The parts
The frame carries a curved arc marked in degrees. The index arm swings along the arc and carries the index mirror at its pivot. The horizon mirror, fixed to the frame, is half mirror and half clear glass (or one lightly silvered glass). Through the telescope you see the horizon directly, and the body reflected from the index mirror to the horizon mirror. A micrometer drum on the arm reads minutes; hinged shades cut the Sun's glare.
Before the sight: index error
Set the sextant to 0° and look at the horizon. If the direct and reflected horizons do not form one unbroken line, turn the drum until they do, and read it. That reading is the index error. If it is on the arc (a small positive reading), the sextant reads high: subtract it. If it is off the arc (below zero), add it. "On, take it off; off, add it on." Check it every time you take sights.
Taking a Sun sight
- Shades first. Swing shades in front of the index mirror before pointing at the Sun, and one over the horizon mirror if the sea glare is strong.
- Find the Sun. With the sextant at 0°, look straight at the Sun through the telescope.
- Bring it down. Squeeze the release and push the index arm away from you, lowering the sextant as you go, so the reflected Sun stays in view until the horizon appears.
- Rock the sextant. Tilt it a little from side to side around the line of sight. The Sun swings in a shallow arc. Turn the drum until the Sun's lower edge (the lower limb) just kisses the horizon at the bottom of that arc. This makes sure you are measuring straight down, not at a slant.
- Mark the time at the instant of contact, to the second, then read the sextant: degrees from the arc, minutes from the drum, tenths from the vernier beside it.
Take three to five sights a minute or so apart. Their altitudes should rise or fall steadily; one that breaks the pattern was a bad sight.
Stars and planets
Stars are shot at twilight, when both the star and the horizon are visible. A point of light is hard to find by bringing it down, so either preset the sextant to the altitude you expect (from a star finder) and sweep the horizon on the star's bearing, or turn the sextant upside down, aim at the star, and bring the horizon up to it.
From the reading to the true altitude
The sextant altitude, Hs, needs three corrections before it can be used:
- Index correction, from the index error above.
- Dip: your eye is above the sea, so the visible horizon is below true horizontal. Always subtracted; about 3′ at 3 m.
- The main correction from the almanac: refraction, and for the Sun and Moon the half-width of the disc and parallax.
The result is the observed altitude, Ho. The lessons on sextant corrections take you through each, with practice that catches a wrong sign.
Looking after it
Lift it by the frame or handle, never the arc or index arm. Keep fingers off the mirrors. Rinse salt off with fresh water and let it dry before closing the case.