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Navigation · Level R4 · 4 parts · about 13 min

Compass error by the Pole Star, and time: UT, zone time and legal time

4 short parts: The Pole Star · Compass error · UT and zones · Legal time. Each part explains the idea, then gives the points to remember, the common mistakes, a worked case and a practice question; the Academy unit adds all 25 check questions.

1 · The Pole Star

Polaris: a true-north mark in the sky

Polaris lies about 0.6° from the north celestial pole. At mid-latitudes its bearing stays close to north; use the almanac for its true azimuth.

Polaris: a true-north mark in the sky

Why Polaris

Polaris, the Pole Star, is the bright star almost exactly above the Earth's North Pole. Its distance from the celestial pole, its polar distance, is about 0.62° in 2026, roughly 37′, and still shrinking slowly until about 2100. As the Earth turns, Polaris circles the pole in that tiny circle once a day. At mid-latitudes, its true azimuth, or true bearing, stays close to 000°.

How far from north

The azimuth's largest swing depends on your latitude: sin (largest azimuth) = sin (polar distance) ÷ cos (latitude). In 2026 that is about 0.6° at the equator, 0.8° at 40°N, 1.0° at 50°N and 1.2° at 60°N. So taking Polaris as exactly north can give an error of up to about a degree at Spanish latitudes; fine for a rough check, not for a deviation card.

Finding it

Polaris is a medium-bright star, not the brightest in the sky. Find the Plough (Big Dipper), a seven-star ladle shape. Follow the bowl's outer edge from its base through its rim, then continue about five star-separations to Polaris. These two stars are the pointers. Its altitude (angle above the horizon) is almost your northern latitude: about 40° off Valencia, low near the equator. Polaris is normally unavailable for navigation south of the equator.

The exact azimuth

The Nautical Almanac gives Polaris's true azimuth using your latitude and LHA Aries, the local hour angle of Aries. Aries is a reference point on the celestial equator. GHA Aries, its Greenwich hour angle, measures its angle west from Greenwich; LHA measures it west from your meridian. Take GHA Aries for the bearing's UT (Universal Time). Add east longitude or subtract west longitude. Add or subtract 360° if needed to keep LHA from 000° up to, but not including, 360°.

Latitude, in one line

The same tables also correct Polaris's altitude, measured with a sextant, into your latitude; the celestial units explain the method.

Key figures

Polar distance (2026)
About 0.62°, roughly 37′
Largest azimuth swing
sin Z = sin p ÷ cos lat: about 0.8° at 40°N
Altitude
Almost equal to your latitude
LHA Aries
GHA Aries + east longitude − west longitude; wrap to 000°–<360°
Almanac table
Polaris azimuth by LHA Aries and latitude
Spanish terms
Estrella Polar, azimut verdadero, horario de Aries

Remember

  • Polaris is about 0.6° from the celestial pole; its azimuth is close to 000° at mid-latitudes.
  • The swing grows with latitude: about 0.8° at 40°N, 1.2° at 60°N.
  • Its altitude is almost your northern latitude; it is normally unavailable for navigation south of the equator.
  • For an exact true azimuth, enter the almanac's Polaris table with LHA Aries and latitude.

Common mistakes

  • Wrong: Polaris is exactly at true north.

    Right: It circles the pole at about 0.6°. Its azimuth swings up to about a degree either side at mid-latitudes.

  • Wrong: Polaris is the brightest star in the sky.

    Right: It is only medium-bright. Find it from the Plough's pointers or from Cassiopeia.

  • Wrong: Enter the Polaris table with your longitude.

    Right: Enter it with the LHA of Aries, worked out from the almanac's GHA Aries and your longitude, and with your latitude.

Worked case

Off the coast at 41°N, a crew member suggests checking the steering compass by taking Polaris as due north, 000°T. How large an error could that cause, and how do you avoid it?

Show the answer

At 41°N Polaris's azimuth can be up to about 0.8° either side of north, depending on the time. Taking it as 000° could put that much error into your compass check. Note the UT of the bearing, find LHA Aries from the almanac and your longitude, and read the true azimuth from the Polaris table.

Practice question

About how far is Polaris from the north celestial pole in 2026?

