Position fix · Tromsø Sextant Nights
Sextant 69°N

Stage 6

Position fix from three stars

Each star gives one line. Where three lines cross, that's your camp.

★ azimuth ZnAP (your guess)intercept = Ho − Hcline of position
From your guess (AP), draw the star's direction. Move along it by the intercept: toward the star if Ho is bigger than Hc, away if smaller. 1′ = 1 nautical mile. Draw the line of position across it at right angles. You are somewhere on that line.

Six steps per star

  1. Shoot it: take 3–5 sights, note UTC to the second, and keep the one that fits the others best.
  2. Ho: reading − index error, ÷ 2, − refraction (T6).
  3. GHA♈︎ at that second: T1 + T2.
  4. AP: latitude 69° or 70°. Pick the longitude so that GHA♈︎ + longitude comes out as a whole degree (LHA♈︎).
  5. Hc and Zn from T8 (row LHA♈︎, column for your star).
  6. Plot: from the AP, move Ho − Hc along Zn (away from the star if negative), then draw the line of position across it at right angles.
Pick stars from the same row of T8. It gives three stars spread around the sky, all at a good height for the tray.

Worked example · 15 Feb 2027, camp SE of Tromsø

Index error +2.4′ (on the arc), −22 °C. The camp's true position was hidden: 69°09.0′ N, 19°33.0′ E.

Denebola at 20:04:12 UTC
Reading40°13.7′−Index error2.4′÷ 2= Ha20°05.7′−Refraction, −22 °C3.0′table T6=Ho20°02.6′
GHA♈︎
15 Feb, 00h144°46.8′table T1+20 h300°49.3′table T2+04 min 12 s1°03.2′table T2=GHA♈︎86°39.3′
Choose the AP
AP latitude69° Nnearest whole degreeAP longitude19°20.7′ Epicked so GHA♈︎ + AP lon = a whole degree, near your guess of 19°30′=LHA♈︎106°
Look up (table T8, lat 69°)
Hc19°59.7′Zn102°
Intercept
Ho − Hc20°02.6′ − 19°59.7′=Intercept2.9 nm toward
StarUTCReadingHoGHA♈︎AP lonLHA♈︎HcZnIntercept
Denebola20:04:1240°13.7′20°02.6′86°39.3′19°20.7′106°19°59.7′102°2.9 T
Aldebaran20:09:4764°41.6′32°17.8′88°03.3′19°56.7′108°32°18.6′225°0.8 A
Deneb20:15:3051°23.4′25°38.2′89°29.2′19°30.8′109°25°29.7′343°8.5 T

T = toward the star, A = away from it.

69°00′69°05′69°10′19°10′19°20′19°30′19°40′19°50′DenebolaAldebaranDenebtrue campFIXopen dots on 69°00′ = assumed positions (AP), one per star
Each gold arrow is an intercept from its AP. The dark lines are the lines of position, and they cross at the fix. Grid: 5′ of latitude = 5 nm. 10′ of longitude = 3.6 nm at 69°N.
Fix: 69°09.2′ N, 19°34.5′ E, which is 0.6 nm (1.1 km) from the true camp.

Onto the map

  1. 1′ latitude= 1 nm = 1.85 km
  2. 1′ longitudeat 69°N = 0.36 nm = 0.66 km
  3. Topo maplat/lon ticks on the map edge