A course in celestial navigation

Find your position from the sky

Fourteen short modules, from a noon latitude to a three-star fix. Already know a topic? Open it and solve one problem to skip ahead.

Start the course
Begin · Part ILatitude and the nautical mile0 of 13 modules done. Read a latitude, turn a difference in latitude into miles, and keep a dead reckoning position.Open the module
Tonight’s star sights

Stars are shot in twilight, while the horizon is still sharp. Give a position to see when tonight’s window opens where you are. It stays on this device.

Before you start: a concept quiz

12 quick questions on the ideas people most often get wrong. Take it now and again at the end, and see what changed.

Take the quiz
I
Part I

Foundations

0 of 3 modules done
  1. Latitude and the nautical mile (optional)Read a latitude, turn a difference in latitude into miles, and keep a dead reckoning position.Not started
  2. Angles, time and UTConvert between arc and time, and turn zone time into UT.Not started
  3. Where the Sun is: GHA and declinationRead the Sun's GHA and declination from the almanac for any second of UT.Not started
  4. Sextant corrections: Hs to HoTurn a sextant reading into the observed altitude, in the right order.Not started
II
Part II

First sights

0 of 3 modules done
  1. Latitude from the noon SunKnow when noon is by your watch, and work a complete noon sight for your latitude.Not started
  2. Checking the compass by the SunFind the compass error and deviation from the Sun's bearing at sunrise or sunset.Not started
  3. Latitude from PolarisFind your latitude at twilight from the Pole Star, with the Polaris tables.Not started
III
Part III

The line of position

0 of 4 modules done
  1. The circle of equal altitudePlace a body's ground position, and turn one altitude into a circle of position in miles.Not started
  2. The line of positionTurn Ho and Hc into a line of position, and plot it from your DR.Not started
  3. The assumed position and LHAChoose the assumed position the tables need, and find LHA for any body.Not started
  4. The intercept methodReduce a Sun sight with Pub. 249 or by formula on a calculator: Hc, Zn and the intercept.Not started
IV
Part IV

The fix

0 of 3 modules done
  1. Planning a twilight fixFind the time of twilight by your watch, and choose stars that give a good fix.Not started
  2. The fixPlot two or more lines of position, including a running fix, and judge the fix.Not started
  3. Planets and the MoonReduce planet sights at twilight, and (advanced) a Moon sight for a daytime fix with the Sun.Not started

Final assessment

Five full sights from the watch and sextant, no hints, never seen in practice. Pass by putting the boat within 5 nm on four of them.

Take the assessment

Passages: a day at sea

Put it all together. Navigate a whole day of an ocean passage with no GPS, from the morning Sun line to the evening star fix. Every voyage is different, so sail them as often as you like.

  • San Diego to HiloA day in the trade winds with no GPS: a Sun line, the noon sight, a running fix, the current, a compass check and a star fix.
  • Canaries to AntiguaThe Atlantic crossing's last week: a dawn star fix and Polaris, the compass at sunrise, a running fix at noon and a planet at dusk.
  • Newport to BermudaThe Gulf Stream: find the current from two fixes, work the course to steer across it, and check it with the evening stars.
  • Panama to the GalápagosCrossing the equator: a noon latitude a few miles north of the line, Polaris gone, and a star fix that says south.
  • Tahiti to FijiSouthern stars and the Sun to the north, and the date line: lose a day on the ship's calendar between noon and twilight.
  • Azores to FalmouthNorthern waters in June: a Sun–Moon fix by day and the long northern twilight before the star fix.

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