  1. About 0.6°
  2. About 2.5°
  3. About 6°
  4. Exactly 0°
Show the answer

Answer: A. Its polar distance is about 37′, roughly 0.62°, and slowly shrinking until about 2100.

2 · Compass error

Compass error and deviation from Polaris, worked

Take a compass bearing of Polaris, compare it with its true azimuth from the almanac, and you have the compass error. Remove variation to get deviation.

Compass error and deviation from Polaris, worked

Compass error

Compass error is the total difference between a compass reading and true: error = true bearing − compass bearing. Across 000°, add 360° to the smaller bearing before subtracting. A positive result is named east, a negative one west. It is the sum of variation and deviation (Compass basics). Spanish courses call it the corrección total (Ct): Ct = azimut verdadero (Zv) − azimut de aguja (Za).

Taking the bearing

To check the steering compass, steady the boat on the heading you want to test and take Polaris's bearing over the compass, using an azimuth mirror or another sight designed for that altitude. Note the heading and the exact UT. A hand-bearing compass checks only itself. Its deviation can change with your position aboard and the boat's heading; it does not give the steering compass's deviation.

Worked example

At 41°N, on a heading of 090°C, Polaris bears 002.0°C. With the UT of the bearing you find LHA Aries, and the Polaris table gives a true azimuth of 359.4°T. Treat 002.0° as 362.0°: error = 359.4 − 362.0 = −2.6°, so 2.6°W.

Deviation

The chart's compass rose, updated to this year, gives a variation of 0.5°E (+0.5). Deviation = error − variation = −2.6 − (+0.5) = −3.1°: 3.1°W on a heading of 090°C. Write it on the deviation card for that heading. Spanish books write it the same way: desvío = Ct − declinación magnética (dm).

Make it count

One bearing gives the error on one heading only, because deviation changes with heading. Repeat on several headings, about every 45°, to check a whole card. At about 40° elevation, you may need an azimuth-mirror attachment; check that your compass's sight can observe that high. An ordinary bearing ring may not allow it.

Key figures

Compass error
True − compass, wrapped to −180°–+180°; + = E, − = W
Deviation
Compass error − variation (east +, west −)
Worked error
359.4 − 362.0 = −2.6°: 2.6°W
Worked deviation
−2.6 − (+0.5) = 3.1°W on 090°C
Spanish terms
Ct = Zv − Za; desvío = Ct − dm

Compass error from Polaris

  1. Steady on the heading to check and note it. — Deviation belongs to the boat's heading, not to the bearing direction.
  2. Take Polaris's compass bearing and note the UT to the minute. — The true azimuth depends on the time through LHA Aries.
  3. Find LHA Aries: GHA Aries for that UT ± your longitude. — It is the entry, with latitude, to the Polaris azimuth table.
  4. Read the true azimuth from the almanac's Polaris table. — Polaris is up to about a degree off north at mid-latitudes.
  5. Subtract compass from true. If above +180°, subtract 360°; if below −180°, add 360°. Positive is E; negative is W. — This is the total correction, variation plus deviation.
  6. Deviation = error − variation, for that heading. — Variation comes from the chart; what remains is the boat's own error.

Remember

  • Compass error = true − compass: wrap across 000° if needed; positive east, negative west.
  • Deviation = compass error − variation, for the heading you were on.
  • Check the steering compass on the heading you are testing, with the exact UT.
  • Repeat on several headings: one bearing gives one heading's deviation.

Common mistakes

  • Wrong: Compass error = compass bearing − true bearing.

    Right: Error = true − compass. Treat 002.0° as 362.0°: 359.4 − 362.0 = −2.6°, so 2.6°W.

  • Wrong: Record the deviation against the bearing of Polaris, about north.

    Right: Deviation depends on the boat's heading. Record it against the heading you were steering, here 090°C.

  • Wrong: Use a hand-bearing compass to find the steering compass's deviation.

    Right: Each compass has its own deviation. Take the bearing over the compass you are checking.

Worked case

On heading 200°C at 38°N, Polaris bears 358.5°C over the steering compass; its true azimuth is 000.7°T. Variation is 1.0°W. A crew member wants to record the deviation against 000°C. Calculate the error and deviation, and decide where to record it.

