MARITIME – CELETIAL NAVIGATION CALCULATOR
Sextant Altitude Correction
A precise tool.
What is the Sextant Altitude Correction & How does it work?
In celestial navigation the altitude read on a sextant (Ho) must be corrected for several systematic errors before it can be used in sight reduction. The most common corrections are the dip of the horizon, atmospheric refraction, and the semi‑diameter of the observed body (the Sun’s radius or the Moon’s apparent radius). Each correction is expressed in minutes of arc and is subtracted from the raw sextant reading. The dip correction (D) accounts for the observer’s height above sea level; the higher the eye, the farther the visible horizon, which makes the apparent altitude slightly lower. Refraction (R) bends light rays toward the observer, making celestial bodies appear higher than they truly are; this effect grows rapidly near the horizon. The semi‑diameter (S_D) is added when the observed limb of the body is used (e.g., the upper limb of the Sun) and must be subtracted to obtain the centre altitude. The corrected altitude (Hc) is therefore calculated by removing all these biases from the observed sextant altitude. The relationship is compactly expressed as:
H_{c}=H_{o}-D-R-S_{D}
Hc = corrected altitude, Ho = observed sextant altitude, D = dip correction, R = refraction correction, S_D = semi‑diameter correction
Parameters
Result
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Frequently Asked Questions
What is the purpose of sextant altitude correction?
Sextant altitude correction adjusts the raw reading from a sextant to account for atmospheric conditions and observer height, improving navigational accuracy.
How do I calculate the dip correction for my sextant reading?
Dip correction is calculated using the formula D = 1.74′ * sqrt(h), where h is your height above sea level in feet.
What is atmospheric refraction, and how does it affect sextant readings?
Atmospheric refraction is the bending of light through Earth’s atmosphere, causing celestial bodies to appear higher than they actually are. It needs to be subtracted from the sextant reading.
How do I account for the semi-diameter of the observed body in my corrections?
Subtract the semi-diameter (radius) of the observed celestial body from your corrected sextant altitude to get the true altitude.
Can you explain why these corrections are necessary in maritime navigation?
These corrections ensure precise celestial observations, which are crucial for determining a ship’s position using celestial navigation techniques.
What is the order of applying these corrections to my sextant reading?
Apply dip correction first, then atmospheric refraction, and finally subtract the semi-diameter of the observed body.
How do I convert minutes of arc into degrees for easier calculations?
To convert minutes of arc to degrees, divide by 60. For example, 30′ is equal to 0.5°.
Results are for informational purposes only and do not constitute professional advice.