How to Use a Sextant

Captains Brass Sextant

 Using a sextant involves measuring the angle between a celestial object (like the sun, moon, or stars) and the horizon. This angle is then used in navigation to determine your position on Earth. Here's a basic step-by-step guide on how to use a sextant:

1. Prepare the Sextant

• Check Calibration: Ensure the sextant is calibrated properly (it should read zero when the horizon and the instrument are aligned).
• Adjust the Mirrors: The sextant has two mirrors, the horizon mirror and the index mirror. Make sure both are clear and properly aligned.
• Adjust the Micrometer Drum: This is used for fine adjustments when measuring angles.

2. Choose the Celestial Object

• For daytime navigation, the sun is the most common object to measure.
• At night, you can measure stars, planets, or even the moon.

3. Align the Sextant

• Hold the Sextant: Hold the sextant in your hand at eye level.
• Find the Horizon: Look through the eyepiece and locate the horizon. The horizon mirror reflects the lower half of the horizon into the field of view.

4. Measure the Angle

• Sight the Celestial Object: Using the adjustable arm (the index arm), bring the celestial object (like the sun or a star) into the line of sight by moving the arm.
• Align the Object with the Horizon: Adjust the micrometer or fine-tune the arm until the celestial object is aligned with the horizon. The goal is to get the object directly on the horizon line as seen through the instrument.

5. Record the Reading

• Once the celestial object is aligned with the horizon, read the angle displayed on the sextant’s scale (usually in degrees, minutes, and seconds). The angle is called the altitude of the celestial object above the horizon.

6. Correct for Refraction and Instrument Errors

• You may need to apply corrections for factors like atmospheric refraction, which causes objects near the horizon to appear higher than they are.
• Some sextants have built-in corrections for these, but others might require manual adjustments. You may also need to account for the height of the observer's eye above the horizon, known as the "sextant height."

7. Use the Angle to Calculate Your Position

• Time and Location: To calculate your position, you also need the exact time of your measurement (usually in Universal Time, UTC), the date, and the location (longitude).
• Nautical Almanac: Using the altitude you recorded and the time of the observation, consult a Nautical Almanac to determine the position of the celestial object at that time.
• Use a Sight Reduction Table or Calculator: These tools will help you convert the altitude into your position on a nautical chart.

Tips:

• Practice: It takes time to become proficient at using a sextant. Practice is key to getting accurate readings.
• Stable Environment: Make sure you're in a stable position (such as on a steady ship or a calm environment) to avoid errors from movement.