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Examples of correcting a sight

Feb 28, 2019

  
Here is a sample problem of sight correction. On June 15, you take a lower limb sight on the sun and get an Hs of 62° 12.6’. The sextant had an index error of 4.1’ off the arc and the height of eye was 10 feet.

The first correction to apply to Hs is the index correction. Remember: If the error is off the arc, put the correction on or add it to Hs. Next, take the height of eye to the dip table located in the front inside cover of the Nautical Almanac. Follow down the column for feet until you reach 10.5 (which is the first number larger than our 10). Because 10 is between 9.8 and 10.5, move to the left from between those two numbers. The dip correction is -3.1’.

Total up Hs, the index correction and the dip to get the Ha of 62° 13.6’. Now take that Ha to the main sun correction table. Because we took the sight in June, choose the right-hand column for sights taken from April to September. Move down the “App. Alt.” column to between 61° 51’ and 67° 17’. Move right to the column for the lower limb and find the correction as +15.5’. Apply this to Ha, and the result is an Ho of 62° 29.1’.

The arithmetic for the problem follows:

Hs
index correction
dip
Ha (app. alt.)
main sun correction
Ho (corrected sight)
  62°12.6’
+        4.1’
–        3.1’
62°13.6’
+      15.5’
62°29.1’


Here are some additional problems:

1. On Oct. 3, you take a lower limb sight of the sun. The Hs is 43° 19.5’. Your sextant has an index error of 7.2’ off the arc and your height of eye is 9 feet. What is the Ho?

2. On May 23, you take a lower limb sight of the sun. The Hs is 8° 16.8’. The index error is 3.6’ on the arc and the height of eye is 10 feet. What is the Ho?


Answers:

1. 43° 39.0’

2. 8° 19.7’

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