Local Apparent Noon Time at sea

Calculating the local apparent noon time at sea can have two different objectives.

The first one is to gain sufficient insight for day planning and finding the ideal interval between the two lines of position (LOPs), which is essential for a good running fix.

This can be done very simply if we don’t take into account the vessel’s own movement.


The second objective is for the mariner who wants to take a noon sight and needs a precise time, allowing them to present themselves about 10 minutes before and start taking sun sights at short intervals just up until the sun reaches the culminating point.

This time, we need to take into consideration the vessel’s own movement

local apparent noon time image
local apparent noon time

Remember, the main purpose of this website is to determine the sun’s Line of Position (LOP) at any time of the day using the Marcq St. Hilaire method, with the ‘La Rochelle’ exercise.

It is essential to understand that the local apparent sight technique as explained on this page can only be applied at a specific time of the day

Moonrise somewhere in Indian ocean. Author image: Anupam291

Initiating the local apparent noon time calculating for day planning

As I have advocated in my course, we start the ocean passage with the downloaded daily pages from the nautical almanac.

Also we have a reliable watch set to UT time. !!

Early in the morning, for example after breakfast, we take the UT time of the next whole hour (T1) and the estimated longitude.

With this UT time (T1), we will look for the GHA in the nautical almanac and calculate P (local hour angle)

It’s better to use a whole hour (T1) so that we don’t have to calculate the increment from the minutes and seconds.

But you can also use the UT time the first sun sight as T1 and use this P (local hour angle)

The local hour angle of the local apparent noon is zero°

How to calculate the local apparent noon, image local apparent noon

let’s see how it works:

Date 06/03/2022

T1 = UT time whole hour = 18 hour ( This was the closest UT time on our watch set to UT time after the breakfast)


L = 33°21’ N; G = 150°18’W

estimated position at 18.00 UT, corresponding to 08.00 local time.

LT = TU – (Time zone number) = 18.00 – (+10) = 08.00


GHA = 87°12’,2 at 18.00 UT time (from nautical almanac)

The Local Hour Angle (LHA) of the sun, representing the angular distance between the observer’s meridian and the meridian containing the sun, increases from an initial value of 296°54’,2 to 360° (or 63°05’,8). This change corresponds to the Polar angle (P).

The hour angle of the sun increases by an average of 15° per hour.

How to calculate the local apparent noon, image explains Time of the local apparent noon
P = 63° 05′,8 and T1 = 18h UT

T lan = TU ⎻ ( + 10) = 12h 12m LT

Influences of Vessel Motion on lan determination:

The precision of determining local apparent noon (lan) through this method is compromised by the influence of a vessel’s own motion, especially when navigating on a course close to east or west.

As a matter of fact, this movement noticeably affects the time interval between T1 and local apparent noon.

Certainly, for individuals taking a noon sight, it is not the complete method.

However, our objective is to gain sufficient insight for planning the day.

Establishing the optimal time interval between the two lines of position (LOPs) is critical for obtaining an accurate running fix.

Local Apparent Noon Time at sea:

The exact method of how to calculate the local apparent noon:

I also expose the exact local apparent noon calculation here.

The uncertainty of the vessel’s speed, course, and the exact time of your watch can still affect the accuracy of this method.

Example 1:

Date 06/03/2022

T1 = UT time whole hour = 18 hour( This was the closest UT time on our watch set to UT time after the breakfast)

estimated position at 18.00 UT:

L = 33°21’ N; G = 150°18’W

SOG (speed on the ground) = 12 kn.

COG (course on the ground) = 300° 

GHA = 87°12’,2 at 18.00 UT time (from nautical almanac daily page)

We have previously established that the accuracy of determining Local Apparent Noon (LAN) is hindered by the impact of a vessel’s motion, particularly when navigating on a course close to east or west.

In fact, this movement alters the time interval between T1 and Local Apparent Noon.

Instead of using the sun’s rate of 15°, we adopt a relative rate that incorporates the vessel’s motion, as indicated by the formula below.

How to calculate the local apparent noon, final formula to calculate the exact lan
How to calculate the local apparent noon, the exercise to calculate the time lan
T local apparent noon = 22h 45m 56s UT

T lan = 12h 15m 56s local time

In this case, we observe about a 4 minutes difference compared to the first method, which did not account for the vessel’s movement


Calculate Local Apparent Noon Time:

You can even improve it by considering the exact increase in the hour angle for this day instead of the average 15° increase.

The resulting error is approximately 4 seconds.

However, it is unimportant for practical noonsight purposes.

How to calculate the local apparent noon, how to calcul the improved increase of the hour angle for a given day

Local Apparent Noon Time at sea:

In conclusion

Typically, during a noonsight, the observer measures the altitude of the sun when it is at its highest point in the sky (culmination). The altitude of the sun at this moment is directly related to the observer’s latitude.

In fact, if you are observing the sun just as it rises and reaches its culmination point, you are effectively skipping the need for time measurements, as the sun’s position at culmination is what matters for latitude determination.

Instead, you can focus on accurately measuring the sun’s altitude when it is at its highest point.

By doing this, you eliminate the need for precise timekeeping during the observation, saving time and potentially simplifying the process.

However, it’s crucial to have a reliable and accurate method for measuring the sun’s altitude, as this measurement is key to determining your latitude using celestial navigation techniques

Steller’s Sea Eagles visit Japan’s northern islands. Author image: Saschathegerman