Show the answer

Error = true − compass = 000.7 − 358.5. Treat 000.7 as 360.7: 360.7 − 358.5 = +2.2°, so 2.2°E. Deviation = error − variation = +2.2 − (−1.0) = +3.2°, so 3.2°E on a heading of 200°C. Record 3.2°E against 200°C, not 000°C: deviation belongs to the boat's heading.

Practice question

Polaris's true azimuth is 359.4°T and it bears 002.0°C. What is the compass error?

  1. 2.6°E
  2. 2.6°W
  3. 1.4°E
  4. 1.4°W
Show the answer

Answer: B. Treat 002.0° as 362.0°: 359.4 − 362.0 = −2.6°, so 2.6°W.

3 · UT and zones

UT, zone time and the zone description

For nautical zone time, UT = ZT + ZD. The zone description (ZD) is positive west and negative east of zone 0. Ships may keep UT instead.

UT, zone time and the zone description

Universal Time

Universal Time (UT) follows Earth's rotation and is referenced to Greenwich. The almanac's tables use it. Your satellite navigator and radio time signals give UTC (Coordinated Universal Time). Occasional leap seconds keep UTC within 0.9 s of UT1, the rotation-based time scale. For this compass check, treat UTC as UT. GMT is the older name you may hear.

Time zones

At sea the world is divided into 24 time zones, each 15° of longitude wide, centred on multiples of 15°. Zone 0 runs from 7.5°W to 7.5°E. Each zone keeps the time of its central meridian: zone time (ZT). Divide your longitude by 15 and round to the nearest whole number to get your zone.

The zone description

The zone description (ZD) is the number of hours you add to zone time to get UT: UT = ZT + ZD. West of Greenwich clocks are behind UT, so ZD is plus; east of it they are ahead, so ZD is minus. A zone at 30°W is +2; one at 135°E is −9. Turn the rule round for the other way: ZT = UT − ZD.

Worked examples

At 024°20′W, 24.3 ÷ 15 = 1.6, so ZD is +2. Zone time 2215 on 3 March gives UT 2215 + 2 h = 0015 on 4 March: watch the date. At 140°E the ZD is −9: ZT 0800 on 5 June gives UT 2300 on 4 June. Log the UT of every bearing you will work with the almanac.

Changing zones

If your ship keeps nautical zone time, put clocks back an hour when moving one zone west, or forward when moving one zone east. The master decides when, often at night or a change of watch. Many ocean sailors keep the log in UT throughout to avoid mistakes.

Key figures

UT
Time at Greenwich; UTC from radio and GNSS, within 0.9 s
Zone width
15° of longitude; zone 0 from 7.5°W to 7.5°E
Rule
UT = ZT + ZD; ZT = UT − ZD
Sign of ZD
West longitude +, east longitude −
Example
024°20′W: ZD +2; ZT 2215 → UT 0015 next day
Spanish terms
Tiempo universal (TU), huso horario, hora del huso

Remember

  • The almanac runs on UT; GNSS and time signals give UTC, close enough for navigation.
  • Each zone is 15° wide; zone = longitude ÷ 15, rounded.
  • UT = ZT + ZD: ZD is plus in west longitude, minus in east.
  • Check the date whenever adding the ZD passes midnight.

Common mistakes

  • Wrong: East longitude zones have a positive zone description.

    Right: East of Greenwich, clocks are ahead of UT, so you subtract: ZD is minus.

  • Wrong: Change the hour but not the date when converting near midnight.

    Right: 2215 ZT on 3 March with ZD +2 is 0015 UT on 4 March.

  • Wrong: Use the zone time of a bearing to enter the almanac.

    Right: Convert to UT first. The almanac's tables are in UT.

Worked case

At 038°10′W, your watch reads 2230 ZT on 10 August. A crew member labels the Polaris bearing 0130 UT, 10 August. Can you use that almanac entry? Explain.

Show the answer

No. 38°10′ ÷ 15 rounds to ZD +3. UT = 2230 + 3 h = 0130 on 11 August. The hour is right but the date is wrong. Correct the date before using the almanac.

Practice question

What is the zone description of a ship keeping zone time at 052°W?

  1. −3
  2. +4
  3. +3
  4. −4
Show the answer

Answer: C. 52 ÷ 15 ≈ 3.47, rounding to zone 3 (zone 3 spans 37.5°W to 52.5°W). West, so +3.