Lesson previewBuild the Hs-to-Ho Evidence Chain1 section shown
Preview lesson
Build the Hs-to-Ho Evidence Chain
What you will learn
Name every stage from measured sextant altitude Hs to observed altitude Ho and preserve enough evidence to audit each correction.
A useful sextant record is more than one angle. It links the identified body and limb, exact observation time, Hs, index evidence, height of eye, horizon method, dated correction data and signed working to the final Ho.
Use three named altitude states. Hs is the angle read from the sextant. Applying index correction and dip forms apparent altitude Ha. Applying the body- and limb-specific correction forms observed altitude Ho. USNO describes the same separation: instrumental and dip corrections produce apparent altitude, then refraction, semidiameter and parallax components produce Ho.
The labels are an error-control system. They prevent a correction from being silently applied twice, make a sign reversal visible and show whether the value belongs to the instrument, visible horizon or celestial body. Preserve the original Hs even after deriving Ha and Ho.
A worksheet is only as sound as its inputs. This lesson supplies constructed correction values so you can practice the chain; it does not generate values for a real sight. For an observation, use the exact current publication and its stated body, limb, altitude, date and conditions.
Sextant correction workbench
Keep every correction named, signed and traceable
Compare six worked states. Each one shows its inputs, arithmetic, result and non-proof in text, so no meaning depends on colour or on a remembered mnemonic alone.
- 1Measured altitude Hs
- 2Index correction
- 3Dip
- 4Apparent altitude Ha
- 5Body and limb correction
- 6Observed altitude Ho
Inputs to preserve
Illustrative Sun lower-limb exercise: Hs 48°00.0′, index correction +1.0′, dip −2.5′ and the supplied daily lower-limb correction +15.2′.
- 1Measured sextant altitudeHs 48°00.0′
- 2Apply index correction+1.0′ → 48°01.0′
- 3Apply dip−2.5′ → Ha 47°58.5′
- 4Apply supplied Sun lower-limb correction+15.2′ → Ho 48°13.7′
Result
The worksheet preserves two subtotals: Ha after instrument and visible-horizon corrections, then Ho after the body-and-limb correction.
What it does not prove
The correction values belong only to this teaching exercise. A real sight needs the exact current publication, body, limb, altitude, date and stated conditions.
Worked example
A constructed Sun lower-limb exercise gives Hs 48°00.0′, index correction +1.0′, dip −2.5′ and a supplied daily main correction +15.2′.
- 1Apply index correction once: 48°00.0′ + 1.0′ = 48°01.0′.
- 2Apply dip: 48°01.0′ − 2.5′ = Ha 47°58.5′.
- 3Apply the supplied Sun lower-limb correction: 47°58.5′ + 15.2′ = Ho 48°13.7′.
- 4Retain the body, lower-limb label, UTC, Hs, both subtotals and the source of every correction.
Sense check: The signed corrections total +13.7′, so Ho must be 13.7′ greater than Hs. That independent net check agrees with 48°13.7′.
| State | What has been applied | Evidence to retain |
|---|---|---|
| Hs | No altitude corrections | Body, limb, exact time and sextant reading |
| Ha | Index correction and dip | Index evidence, height of eye and horizon method |
| Ho | Body- and limb-specific main correction | Dated table or exact maintained method and arguments |
Write an evidence-first correction ladder
Before reducing a sight, rule one line each for body and limb, UTC, Hs, index correction, dip, Ha, main correction and Ho.
1. Name
Write the body, limb and altitude state instead of carrying an unnamed number.
2. Sign
Copy each correction with its plus or minus sign and source argument.
3. Check
Compare the direct Hs-to-Ho change with the sum of the signed corrections.
Sense check: A second person should be able to reconstruct Ho without guessing which body, limb, table, sign or height was used.
Common mistake or limitation
- Calling Hs, Ha and Ho simply 'altitude' and losing track of which corrections are already included.
- Using a plausible main correction without retaining its dated source and arguments.
- Overwriting Hs after calculating Ho and destroying the independent audit trail.
Recap
- Index correction and dip form Ha.
- Body- and limb-specific corrections form Ho from Ha.
- Names, signs, inputs and sources are part of the answer—not optional paperwork.
Optional quick check
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