All these celestial essentials elements are intricately woven into the two fundamental pages: the “LaRochelle Exercise page“, guiding you to the “Interactive Worksheet”

CELESTIAL ESSENTIALS, image time zones
Time zones of the world-UTC. see full scale image from Author Goran tek-en


THE PRINCIPLE of the sun sight

The use of the sextant

_ Apparent sextant height of the sun

_ Observed sextant height of the sun

UT time

UT time keeping at sea

CELESTIAL ESSENTIALS, image time clock
Author: F1jmm

The nautical almanac

D.R. (dead reckoning)

The nautical mile

theory Zv (azimuth)

The celestial plotting sheet

_ Universal plotting sheet

CELESTIAL ESSENTIALS, image universal plotting sheet


Declination of the sun

_ How to calculate the declination of the sun

intercept and line of position

GHA (Greenwich hour angle)

_ How to calculate the GHA at the exact time of the observation

LHA and P (Local hour and polar angle)

_ How to calculate the sun’s local hour angle



Local apparent noon sight latitude

Local apparent noon time at sea

CELESTIAL ESSENTIALS, image local apparent noon sight principe


Interval between two sun sights

Sun’s LOP running fix

CELESTIAL ESSENTIALS, image of a transferred LOP
CELESTIAL ESSENTIALS light house Nova Scotia
Nova Scotia, Canada. Author image: Kevin Finch / finchkh

CELESTIAL ESSENTIALS: The essentials above are necessary for understanding the line of position.

CELESTIAL ESSENTIALS image stairs lightouse Baleines
This is the internal stairs of the “Baleines” lighthouse on the Ré island in France. author image: Galano

On a nautical chart, someone locates themselves on a straight line called a line of position.

In summary, while a Line of Position indicates that the vessel’s location is somewhere along that line, it does not provide a precise fix on its location.

Indeed, to determine a vessel’s location accurately, you need to intersect at least two LOPs.

Firstly, take an initial observation with the sextant to measure the height of the sun above the horizon and calculate the LOP.

Then, Take a second observation a few hours later when the sun moves from east to west and changes its azimuth angle.

Finally, calculate the new LOP and intersect these two position lines to determine your location.

Mountain water and sky, Black Sea. Author image: Pavel.Shulekin


Hence, the main goal of this website is to provide information on how to calculate the LOP (Line of Position).

In fact, a minimum level of navigational knowledge is required for this course.

That is to say, placing a geographical point on a map, drawing a course line, and measuring a distance in nautical miles.

F/V Northwestern docked at the Trident shore plant in Akutan, Alaska. author: Drew Koshar