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Celestial navigation

Meridian passage – when does the sun cross the meridian?

For the noon sight you need to know when the sun reaches its highest point at your position. NauticTools2 calculates the sun's meridian passage at Greenwich for any date, plus the equation of time and – with your own longitude – the longitude in time and the local meridian passage.

• Mer. Pass.
• Equation of time
• Longitude in time
• Local meridian passage
• Arc to time
• Any date

What is meridian passage?

Meridian passage – “Mer. Pass.” in the Nautical Almanac – is the time in UT at which the sun crosses the Greenwich meridian. If the sun moved evenly, it would be 12:00 every day. Because the Earth's orbit is elliptical and its axis is tilted, the true sun runs ahead of or behind the imaginary mean sun. The difference is the equation of time; over the year it varies between about −14 and +16 minutes.

When do you need it?

• Noon sight: The sun's altitude is measured at meridian passage. The time at your position is the Mer. Pass. plus or minus the longitude in time.
• Planning a sun-run-noon fix: Morning position line, then noon latitude – knowing the time of transit lets you plan the sights.
• Arc to time: The longitude in time is also needed for local and zone time.

Formulas and working

1. Mer. Pass.

   Calculated to the second for the date, and below that rounded to the whole minute as in the almanac.

2. Equation of time

   equation of time = 12:00 − Mer. Pass.; positive when the true sun is ahead of the mean sun.

3. Longitude in time

   360° = 24 h, so 1° = 4 min and 1′ = 4 s.

4. At your position

   In west longitude add the longitude in time – the sun comes later; in east longitude subtract it – it comes earlier.

Worked examples

Things to watch out for

• Times in UT: Add the zone time for ship's time – at Kiel in October two hours, so noon is at 13:03 ship's time.
• DR longitude is enough: One minute of longitude error is only four seconds – plenty for the time of the sight. The noon altitude hardly changes around transit anyway.
• Steaming east or west: Fast passages east or west meet the sun earlier or run away from it. Then use the DR longitude at the expected noon.

How it works in NauticTools2

1. Enter the date

   Meridian passage and equation of time appear straight away.

2. Enter longitude (optional)

   With your own longitude NauticTools2 also calculates the longitude in time and the local meridian passage.

3. Carry on

   Measure the sun's altitude at transit and find your latitude with the sextant altitude corrections and the noon latitude. Inputs can be saved and the result printed.

Frequently asked questions

What is meridian passage?

The time in UT at which the sun crosses the Greenwich meridian – between about 11:44 and 12:14 depending on the season. In the Nautical Almanac it is listed as “Mer. Pass.”.

How do you convert longitude to time?

1° is 4 minutes, 1′ is 4 seconds. 10°08′ is therefore 40 minutes and 32 seconds.

Why isn't noon always at 12 o'clock?

Because the Earth moves on an ellipse around the sun and its axis is tilted. The true sun is therefore sometimes ahead of and sometimes behind the evenly moving mean sun – that is the equation of time. On top comes your own longitude.

Related tools

• Rising and setting times – meridian passage is listed there too
• Time systems on board – UT, zone time and local time
• Meridian diagram – how high the sun stands at transit