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Celestial Navigation – Understanding the Basics

By Jeff Delaney

In age of the infallible GPS, most sailors never give serious consideration to mastering the practice of celestial navigation. However, celestial navigation is not only a rewarding skill to hone, but is can be quite fun. After spending a frustratingly long time learning the basics, I’ve decided to map out an easy to follow guide to help you impress your friends using this “primitive” form of navigation.

Everything the Beginner Needs to Know

There are several key concepts that you’ll need to understand before becoming a successful celestial navigator. It’s assumed that you already understand the most basic aspects of navigation, such as how to read latitude/longitude, nautical charts, and a compass. Print this article out and use it when you are taking measurements in the field.

The Theory of Celestial Navigation

So how does celestial navigation work? At this exact moment in time the heavens are aligned in a way that is unique to your position on earth. If you you could be magically teleported to your in-law’s house 200 miles away at this exact moment, the heavens would appear slightly different. Or if you just look in the sky a few hours from now, you will find the sun, moon, and stars in different spots. If you were to measure the position these heavenly bodies at a specific moment in time (accurate to the second), you could look up their positions in the nautical almanac, then plot your position on a nautical chart. It’s helps to imagine the Earth as the center of the universe, with the Sun, Moon, planets, and stars rotating around it.

Navigators are able to obtain a precise fix on their location by measuring at least 2 celestial bodies with a sextant. The sextant measures the angle of a celestial body from the horizon and a wrist watch measures the exact moment in time. After performing some basic mathematic calculations and referencing figures in the nautical almanac, the end result is a “position line” plotted on the nautical chart.

Taking a single celestial measurement will result in a giant circle thousands of miles in diameter encompassing some part of the globe. Your boat is at some spot along this great circle. Therefor, we must take at least two measurements to obtain our precise fix. The second gigantic circle will intersect with the other circle, giving us an exact geographic position (X marks the spot). You will understand how this works as you start taking measurements and plotting position lines on a chart.

A Visual Description of How it Works

This diagram shows the result of taking one celestial observation. In theory, your vessel could be anywhere along the position line, which is represented by the black circle.

This diagram shows the result of taking one celestial observation. In theory, your vessel could be anywhere along the position line, which is represented by the black circle.

As you can see, the position line circles intersect in two spots. Since we know our vessel is not in Eastern Europe, it must be the point of intersection in the Atlantic Ocean.

As you can see, the position line circles intersect in two spots. Since we know our vessel is not in Eastern Europe, it must be the point of intersection in the Atlantic Ocean. This is how navigators obtain a fix on their precise geographic coordinates.

More Key Concepts

You should also have a strong understanding of the concepts below.

Greenwich Mean Time GMT (Universal Time UT) – Greenwich Mean Time refers to the timezone that runs through the Royal Observatory in Greenwich, England and is the global standard for timekeeping. You will need to set your watch to GMT or convert your time to GMT after taking observations. Personally, I prefer to set my watch to GMT in advance. Get the exact GMT here.

Dead Reckoning (DR) Position – Basically, this is an estimated position based on a course you’ve been following. For example, if you started at position N35° W120°, then sailed on a heading of 030° at 6 knots for 5 hours, you would use this data to plot a DR position. It intentionally does not take into account fluctuations in steering, speed, etc.

Geographic Position (GP) – Imagine a straight line that travels through the center of the earth to the center of the sun (or any other celestial body for that matter). The coordinates of this line on the surface of the earth are  considered geographic position of celestial body.

Declination – Declination is simply the latitude of a celestial body. If you live in North America, you should notice that the days are longer in summer and shorter in winter. This is because the sun is at a declination of N23° during the summer solstice, then sinks to S23° during the winter solstice. In the spring and fall equinoxes when days are equal to nights, declination is exactly 0.0°. You will be looking up declination in the almanac and using it as one of the arguments, or variables, when you use your sight reduction calculator.

Hour Angle – Hour angle is way we measure the longitude of a celestial body. This angle is measured from the Greenwich meridian and is called the Greenwich Hour Angle or GHA. From here, we can use some addition or subtraction to get the Local Hour Angle or LHA. Don’t worry if this doesn’t make sense yet, you’ll see it in action in the next article.

Altitude – Altitude is the angle between YOU (the navigator holding the sextant) and the SUN (or moon, planet, or star). This angle is what we measure with the sextant and will be somewhere between 0° and 90°. Directly above your head would be 90°, and directly on the horizon would be 0°.

Celestial Navigation FAQs

Is celestial navigation difficult? In my opinion, yes! It does not require any exceptional math skills, but it’s a process with many steps. Compared to luxury of GPS, which takes a few seconds to read, a celestial fix may take a couple hours to obtain depending on the position of the heavenly bodies you’re measuring.

How accurate is celestial navigation? I’ve been able to consistently locate my position within 1 to 3 nautical miles when checked to the GPS.

What does a Sextant do, and how does it work? A sextant is a tool for measuring the altitude (or angle) of celestial bodies from the horizon. It works by allowing the user to look through an eye piece which goes to a split screen of two images. The left image is fixed and kept on the horizon. The right image goes through a series of mirrors that can be moved by the user to view a celestial body. The user then brings the celestial body to the same level as the horizon, thus providing a measurement for it’s altitude.

What Sextant should I buy? I purchased a Davis Mark 15 sextant on eBay for under $100, which is a great deal. It’s plastic and feels fragile, but produces very accurate measurements and is one of the most popular models around. Metal sextants are awesome, but expensive. Old WWII sextants can be found for several hundred bucks, but they may need calibrating and/or maintenance.

Tools of the Trade

Hopefully you’re ready to get started with the rewarding skill of navigating by the stars. Below is your shopping list. The total damage should be somewhere around $200 if you’re starting from scratch. It’s worth noting that sextants hold their value well and you can easily resell them on eBay for near break-even amounts.

  • Sextant – $100 to $500
  • Wrist Watch – $10
  • Notepad and Pen – $3
  • Commercial Nautical Almanac – $20
  • iPhone Sight Reduction Calculator – $1.99
  • (Optional) Sight Reduction Tables H.O. 229 – $50
  • Nautical Chart for your Area – $20

In part two I will explain how to use your sextant to obtain an accurate celestial body measurement.