Calculating the local apparent noon time at sea: this can be done very simply if we don’t take into account the vessel’s own movement.
But 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
To calculate the noon time, you need to know how to determine the polar angle (P) beforehand !
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
Also, with the apparent noon time, the observer’s latitude, and the declination of the sun, you can plan the appropriate intervals between two or more sun sights for that day.
Initiating the local apparent noon (LAN) time , without the vessel’s own movement
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.
It’s better to use a whole hour (T1) so that we don’t have to calculate the increment from the minutes and seconds.
Remember to calculate the noon time, you need to know how to determine the polar angle (P) in advance!
With this UT time (T1), we will look for the GHA in the nautical almanac and calculate P (polar angle)
let’s see how it works:
T1 = UT time whole hour = 18.00 hours
Day X
L = 33°21’ N; G = 150°18’W
18.00 UT, corresponding to 08.00 local time (LT) on board.
150°18′ W is between 142,5° and 157,5 → time zone number = + 10
LT = UT ⎼ (Time zone number) = 18.00 ⎼ (+10) = 08.00 (time on board)
GHA = 87°12’,2 at 18.00 UT time (from nautical almanac on this day.)
P will decrease to zero at a rate of 15° per hour, reaching zero at local apparent noon time.
18.00.00 + 04.12.00 = 22.12.00 UT Time
T local apparent noon (lan) = UT ⎼ ( + 10) = 12h 12m LT. (local time)
Influences of Vessel Motion on lan determination:
The precision of determining local apparent noon (LAN) using this method is compromised by the influence of a vessel’s own motion, especially when navigating on a course close to east or west at high speed
As a matter of fact, this movement affects the time interval between T1 and local apparent noon.
Certainly, for individuals taking a noon sight, it is better to take into consideration the influence of the vessel’s own motion.
Local Apparent Noon Time at sea:
The exact method of how to calculate the local apparent noon:
Also, here we expose the exact local apparent noon calculation .
The uncertainty of the vessel’s speed and course can still affect the accuracy of this method.
Day X
Additionally, T1 is the UT time at the whole hour, which in our case is 18:00:00, corresponding to 08:00:00 local time, which is early in the morning on board of our vessel.
Remember, you can take any time as T1 before apparent noon; the final answer will be the same.
estimated position at 18:00:00 UT:
L = 33°21’ N; G = 150°18’W
SOG (speed over ground) = 12 knots.
COG (course over 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.
T lan = 12h 15m 56s local time
Finally, the difference between the method that takes into account the vessel’s own movement and the one that does not is about 4 minutes.
But if you are prepared 10 minutes before noon with your sextant outside and start following the sun’s ascension until its culmination, you will be able to find your latitude safely.
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.
By focusing on accurately measuring the sun’s altitude at its highest point, you eliminate the necessity for precise timekeeping during the observation, thereby saving time and simplifying the process. In other words, you do not need to record the time at that very moment; it’s just you, the sun, and the sextant.
However, it’s crucial to have a reliable and accurate sextant for measuring the sun’s altitude, as this measurement is key to determining your latitude.
Local Apparent Noon Time at sea: Exercise
In fact, you are sailing offshore the Guadeloupian islands on a beautiful day with almost no clouds.
Your Dead Reckoning (D.R.) position is 15°54.4′ N, 056°34′ W.
At 09:00 local time on board 6 June 2022
1)Calculate the local apparent noon time
2)Do I face the sun in the northern or southern direction at this moment?
Also, assume the vessel’s movement is not taken into account.
Firstly, we have to calculate the UT
UT time = LT time + Time zone number
UT time = 09.00 + (+ 4) = 13.00
GHA = 15° 19’,1 (T1 = 13.00 UT)
G = 56° 34’0
___________________ ⎼
LHA = 318° 45’,1 ⇒ P = 41° 14’,9
△t = P/15°
⇔ △t = 41’,9 / 15° = 2h 45m
local apparent noon time = T1 + △t
⇔ local apparent noon time = 09h00 +2h45 = 11h45
Sun in the north or south?
Latitude = 15° 54’,4 N
Declination = 22° 41’ N
⇔ D ＞ L (hemisphere North)
We will face the sun in the North
Now we do the same exercise but take the vessel’s movement into account.
COG (course over ground) = 065°
SOG (speed over ground = 8 knots
In conclusion, the method that accounts for the vessel’s movement differs from the one that does not by approximately one and a half minutes.
However, if you begin following the sun’s ascension ten minutes before the culmination point, this discrepancy is inconsequential.