
| 7 Computed Altitude and Azimuth | 15 Amplitudes | 23 Deviation Table Construction |
| 8 Interpolation | 16 Time of Local Apparent Noon | 24 Sample Final Exam |
Corrections to Sextant Readings
When you use your sextant to shoot a star or planet, you obtain the angle between the horizon and the celestial body. This is called the sextant altitude, which is abbreviated as HS. The angle will be expressed in degrees, minutes, and tenths of a minute, as shown here.
In order to use this information, it must be corrected for a number of factors. After the corrections are made, the resulting angle will be observed altitude (Ho), which is needed to find distance from the ship to the G.P. the first error to be aware of, is instrument error, which may be found in some less expensive sextants. It is a built in error resulting from the manufacturing process and cannot be removed. A simple mathematical calculation can compensate for it. Good sextants usually have no instrument error. Because the coast guard exam assumes you are using a perfect sextant, all their problems and those in this course will ignore it.
Even a perfect sextant can develop index error (I.E.), which may change every day. The index and horizon mirrors getting slightly out of adjustment cause this error. You check for this each time you use the sextant by lining up the actual horizon with the reflected image of the horizon. When the view through the sextant looks like this, the reading should be 0˚-00'. 0.
Chances are, however, that the sextant may be reading 0˚ 02.5' meaning that it has an index error (I.E.) of plus 2.5'. This is sometimes stated as an i.e. of 2.5' on the arc.
Another sextant might show a reading of 1.2' less than zero, or off the arc. This could also be expressed as an i.e. of -1.2'.
Rather than continually adjusting the mirrors of your sextant, it is easier to apply an index correction (I.C.) to compensate for index error. Let's suppose your sextant was reading 2.5' too high (on the arc) when it should have read zero. In order to bring it back to zero must subtract that amount. You're index correction (I.C.) would therefore be -2.5'
If you're sextant was 1.2' off the arc. Too low, your (I.C.) would be plus 1.2'.
Coast guard problems will give you either the index error or the index correction, so make sure you keep them straight. Here are some practice problems:
1. Your sextant reading is 22˚ 28.6' index error is 2.5'on the arc. What is the corrected reading? Answer ______________________
Answer: because an index error on the arc is too high, the number must be subtracted:
Hs 22˚ 28.6
I.C. -2.5
22˚ 26.1
2. Your sextant is reading 42˚- 13.8'. Index correction is -1.2' what is the corrected reading? Answer____________________
Answer: An index correction is applied directly by following the + or - sign.
Hs 42˚ 13.8'
I.C. -1.2
42˚ 12.6'
3. Your sextant reading is 35˚-29.6'. I.E. is 1.8' off the arc. Corrected reading: _________________
Answer: if the index error is off, you add it.
Hs 35˚ 29.6'
I.C. +1.8
35˚ 31.4'
4. Your sextant reading is 42˚- 05.4'. I.C. IS +2.1, corrected reading________________
Answer: Follow the plus or minus sign when working with I.C. (Index correction).
HS 42˚ 05.4'
+ 2.1
42˚ 07.5'
Try these for practice.
Last edited on 25-Sep-2009 04:19:01 -0500
