Key Elements and Basic Principles of Celestial Navigation

by | Last updated May 9, 2024 | Navigation, Shipboard Operations | 0 comments

I have mad respect for our early sailors who brave the seas and rely on celestial navigation to find their way. More than anyone else, these seafarers heavily studied and understood this subject matter to conquer the unknown.

Back in the day, before fancy GPS and electronic gadgets came along, sailors had their own tricks up their sleeves. They didn’t have satellites beaming coordinates to their fingertips. Nope! They relied on something truly awe-inspiring: the stars. 

By looking up at the night sky, they could figure out their position on Earth. Cool, right!? It’s like a cosmic game of connecting the dots! They paid close attention to the sun, moon, stars, and planets, carefully measuring their positions and angles. 

With this celestial roadmap, they could navigate the open seas with mind-boggling accuracy. It’s like having your very own cosmic GPS, centuries before it even existed!

What is Celestial Navigation?

Celestial navigation is a type of marine navigation that uses the positions of celestial bodies, such as the sun, moon, and stars, to determine the position of a ship on the Earth’s surface.

Imagine yourself as a sailor on a vast ocean, surrounded by nothing but water stretching as far as the eye can see. No GPS, no cellphones, just you, your ship, and the ever-changing sky above. How do you know where you are and where you’re heading? That’s where celestial navigation or CelNav comes in.

You see, the positions and movements of these celestial bodies can tell sailors a lot about their own position on Earth. By studying their patterns and measuring their angles and positions relative to the horizon, mariners can determine where they are on the globe.

Celestial navigation is also called astronavigation and we will use these terms interchangeably.

Why Should I Learn Celestial Navigation?

Here’s a no sugar-coat reason why many students and cadets have no interest in learning even the basics of celestial navigation: the subject is too complicated and overwhelming! I’ve been there and it’s difficult to learn at first. 

Besides, they flex their GPS, ECDIS, phones, and all those electronically-dependent devices. But those devices have limitations. They are too dependent on electricity and if their power source runs out including the emergency generators, the ship is technically back to the 1600s.

A man taking a sight of a star with his sextant during night time.
Taking a sight of a star during night time. Image: Wikimedia Commons.

Aside from that, learning how to find your position using celestial navigation is a lifetime skill that can’t be stolen. By understanding its principle, you can apply them not only in the middle of the sea but practically anywhere on Earth.

Lastly, you’ll learn more about the stars. And guess who else is interested in those tiny lights in the night sky? Your crush. Telling her about the constellations and how to use them in finding your way can make a pretty interesting topic.

So yeah, there are still rooms for celestial navigation on board (and in your life).

Key Elements and Basic Principles

Remember my huge respect and admiration for the ancient mariners? It’s because of their true understanding of celestial navigation. They mastered the key elements at heart and understood the basic principles of this art.

They don’t have any fancy calculators or computers to cross-check their work. Every sight that they make is a do-or-die situation. They must calculate their position accurately to arrive at their intended location.

By knowing the basic principles and key elements, they know how to harness these celestial markers. This article will help you achieve just that. Let’s dive into them now.

1. Celestial Bodies

The most basic principle of celestial navigation makes the use of celestial bodies. Celestial bodies are objects in space that come in all shapes and sizes. For the purpose of celNav, the celestial bodies that we can use are the sun, moon, stars, and planets.

Celestial navigation relies on observing and measuring the angles and positions of celestial bodies in relation to the observer’s location.

2. Celestial Sphere

The concept of celestial navigation assumes that the Earth is the center of all these celestial bodies. The sphere on which these bodies can be found is called the celestial sphere.

The celestial sphere is an imaginary infinite sphere surrounding the Earth and where the astronomical bodies appear to be located. All the planets, moon, sun, and stars in this sphere are also assumed to have the same distance to the earth’s surface.

Understanding this is important because the position of a celestial body in the celestial sphere corresponds to a geographical position (GP) on the earth’s surface.

And like the latitude, longitude, Prime Meridian, and Equator on Earth, the celestial sphere also has equivalent lines or celestial coordinates. These lines extend from the Earth and projected into the celestial sphere.

The celestial sphere and its corresponding parts in the earth's coordinates.
The celestial sphere.
EquatorCelestial Equator
Prime MeridianCelestial Greenwich
LongitudeRight Ascension (RA)
North PoleNorth Celestial Pole
South PoleSouth Celestial Pole

3. Marine Sextant

A marine sextant is the primary tool for celestial navigation. It allows sailors to measure the altitude or elevation of celestial bodies above the horizon.

