How To Find Sextant Index Error By Celestial Observations

How to find Index Error of the Sextant by Sun, star or by using the horizon? This is one of the most common questions that is asked by the Examiners when one appears for the Competency Exams. The candidate often gets confused when this type of question is asked. Its answer is pretty simple, let us understand how this question should be answered.

First of all, one should be aware of the names of various parts of Sextant. Refer following image explaining the names of all parts of sextant:

Let us understand what is Index Error:

Index Error is caused when the Index glass and Horizon glass are not parallel to each other when the index bar is set to zero.

Now, let us see what is the answer to the above question.

Checking Index Error By Horizon Observation

• Clamp the index bar at zero.
• Hold the sextant vertically and view the horizon through the telescope.
• If true horizon and its reflected image appear in one line, Index Error is not present.
• If they appear vertically displaced that means Index Error is present.
• To eliminate the Index Error, clamp the Index bar at zero. Look through the telescope, turn the third adjustment screw, till the true horizon and its reflection appear in alignment.

Checking Index Error By Sun Observation

• Clamp the index bar at zero.
• Hold the sextant vertically.
• Using necessary shades, view the sun through the telescope.
• Turn the micrometer “On The Arc” until the Upper limb of the reflected Sun touches the lower limb of the True Sun.
• Note down the reading “On The Arc”.
• Turn the micrometer “Off The Arc” until the Lower limb of the reflected Sun touches the Upper limb of the True Sun.
• Note down the reading “Off The Arc”.
• If the two readings are same, it indicates no Index Error is present.
• If there is a difference between the two readings, it means Index Error is present.
• To find the value of the Index Error, take the difference of the two values obtained by above method and divide it by 2.
• “On the Arc” or “Off the Arc” should be given according to whether the “On the Arc” or “Off the Arc” reading was larger.

Example:

Reading on the arc= 31.6

Reading off the arc= 31.2

Hence, Index Error=(31.6-31.2)/2= 0.20 On the Arc (as On the Arc reading is greater than Off the Arc reading here.)

When Index error is calculated by above method, it’s accuracy can be verified by the following method:

• Obtain the Semi-diameter of the sun = Sum of two readings/4 i.e. (31.6+31.2)/4 = 15.7′.
• Get the True Semi-diameter of the sun from the almanac for that date.
• If the value of Obtained Semi-diameter and True Semi-diameter of the sun are equal, it means the value of Index Error obtained is correct.

Checking Index Error During Night Time By Star Observation

• Clamp the index bar at zero.
• Hold the sextant vertically, observe a star through the telescope.
• If the star and its reflection are not displaced vertically, it indicates Index Error is not present.
• If they appear vertically displaced that means Index Error is present.
• To eliminate the Index Error, clamp the Index bar at zero. Look through the telescope, turn the third adjustment screw, till the true star and its reflection appear in alignment.

Hope this will prove to be helpful for you. Do let us know your views in the comment section. We will love to hear from you..!!