Oil horizon · Tromsø Sextant Nights
Sextant 69°N

Stage 2

The oil horizon

No sea horizon inland, so you make a level one. A still liquid surface is always level.

★★ same stareyeh aboveh belowsextant measures 2 × htray of oil (level)level
The star's reflection is exactly as far below level as the star is above it. You measure the full angle between them: altitude = reading ÷ 2. Eye height doesn't matter, so no dip correction.
  1. Readingsextant
  2. Index error
  3. Ha
  4. Refractiontable T6
  5. Hoaltitude
0°30°60°90°120°150°end of arc ≈ 120°usable: stars 15°–60° highDenebreads 53°Polluxreads 97°Capellareads 127° ✗Polarisreads 140° ✗
With a tray the sextant reads twice the altitude. Polaris (70° high at Tromsø) would need 140°, past the end of your arc. Use it only against the sea horizon.

Four ways to build one

A
★wind

Open tray in a snow pit

Start here. Most accurate. Needs calm air at the surface.

B
★glass roof

Tray + glass roof

For wind and drifting snow. Turn the roof 180° for half the sights and average.

C
★

Cup horizon

Black mug or tin, 8–12 cm, dug in flush. Lightest. Stars only.

D
★

Levelled mirror

No liquid. Windproof. Sight in pairs, turning the plate 180°, to cancel tilt.

Which liquid

LiquidGood toVerdict
0W-20 / 0W-30 synthetic motor oil≈ −40 °C✓ best: dark, damps ripples
−40 °C windscreen washer fluid≈ −40 °C~ works in a black tray; faint, ripples
Water0 °C✗ freezes (home practice only)
Mercury—✗ toxic, never

How bright is the reflection?

1.515°2.020°2.325°2.730°3.035°3.240°3.345°3.450°3.555°3.660°01234magnitudes fainterstar altitude
In oil, a star's reflection is 1.5–3.6 magnitudes fainter than the star (low stars reflect more). Use planets and stars of magnitude 2 or brighter.

Setting up on snow

  1. Pick a spot 20 m or more from the dogs. Their shaking ripples the oil.
  2. Stamp the snow flat, put down plywood, set the tray on it, pour 3–6 mm of oil at air temperature.
  3. Build a snow wall on the windward side (or put the roof on). Wait 5 minutes for the oil to go still.
  4. Get into position as below. Cover the tray between sights so spindrift doesn't land on it.
  5. Pour the oil back into its bottle, wipe the tray, and pack everything out.
lying0.6 mkneeling low1.3 mkneeling up2.1 mtray★ star, 25° up
Your eye goes on the line that runs from the tray up toward the star, but on the side away from the star. Distances shown for a star 25° high. Low star: get low and further back. High star: get right over the tray.
Test at home first: shoot Jupiter 5 times in 5 minutes. If the readings scatter by more than 1′, the oil isn't still enough. Add a windbreak, pour deeper oil, or wait longer.