These sextant readings are then calculated and tabulated using the nautical almanac.

After applying a little paperwork and math, you can get your LOP or line of position and plot it on the nautical charts.

This equipment is one of the navigational instruments you can find on board.

4. Timekeeping Device

Precise timekeeping is necessary for celestial navigation. It helps in calculating the Greenwich Hour Angle (GHA), which is essential for determining the position of celestial bodies.

The celestial bodies used for navigation appear to move across the sky throughout the day and night. For accurate calculations, make sure that the time of the observation is with great precision. 

This is because the apparent positions of celestial bodies change slightly over time. The sun, for example, moves 15 degrees per hour or 1 degree every 4 minutes. So even a small error in timekeeping can lead to significant inaccuracies in determining position.

To keep track of the correct and accurate time in GMT, mariners use a very handy timekeeping device called a marine chronometer.

5. Horizon

A horizon is an apparent line where the sky meets the Earth or sea and is used to get the altitude of a body. It serves as a reference point for celestial observations. Navigators align the horizon with the sextant’s arc to measure the angle between a celestial body and the horizon accurately.

This is well appreciated when you are in the middle of the sea and there’s a 360-degree horizon around you. You can take sight of any celestial object in any direction unlike when you are on land where there are obstructions.

A seawoman taking the altitude of the sun using a sextant behind the vessel. This is oldest method of position fixing in marine navigation.
Taking the altitude of the sun from the horizon using a sextant. This is the oldest method of position fixing in marine navigation.

6. Nautical Almanac

A nautical almanac is a publication that provides essential data astronomical data, such as the predicted positions of celestial bodies, which allows navigators to calculate their positions accurately.

Since celestial bodies move in a predictable manner, you can find their positions and other important data tabulated in this book. Here are some other details that you can find in the nautical almanac.

  • Positions of celestial bodies
  • Times of sunrise and sunset:
  • Twilights:
  • Moonrise and moonset
  • Eclipses
  • Earth’s rotation
  • Refraction of light.

7. Sight Reduction Tables

The most common book found on merchant ships using sight reduction tables for marine navigation is Norie’s Nautical Tables. It’s always on every ship that I sailed with including new and old. Here, you’ll find the ABC Tables (Table 249) very useful.

The tables offer various calculations and procedures for reducing celestial sights. Navigators use these calculations to convert observed angles between celestial bodies and the horizon into lines of position (LOPs) and intercepts, enabling them to determine their position.

8. Dead Reckoning (DR)

Dead reckoning is the process of estimating a vessel’s current position based on a previously known position, course, and speed. It is basic navigation technique that has been used for centuries and even today.

You can combine celestial navigation with dead reckoning to improve your position’s accuracy.

9. Plotting

Plotting allows navigators to determine their position relative to the true azimuth of the celestial object and its intercept. It is the culmination of all the key elements and basic principles applied on paper.

Additionally, it feels so great when you’re practicing and getting your positions very close to your primary position-fixing method.

How Celestial Navigation Works

With the principles and key elements listed above, we can now apply them to get the position of your ship in the middle of the sea. Not only ship captains and officers are required to obtain this skill but the cadets as well.

You should have a fair idea of how celestial navigation works if you understand those principles. But knowing them better requires practice. Here are methods to understand better how astronavigation works.

1. Plan your sight. Know which celestial body you’re planning to “shoot” and what time. Taking sights on the spot makes you prone to errors. By planning ahead, you will be better prepared and composed.

2. Observe accurate timekeeping. As we discussed above, accurate timekeeping can get you closer or much farther away from your precise location. A few-minute difference between taking the sight and the exact time of taking it can lead to a significant deviation in your calculated position.

Use the marine chronometer to keep track of precise timekeeping.

A marine chronometer which tells accurate time on board.
Image: Bautsch | Wikimedia Commons.

3. Measure the altitude and azimuth of a celestial body. This basically means determining its angle above the horizon and its angular distance from a reference point. By using a sextant, you can make these measurements with surprising accuracy

4. Crunch some numbers. This is when the nautical almanac and sight reduction tables come into play. By comparing the observed angles with the known positions of celestial bodies at specific times, you can determine your own position relative to those celestial bodies.

5. Apply dead-reckoning and plot your position. After your calculations, interpolations, and paperwork, it’s time to plot your position in the nautical charts.

We many navigational equipment on board that

By practicing these techniques, you will understand better how celestial navigation functions onboard. It works hand in hand with the basic principles and key elements.

Furthermore, the more you understand one of them, the deeper you’ll learn about the subject.

May the winds be in your favor.



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