NauticTools2Celestial navigation
Index error – checking the sextant with the sun
Hardly any sextant reads exactly zero when set to zero. The index correction compensates this residual error and goes into every set of sextant altitude corrections. NauticTools2 finds it from two readings on the sun – off the arc and on the arc – and checks the sights straight away via the measured sun's radius.
- Off the arc
- On the arc
- Index correction IC
- Sun's radius
- Accuracy check
- Saved to Settings
How is it measured?
With the sun shades in place, point the sextant at the sun and set it near zero. Two sun images appear in the telescope: the direct and the reflected one. First move the reflected image across the direct one until the limbs touch and read off – the index arm is then just off the arc, before zero. Then do the same the other way, just past zero on the arc. The two readings are one sun's diameter either side, four radii in total; the midpoint between them is the true zero.
When do you need it?
- Before every round of sights: The index error can change with temperature and knocks. Found shortly before the sights, it is most reliable.
- Sextant altitude corrections: The index correction is the first correction – an error of 1′ shifts every position line by 1 nm.
- Adjusting the sextant: If the index error grows beyond a few minutes, it is worth adjusting the horizon mirror.
Formulas and working
Index correction
Half the difference of the two readings. If the reading off the arc is larger, the true zero lies off the arc and IC is positive; otherwise negative.
Measured radius
A quarter of the sum of both readings.
Check
Compare with the true sun's radius, about 15.8′ in July to 16.3′ in January. If they differ by more than about 0.2′, repeat the sights.
Worked example
On 22 October 2026 you read 31.2′ off the arc and 33.2′ on the arc. What is the index correction?
IC = (31.2′ − 33.2′) / 2 = −1.0′
Measured radius = (31.2′ + 33.2′) / 4 = 16.1′
True sun's radius 16.1′ – the sights were good
Things to watch out for
- Sun shades: Never look at the sun without adequate shades in front of the telescope and mirrors – it damages your eyes permanently.
- Reading off the arc: Off the arc the micrometer drum runs “backwards”. Read the amount before zero, not the figure on the drum itself – this is where most mistakes happen.
- Take several readings: Three readings each way and their mean give a much more accurate result.
- Sign: The index correction is the opposite of the index error – NauticTools2 consistently works with the correction.
How it works in NauticTools2
Enter the readings
Off the arc and on the arc in minutes of arc, plus the date for the sun's radius.
Calculate
Index correction, true and measured sun's radius appear in the result.
Save it
“Use as reference value in Settings” stores the index correction; sextant altitude corrections and fix from altitudes then fill it in automatically.
Frequently asked questions
How do you find the index error with the sun?
Bring the direct and reflected sun images into contact once off the arc and once on the arc and read both values. The index correction is half the difference: (off − on) / 2.
What is the measured sun's radius for?
It is a built-in check. If it agrees within about 0.2′ with the true radius, both readings were good.
Can you do it without the sun?
Yes, on the horizon or a star: bring the reflected and direct images into line and read off. The sun method is more accurate, though, because it gives two readings and a check.
Related tools
- Sextant altitude corrections – from the reading Hs to the observed altitude Ho
- Vertical sextant angle – the sextant in coastal navigation
- Fix from 2 or 3 altitudes – position lines and the fix
NauticTools2 is free and runs on Windows 10 and 11. An Android version is in preparation.
Get it from the Microsoft Store