A sextant reads the angle between a body and the horizon to a tenth of a minute, and a minute is a mile on the chart. Instrument, handling and corrections decide whether the fix is any good.
How do I read a sextant micrometer without getting the degree wrong?
Degrees from the last mark the index passed on the arc, minutes from the micrometer drum, tenths from its vernier. A drum near 57 belongs to the degree not yet reached: just short of 42° reads 41°57.3′.
The micrometer drum moves the index arm along the arc, and the index mirror turns with it. The horizon mirror and the telescope are fixed to the frame: through the clear half of the horizon mirror you see the sea horizon, in its silvered half the body, reflected from the index mirror. Shades sit in front of each mirror.
| Reading | Where |
|---|---|
| Degrees | The last mark passed on the arc |
| Minutes | The micrometer drum |
| Tenths | The drum's vernier |
A degree misread is 60 miles: a drum near 57 belongs to the degree the index has not quite reached. Lift the sextant by its handle or frame, never by the arc, arm or mirrors; dab spray off, dry it, and stow it boxed, wedged low amidships. A gritty drum wants the rack cleaned and a drop of light oil. A metal sextant keeps its index error; a plastic one drifts with heat: check it at every sight.
Do I add or subtract sextant index error?
Index error read on the arc, above zero, is subtracted; read off the arc, below zero, it is added. On the arc, take it off; off the arc, put it on.
Each adjustment can upset the next, so the order is fixed: perpendicularity, side error, then index error. The first is cured by the screw behind the index mirror, the second by the side screw of the horizon mirror. Collimation, the telescope not parallel to the frame, is rare.
A small index error (IE) is not adjusted away but applied to every reading. Read on the arc, above zero, it is subtracted; read off the arc, below zero, it is added, and the drum then counts backwards: 57.0 means 3.0′ off. After a knock, measure it again before every sight.
On the arc, take it off. Off the arc, put it on.
How do I know my sextant is vertical when I take a sight?
Rock the sextant: the body swings in an arc, and only at the lowest point of that arc is the sextant vertical. Tilted, it reads too high.
Swing in the index shades before you look for the Sun; a horizon shade only against glare. Find the body near zero and move it down to the horizon, or, sextant inverted, bring the horizon up to a star. Rock the sextant: the body swings in an arc, and only at its lowest point is the sextant vertical; tilted, it reads too high.
Bring the lower limb to the horizon; the upper when cloud hides the lower, and on the Moon always the lit one. Use the sea horizon straight under the body, never a coastline or a near crest: in a swell shoot from a crest, in haze from lower down, with the dip for that height. Alone, start a stopwatch at the sight and stop it on a whole minute of the watch.
Stars need twilight, when star and horizon show together. Start in the east, where the horizon fades first at dusk and appears first at dawn; take the bright stars first at dusk, the faint ones first at dawn. Preset the planned altitude and sweep the azimuth. Take three to five sights, plot altitude against time, drop any off the line and average the rest.
In what order do I correct a sextant altitude from Hs to Ho?
Index error and dip turn Hs into the apparent altitude Ha; Ha then enters the almanac's main correction table, which gives Ho. For the Sun that correction combines refraction and semi-diameter.
Index error and dip turn the sextant altitude Hs into the apparent altitude Ha; Ha enters the almanac's main correction table, which gives the observed altitude Ho. Your eye is above the sea and sees the horizon below the true horizontal; that is the dip: 1.76′ × √(height of eye in metres), more the higher you stand.
| Correction | Sign | Applies to |
|---|---|---|
| Index error | − on the arc, + off | everything |
| Dip | always − | everything |
| Refraction | always − | everything |
| Semi-diameter, about 16′ | + lower limb, − upper | Sun and Moon |
| Parallax | always + | the Moon |
The Sun's main correction combines refraction and semi-diameter, in lower- and upper-limb columns and by season, since the semi-diameter follows the date. Stars and most planets get refraction alone. Refraction grows fast low down, 5′ at 10° and 10′ at 5°, and cold air or high pressure change it: wait for a body to climb above 10°; the temperature and pressure correction matters only below that.
Is the Moon's parallax correction added or subtracted?
Always added: from the Earth's centre the Moon stands higher than from the surface, so parallax, HP × cos Ha, goes on. Use the Moon's own table, and subtract the SD for the upper limb.
The Moon is close enough for parallax: from the Earth's centre it stands higher than from the surface, by the horizontal parallax HP × cos Ha, always added. Use the Moon's own table with the day's HP and SD, not the Sun's, and when the lit limb is the upper one, subtract the SD.
Venus and Mars get a small additional correction of their own; Jupiter and Saturn are corrected like stars.
How far out is my position line if the sextant altitude is one minute wrong?
One mile: every minute of altitude is a mile on the position line. Too high puts the line too far towards the body, too low, away; a sign applied the wrong way counts twice.
Every minute of altitude is a mile on the position line. An altitude too high puts the line too far towards the body, too low, away; a sign applied the wrong way counts twice. A constant error in all the sights of a fix, an index error in doubt or a dip spoilt by strange air, shifts every line the same way.
So spread the stars round the sky. With three 120° apart your position is the centre of the cocked hat; with two on opposite bearings half the sum of the intercepts is the error, and the rest is the true offset.
Questions from this topic, as the app asks them. Open one to see the answer.
You read your micrometer sextant after a sun sight. What is Hs?
B. 41°58.6′
Degrees from the arc (the last whole degree passed, 41°), minutes from the drum (58′), tenths from the vernier (0.6′).
You take a sight with the plastic backup sextant. What do you do about its index error?
B. Measure it again now, just before the sight
A plastic frame bends with temperature, so its index error changes within hours: check it at every sight.
You are about to bring the Sun down to the horizon. What do you swing in first?
C. Index shades, enough to dim the Sun's image
The Sun's image comes via the index mirror: dim it with index shades before you look. Horizon shades only cut the glare on the sea.
You are working up a sun sight. Which altitude do you enter the main correction table with?
A. Ha — Hs after index error and dip
Hs ± IE − dip = Ha (apparent altitude); refraction and SD depend on Ha, so the main table is entered with Ha to give Ho.
You take the sight on the Sun's upper limb. How does the semi-diameter go into the correction?
A. Subtracted — the centre is below the upper limb
The tables give the altitude of the Sun's centre: add SD to a lower-limb sight, subtract it from an upper-limb sight.
You take your sights standing at the shrouds of a 15 m ketch. What dip do you apply?
C. 3.1′
Dip = 1.76 × √3.2 = 1.76 × 1.79 = 3.1′, always subtracted from Hs.
All 100 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.