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Yachtmaster Ocean revision module 2 of 10

TIME

Celestial Sphere and Time

Celestial coordinates, apparent movement, UTC, zone time, chronometer error, and sight timing.

celestial sphereUTC

3 guided lessons · 8 practice questions · 8 flashcards

3 outcomes · self-guided revision
  • Explain celestial coordinates, apparent movement, UTC, zone time, chronometer error, and sight timing.
  • Apply celestial sphere to a realistic onboard situation.
  • Recall the core check: gP means geographical position: the point on Earth under the body.

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Lesson 1 of 3

Celestial Sphere Language

Astro navigation turns the position of a celestial body into a line of position on Earth. The language can feel abstract, but the aim is practical: identify where the body's geographical position is and compare your measured altitude with the altitude expected from an assumed position.

Declination is like latitude on the celestial sphere. Greenwich Hour Angle is like longitude measured westward from Greenwich. Local Hour Angle relates the body to your assumed longitude.

  • GP means geographical position: the point on Earth under the body.
  • Declination is north or south of the celestial equator.
  • GHA is measured westward from Greenwich.
  • LHA connects GHA with the observer's assumed longitude.
TermWorking meaningUsed for
DeclinationCelestial latitudeSight reduction and meridian altitude
GHAAngular distance west of GreenwichFinding the body's longitude reference
SHAStar reference angle from AriesStar sight data
LHAHour angle from assumed longitudeSight reduction tables/calculation
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Lesson 2 of 3

Time Discipline

A few seconds of time error can create a meaningful position error because the Earth rotates 15 degrees per hour. Ocean sights need UTC, exact sight time, and known chronometer or watch error.

Record time at the instant the altitude is taken, not after the worksheet is complete. If using a phone or GPS time source, know what happens if electronics fail and keep a backup time plan.

  • Use UTC for almanac work.
  • Record time immediately and clearly.
  • Apply chronometer error with the correct sign.
  • Do not mix local time, zone time, and UTC casually.
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Lesson 3 of 3

Time Zones and Date Traps

Ocean passages cross time zones and sometimes the International Date Line. The almanac does not care what meal time the crew uses; it needs the correct UTC date and time.

Many sight errors start with the wrong date page, wrong hour, or wrong sign for watch error. Build a routine that checks date, UTC, DR longitude, and whether the sight occurs near midnight.

  • Check UTC date as well as time.
  • Beware sights near midnight or date-line changes.
  • Keep ship's time separate from calculation time.
  • Write watch error as fast or slow, then convert deliberately.
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