Sextant Simulator
Look through a working sextant at the real sky. The sun, moon, planets and 30 navigational stars are placed by a true ephemeris for the date and position, with refraction, dip, parallax and a rolling deck. Learn the instrument, then bring each body down to the horizon, rock the arc and mark.
How a sextant works
A sextant measures the angle between a celestial body and the horizon. Light from the body reflects off the index mirror, then off the half-silvered horizon mirror into the telescope, while the horizon is seen straight through the clear half. Moving the index arm turns the index mirror; because a mirror turned through an angle deflects a ray through twice that angle, the 60° arc is engraved from 0° to 120°.
Taking a sight
- Check the index error against the horizon before you start.
- For the sun, swing in the index shades first.
- Set the arc to zero, look at the body, then release the clamp and move the arm while you lower the sextant until the horizon appears.
- Fine-tune with the micrometer drum to put the lower limb (or a star) on the horizon.
- Rock the sextant gently: the body swings in an arc. Its lowest point is the true vertical. Mark the time at that moment.
- Read the arc, drum and vernier, and note the time to the second.
From sextant altitude to observed altitude
- Error de índice:
- The reading when the two horizon images line up. On the arc, subtract it; off the arc, add it.
- Dip:
- Your eye is above the sea, so the visible horizon lies below the true horizontal. Dip is about 1.76′ × √(height of eye in metres), always subtracted.
- Refracción:
- The atmosphere bends light down, lifting every body. It is about 34′ on the horizon, 5′ at 10° and under 1′ above 45°, always subtracted.
- Semidiámetro:
- The sun and moon are discs about 32′ across. Add 16′ for a lower-limb sight, subtract for the upper limb, to get the centre.
- Paralaje:
- You stand on the Earth's surface, not at its centre. Tiny for the sun (0.1′), but up to a degree for the moon: added.
One minute of altitude error moves the position line one nautical mile. At noon, latitude = 90° − Ho ± declination; from Polaris, latitude is the corrected altitude with a small correction for the star’s distance from the pole.
Positions here come from a low-precision ephemeris accurate to about an arcminute for the sun and stars. Use the Nautical Almanac for real navigation.