What It’s Like to
Do Celestial Navigation

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In Short: Fun, but hard work.

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Read Time: 3 minutes

During my Navy career I was intrigued by celestial navigation: figuring out where you are by looking up at the sky. As Navigator on one ship, I made sure we were proficient at celestial navigation, even though we had every form of radio navigation available, and then some.

To figure out where you are, you need to know exactly what time it is, and you need a Nautical Almanac that lists the position in the sky of every celestial body used for navigation. Generally, this means the Sun, the Moon, Venus and about 57 selected stars, being well scattered around the sky. Also, you need a clear horizon.

To get the basic idea, suppose you are standing under a street light. Look up at it and measure the angle from the ground to the light. Take a map of which shows where that light is located and draw a cone downward from the light using the angle you observed. This cone makes a circle on the ground. If you have three street lights and three circles, you are at the position where the circles intersect.

A sextant (3D rendering by Lars Plöger, Pixabay)

So much for the easy part.

To take a celestial fix, you use a sextant and measure the angle between the star and the horizon. This is done by peering through a small telescope and moving the rocking arm until the star appears to sit right upon the horizon, because the view is split in half, partly by a mirror that bounces up to the star and partly by clear pane. When the star is sitting on the horizon, you look at the side of the sextant and the rocker arm tells you the angle to the star.

Actually, the hard part is figuring out which star is which. First you start with the celestial almanac and determine, from your estimated position, which of the useful stars will be available. Then you go topside with your sextant, and an assistant who has a calibrated stopwatch with the exact time.

You select a star which is bright and clear, for which you can see a crisp horizon line below. You then line up the sextant, move the rocker arm to bring the star to the horizon, and spin a little caliper to make it exactly kiss the horizon. Rocking the sextant side to side helps, as the star appears to swing like a pendulum. When it’s exact you call out “mark,” and the assistant writes down the time. Being off by even one second means an error of 60 nautical miles.

In my day we used a worksheet to record five or six (or more) star sightings, then worked down through the sheet. The mathematics needed is thus made simple. At the bottom each column of the worksheet gives a line which is plotted on the chart.

Often, many of your sightings will fail, most likely due to the fact that you looked at the wrong star. You hope you have enough sightings to give at least three intersecting lines on your chart.

You can also use the Sun and Moon, which are much larger and closer. Larger requires you to choose whether to have the top or the bottom kiss the horizon. Closer means they appear to move much faster, so you have to be quick to get an exact measurement. But it was always fun to say we were “Shooting the Moon.”

Bill Cupp is a retired Naval Officer, and a retired Computer Science professor. He likes to “retire early and retire often.”

Editor’s Note: Bill notes that this motion gif on Wikipedia is a good visual summary of how a sextant works.

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2 thoughts on “Do Celestial Navigation”

  1. This is a very well-written and helpful article about how a sextant works. How interesting that being off by one second could mean an error of that size! When bobbing around in the ocean, 60 miles can be a large distance if any type of search is being done. Thanks for the very informative article!


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