Celestial navigation is a way to find a ship’s position using the Sun, Moon, planets, or stars. It matters because it works anywhere on the ocean without radio signals, satellites, or nearby landmarks. For centuries, sailors used the sky as a natural map to cross oceans safely.
Even today, ships and submarines study celestial navigation as a backup to GPS and electronic navigation.
Key Facts
- Altitude angle = angle between a celestial body and the visible horizon.
- Latitude from noon Sun is found using observed solar altitude, date, and the Sun’s declination.
- Longitude depends on time: Earth rotates 15 degrees of longitude per hour.
- Observed altitude must be corrected for instrument error, horizon dip, refraction, and the Sun’s or Moon’s size.
- A line of position is a curve on Earth where the measured altitude of a celestial body would be the same.
- A position fix is found where two or more lines of position intersect.
Vocabulary
- Sextant
- A sextant is an optical instrument used to measure the angle between a celestial body and the horizon.
- Altitude
- Altitude is the angular height of the Sun, Moon, planet, or star above the horizon.
- Chronometer
- A chronometer is an accurate clock used to compare local observations with a standard time such as Greenwich Mean Time.
- Line of Position
- A line of position is a plotted line showing all possible places where an observer could be based on one celestial measurement.
- Declination
- Declination is the celestial coordinate that measures how far north or south a celestial body is from the celestial equator.
Common Mistakes to Avoid
- Using the sextant reading without corrections is wrong because refraction, horizon dip, and instrument error can shift the true altitude.
- Confusing altitude with azimuth is wrong because altitude measures height above the horizon, while azimuth measures compass direction along the horizon.
- Forgetting accurate time is wrong because longitude calculations depend directly on Earth’s rotation rate of 15 degrees per hour.
- Trying to get a fix from only one sight is wrong because a single observation usually gives only one line of position, not a unique location.
Practice Questions
- 1 A navigator measures the Sun’s altitude as 62 degrees at local noon. If the Sun’s declination is 10 degrees north and the Sun is south of the observer, estimate the latitude using latitude = 90 degrees - altitude + declination.
- 2 Earth rotates 15 degrees of longitude per hour. If local noon on a ship occurs 3 hours after noon at Greenwich, how many degrees of longitude west is the ship?
- 3 A navigator takes one star sight at night and plots one line of position. Explain why this does not give a complete position fix and describe what additional observation would help.