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Celestial navigation

Rising and setting – when does the sun set, when does nautical twilight begin?

NauticTools2 calculates the rising and setting times of the sun, the moon and the planets Venus, Mars, Jupiter and Saturn for any place and date. For the sun you also get meridian passage and all three kinds of twilight – civil, nautical and astronomical. All times in UTC.

• Sun · moon · planets
• True and visible rising/setting
• Meridian passage
• Civil twilight
• Nautical twilight
• Astronomical twilight

What is calculated?

In each case the tool looks for the moment the centre of the body reaches a certain altitude above the rational horizon. Which altitude depends on the event:

The sun's −50′ consist of refraction at the horizon (about 34′) and the sun's semi-diameter (about 16′). For the moon, its large horizontal parallax more than cancels the refraction – hence +8′. Planets are practically point sources, so only refraction counts. Meridian passage (shown as “Transit” in the app) is the crossing of the meridian, the highest point in the sky.

When do you need it?

• Planning star sights: Nautical twilight sets the window in which horizon and stars are both visible. Then pick suitable stars with the star chart.
• Preparing the noon sight: The sun's meridian passage is the moment for the noon latitude.
• Navigation lights: Navigation lights are shown from sunset to sunrise – here you get the times for any place.
• Planning a night passage: When does the moon rise and set? A bright moon keeps the horizon and the fairway visible at night.

Calculation

1. Target altitude

   h₀ from the table above, e.g. −50′ for visible sunset or −12° for the end of nautical twilight.

2. Local hour angle

   From latitude φ and declination δ the formula gives the local hour angle t at which the body has this altitude – west for setting, east for rising.

3. Time

   From t and the longitude follows the Greenwich hour angle, and from the ephemerides the corresponding time in UTC. Because declination and hour angle change during the day – especially for the moon – NauticTools2 uses the values for the moment in question.

4. Special cases

   If the fraction lies outside −1 to +1, the body does not reach that altitude on that day: it is always visible (midnight sun) or not visible (polar night).

Examples

Things to watch out for

• Times in UTC: Add the zone time for ship's time – at Kiel in October (summer time) two hours. The time systems tool does the conversion.
• High latitudes: In summer the sun may not reach the target altitude at all – then “always visible” or “not visible” appears instead of a time. In northern European waters this mainly affects astronomical twilight in June.
• Moon: It rises on average about 50 minutes later each day, so on some calendar days there is no moonrise or no moonset.
• Height of eye: The times apply to the horizon at sea level. From a high vantage point you see the sun a little longer.

How it works in NauticTools2

1. Choose the body

   Sun, moon or planets using the bar at the top.

2. Enter date and place

   Enter latitude and longitude or pick a saved waypoint; the location icon also saves the place as a new waypoint.

3. Calculate

   The times appear in chronological order – for the sun from the start of astronomical morning twilight to the start of night.

4. Save and print

   Save inputs under a name and load them later, print the result as a record or save it as a PDF.

Frequently asked questions

What is the difference between true and visible sunset?

At true sunset the sun's centre reaches the horizon by calculation. The sun is still visible then, because refraction lifts it and its upper limb is higher. Visible sunset is at −50′ – a few minutes later in mid-latitudes.

When is nautical twilight?

When the sun is 6° to 12° below the horizon. The bright stars are already visible and the horizon can still be seen – the classic time for star sights.

Why does the moon sometimes not rise on a given day?

Because it is about 50 minutes later every day. Roughly once a month the moonrise slips past midnight into the next day.

Related tools

• Star chart – which navigational stars are up at twilight
• Time systems on board – convert UTC, zone time and local time
• Meridian diagram – meridian altitude and circumpolar bodies explained