Lesson previewDefine the Meridian Altitude and Its Evidence1 section shown
Preview lesson
Define the Meridian Altitude and Its Evidence
What you will learn
Distinguish upper meridian transit, corrected meridian altitude and latitude output, and state what one noon sight cannot establish.
A meridian altitude is the corrected altitude of a body when it is on the observer's celestial meridian. For the Sun's upper transit at ordinary observing latitudes, this is local apparent noon and is near the day's greatest altitude, but it is not automatically 1200 by a clock.
At upper transit the body's local hour angle is zero and it lies on the north-south meridian through the zenith. The body may bear north or south; that observed side of the zenith is part of the calculation evidence, not a label to infer later from the expected latitude.
Use Ho, the observed altitude after the applicable instrument, horizon and body corrections. Zenith distance is z = 90° − Ho. Combine z with the Sun's declination for the correct date and transit time using an explicit signed north/south construction.
The result is latitude at the observation time, subject to sight, correction, timing, declination, movement and arithmetic uncertainty. It does not supply longitude or a complete fix by itself. A transferred earlier LOP can provide another constraint, but then the result is a running position that inherits transfer error.
Meridian latitude workbench
Keep transit, altitude, direction and latitude signs in one chain
Compare six worked states. Each names its evidence, signed operation, result and limitation in text, so no meaning depends on colour or a same-name mnemonic alone.
- 1Plan upper transit
- 2Record timed sight series
- 3Correct selected sight to Ho
- 4Zenith distance z = 90° − Ho
- 5Combine z with signed declination
- 6Compare latitude with other evidence
Inputs to preserve
Constructed planning case: the maintained method gives 12:04 UTC for the Sun on the Greenwich meridian; estimated longitude is 030°00′W.
- 1Convert longitude to time30° ÷ 15° per hour = 2 h
- 2Apply west-longitude senseLocal upper transit is about 2 h later than Greenwich
- 3Form the planning estimate12:04 + 2:00 = 14:04 UTC
- 4Bracket and updateBe ready early; update for movement and the exact current method
Result
The estimate creates an observation window. It is not permission to call the single highest reading at 14:04 an exact meridian altitude.
What it does not prove
Clock 1200, ship's noon and local apparent noon are different concepts; use the current publication or service and its sign convention.
Worked example
A worksheet contains Hs, an approximate noon time and a copied declination but no correction chain, body direction or source version.
- 1Keep the raw sight and exact time; do not relabel Hs as Ho.
- 2Recover the named body, limb, height of eye, horizon method and current correction source.
- 3Establish the upper-transit method, declination at the required time and whether the Sun was north or south of the zenith.
- 4Only then form Ho, z and signed latitude; mark any missing input as a stop condition.
Sense check: A latitude answer cannot be independently reconstructed if another navigator cannot identify the observation, correction, time, declination and body-direction inputs.
| State | Meaning | Not interchangeable with |
|---|---|---|
| Hs | Sextant reading at an exact time | Corrected observed altitude |
| Ho | Observed altitude after applicable corrections | Hc or an uncorrected maximum |
| z | 90° − Ho | Declination or latitude |
| Meridian latitude | Signed combination of z and declination | Longitude or a complete fix |
Brief the evidence before the answer
Report the result as body, event, exact time, Ho, declination, body direction and latitude rather than saying only 'the noon sight gave 41 north'.
1. Event
Name upper transit or the exact ex-meridian method used.
2. Inputs
State Ho, current declination and whether the body bore north or south.
3. Limit
Call the output latitude and name the other evidence needed for position.
Sense check: The report should remain accurate even if the longitude is still uncertain.
Common mistake or limitation
- Calling the largest raw sextant altitude Ho without applying the full correction chain.
- Treating 1200 ship's time as proof that the Sun is on the meridian.
- Presenting meridian latitude as a complete longitude fix or exact two-dimensional position.
Recap
- Upper transit places the body on the observer's north-south celestial meridian.
- Meridian latitude uses corrected Ho, z = 90° − Ho, current declination and body direction.
- One meridian altitude supplies latitude, not longitude or a complete fix.
Optional quick check
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