How to Use a Sextant

How to Use a Sextant

A sextant is a navigational instrument used for measuring the angle between two objects in the sky, such as the sun and the horizon. It is primarily used for navigation at sea, and is useful when a reliable reference point is not available, such as in open waters. Operating a sextant requires a certain level of skill, patience and practice. This guide will walk you through the steps on how to use a sextant for basic navigation.

Step 1: Prepare the Sextant

Before you begin, it is important to ensure that your sextant is properly assembled and adjusted. First, check that the frame is securely attached to the index arm. The index mirror can be adjusted with the tension screw. Use the adjustment screws on the arc to adjust the bubble level. Make sure the screws are tight and that the sextant is level. The horizon mirror should be at a 90° angle to the index mirror, and the arc should be tight and well lubricated.

Step 2: Sight the Object with the Sextant

Once your sextant is ready, you can begin sighting the object. Start by ensuring that the object is square in the index mirror. If it is not, you can adjust the index mirror with the tension screw until it is. Then, look through the small telescope while turning the micrometer drum to bring the lighted lines together. When lined up, the index arm will be pointing to the angle you need to record.

Step 3: Take the Measurement with the Sextant

Once the object is in the index mirror, you can begin taking the measurement. There is a scale located on the arc of the sextant, which will indicate the angle between the horizon and the object. You will need to read the scale from left to right, since the object is usually located farther away than the horizon. Alternatively, you can use the vernier scale, which is more precise and can measure to within a few arc seconds. To use the vernier, you will need to determine the distance between the two lines, which can be determined by counting the number of divisions between them.

Step 4: Record the Measurement

Once you have determined the angle with the sextant, you will need to record the value. This can be done by writing the value down in a logbook, which will be used later for calculating your position at sea. It is important to note that the measurement should be recorded with the time of day, as well as the direction (north, east, south, west) in which the object was sighted. This will ensure that you have a reliable reference point when navigating.

Step 5: Make Calculations with the Sextant Measurement

Once you have recorded the measurement, you can use it to calculate your position at sea. This will require calculating the latitude and longitude of the position using the sextant measurement. You will need to determine the latitude by plotting the angle of the sun and the horizon at noon, and then using the angles determined by the sextant to calculate your latitude. Similarly, you can calculate the longitude by determining the time from Greenwich, and then plotting it against the angle of the sun. Once you have determined the latitude and longitude, you can calculate your exact position at sea.

Step 6: Practice Using the Sextant

Using a sextant takes time and practice to perfect. The best way to become proficient with the instrument is to practice on dry land first with the help of a chart. Here, you can get the hang of taking measurements, recording the readings, and making calculations. With time, you will become more accurate with the sextant and can apply the knowledge in a real-world navigation exercise.


Navigating with a sextant can be a tricky skill to master, but with patience and practice, it can be done. To use a sextant, first prepare the instrument by checking for tightness and level. Next, sight the object and take the measurement with the sextant. Remember to record the readings along with the time and direction of the object sighted. Finally, use the measurements to make calculations for your current position at sea. With practice, you will become a pro at using the sextant for navigation.

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