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Celestial navigation

Star chart – which navigational stars are up right now?

The star chart shows the sky for any place and time – with the navigational stars, sun, moon and the four navigational planets, constellations and the current twilight status. A rotatable crosshair helps you pick three well-spread stars for a fix.

• 57 navigational stars + Polaris
• Sun · moon · planets
• Constellations
• Altitude and azimuth
• Twilight
• Crosshair

What does the star chart show?

The chart is the view from the zenith (centre) down to the horizon (edge) – as if you were lying on deck looking straight up. North is at the top, east on the left and west on the right. A horizon grid with altitude circles at 30° and 60° and azimuth lines every 30° helps with estimating. The brighter a star, the larger its dot; the 57 navigational stars of the Nautical Almanac plus Polaris are highlighted in colour, with their almanac names and numbers. The other stars are only there so you can recognise the constellations.

When do you need it?

• Planning star sights: Which navigational stars are at a good altitude at twilight, and how are they spread across the sky?
• Identifying stars: A tap shows name, constellation and almanac number – useful when you can't place a bright star from the deck.
• Telling planets apart: Venus, Mars, Jupiter and Saturn wander through the constellations and are easily mistaken for stars.
• Catching twilight: The chart shows whether it is day, civil or nautical twilight, or night.

The crosshair

For a good fix from three altitudes the bodies should be spread as evenly as possible around the horizon – the position lines then cross at about 60° or 120°. The crosshair has three arms 120° of azimuth apart and can be turned with a slider. The two rings mark 15° and 70° altitude: below that refraction becomes uncertain, above it the altitude is hard to measure with the sextant. If a navigational star lies on each arm between the rings, you have a good selection.

For instructors: create your own celestial exercises

The star chart is the ideal starting point for new exercises on a fix from star altitudes – with stars that genuinely fit the place and time:

1. Choose a position

   Pick any coordinates, for example in the Caribbean off Martinique (14° 30' N, 061° 20' W). This is the true position and therefore the answer to the exercise.

2. Find twilight

   Choose a date and move the time (UTC) until the chart shows “nautical twilight”.

3. Pick three stars

   Turn the crosshair until a highlighted navigational star lies on each arm between the rings. Tap each star and note its name, altitude and azimuth.

4. Set the exercise

   Give your students a DR position a few miles away from the true position and alter the altitudes slightly – a few minutes of arc, just as a real sextant sight would. If the exercise should start from the sextant reading, work the altitudes back to Hs with height of eye, index error and refraction.

5. Check the answer

   Work the exercise yourself with Fix from 2 or 3 altitudes: the cocked hat should lie close to the chosen position.

In a few minutes you have realistic exercises for any sea area – not just the same textbook examples over and over.

How the chart is calculated

1. Ephemerides

   For the chosen time (UTC), Greenwich hour angle and declination of all stars, the sun, the moon and the planets are calculated – as in the ephemeris calculation.

2. Local hour angle

   LHA = GHA + λ (east longitude positive).

3. Altitude and azimuth

   Altitude H from the formula above, the azimuth from the same navigational triangle. Bodies below the horizon are not drawn.

4. Twilight

   From the sun's altitude: above 0° day, down to −6° civil twilight, down to −12° nautical twilight, below that night.

Things to watch out for

• Time in UTC: The chart works in universal time, not ship's time or summer time.
• A finding aid: The star chart is not a planetarium. It deliberately shows only the stars that matter for navigation and for recognising the constellations; the constellation lines are simplified.
• Calculated altitudes: The altitudes shown refer to the rational horizon. For the position line what counts is the measured and corrected sextant altitude.

How it works in NauticTools2

1. Choose place and time

   Enter date, time (UTC), latitude and longitude or pick a saved waypoint. Without your own input the chart uses the current GPS position, if available – so it shows what is overhead right now. Every change is applied immediately.

2. Set up the display

   Show or hide constellation and navigational star names, switch on the crosshair and turn it with the slider.

3. Tap a star

   Name, constellation, magnitude, number in the Nautical Almanac and altitude and azimuth appear in the info panel. On Windows the mouse also shows altitude and azimuth anywhere on the chart.

4. Save and print

   Save the place as a waypoint, or print the chart as a record or save it as a PDF.

Frequently asked questions

Why is east on the left?

Because the chart shows the sky as you see it looking up. With north at the top, east is on the left and west on the right – mirrored compared with a map you look down on.

When do you take star sights?

During nautical twilight, when the sun is between 6° and 12° below the horizon. The bright stars are already visible and the horizon can still be seen. The star chart shows this status directly.

How many navigational stars are there?

The Nautical Almanac lists 57 navigational stars plus Polaris – exactly the stars highlighted on the chart, with their almanac names and numbers. (The German version of the app uses the 80 stars of the German Nautisches Jahrbuch instead.)

Related tools

• Fix from 2 or 3 altitudes – from star altitudes to the ship's position
• Meridian diagram – meridian altitude and circumpolar bodies explained