Where this fits
- Yachtmasterrelevant
- Coastal Skipper extensionrelevant
Free sextant simulator
How a sextant works, from sight to latitude
See how a sextant doubles angles, align the Sun with the horizon, and work a simplified noon-sight latitude calculation. It is an independent revision aid, not official RYA training or assessment.
Direct answer
A sextant measures the angle between a celestial body and the horizon by double reflection. After applying index, dip, and altitude corrections, a noon Sun sight can be reduced to observed altitude, zenith distance, and latitude.
- Best for
- Index error, dip, observed altitude, noon sight
- Includes
- Sextant instrument, telescope view, calculation ladder
- Practice mode
- Alignment, guided steps, latitude quiz
- Cost
- Free, no login required
ALTITUDE, CORRECTIONS, NOON LATITUDE
Sextant simulator
Look through the telescope, bring the reflected Sun down to the visible horizon, then read and correct Hs.
Goal
Main idea: bring the reflected Sun's lower limb onto the visible horizon, then read Hs.
1
Primary interaction
Move the reflected Sun
Drag the Sun or use the slider. Correct alignment is the Sun's lower limb touching the horizon line.
Sun too high — bring it down.
Drag the Sun or use the slider
Sextant altitude Hs
47°00.0′
Fine adjustment
0.1′ · snaps near horizon
Read Hs only after the lower limb is on the horizon.
2
Correct Hs to Ho
Corrections turn Hs into Ho.
Align the lower limb first.
Once the sight is correct, the worked example reveals Hs, the minute corrections, Ho, zenith distance, and latitude.
This is a simplified teaching model, not a substitute for official nautical almanac tables or real navigation checks.
3
Instrument anatomy
See what moves on a sextant
Move the index arm below. The arm, index mirror, and micrometer travel together; the horizon mirror and telescope stay fixed.
47°
Moves with the index arm
Index arm. Pivots across the arc and carries the index mirror with it.
Moves together
Arm, index mirror, and drum
Fixed references
The horizon stays steady
Moves separately
Flip shades in as needed
4
The sextant observation: step by step
1
Move the reflected Sun down
Use the telescope view, not the instrument drawing.
2
Lower limb touches horizon
The bottom edge of the Sun just kisses the line.
3
Read sextant altitude Hs
Write down the arc reading in degrees and minutes.
4
Apply corrections
Index, dip, and Sun correction turn Hs into Ho.
At noon, use Ho to find zenith distance and latitude.
Link to this tool
Use this canonical URL in club newsletters, lesson notes, or student revision plans.