Tuesday, May 6, 2008

Celestial Navigation (Index Error)

Even a perfect sextant can develop index error (I.E.),which may change everyday.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.

Rather than continually adjusting the mirrors of your sextant, it is easier to apply an index correction (I.C.) to compensate for index error. Suppose your sextant was reading 1.5' too high (on the arc) when it should have read zero. In order to bring it back to zero you must subtract that amount. Your index correction (I.C.) would be -1.5'.
If your sextant was 2.2' "off the arc". Too low, your (I.C.) would be +.2.2
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 31-28.6 index error is 2.5 on the arc. What is the corrected reading?
Answer: because an index error on the arc is too high, the number must be subtracted:

Hs 31- 28.6
I.C. -2.5
Hs 31- 26.1

2. Your sextant is reading 26-13.8'. Index correction is -1.2' what is the corrected reading?
Answer: An index correction is applied directly by following the + or - sign.

Hs 26- 13.8'
I.C. -1.2
Hs 26- 12.6

3. Your sextant reading is 45-29.6, I.E. is 1.8' off the arc.
Answer - if the index error is "off," you add it.

Hs 45 -29.6
I.C. +1.8
Hs 45 -31.4

4. Your sextant reading is 12- 05.4'. I.C. is +2.1,
Answer: Follow the plus or minus sign when working with I.C. (Index correction).

HS 12- 05.4
I.C. + 2.1
Hs 12- 07.5

Here are some more for practice.
1. Sextant reading: 32-49.8' 1.8 off the arc corrected: 32-51.6

2. Sextant reading: 18-52 .7 1.9 off the arc corrected: 18-54.6

3. Sextant reading: 58-22.0 2.7 on the arc corrected: 58-19.3