A Detailed Practical Guide to Celestial Navigation

I. Essential Tools for Celestial Navigation

To perform celestial navigation, you’ll need several key tools:

• Sextant: This is the primary instrument for measuring the angle (altitude) between a celestial body and the horizon. A plastic sextant is adequate for learning and emergency use, but a metal sextant offers greater accuracy due to its more precise calibration and stability. Consider purchasing a sextant with a wide field of view and good optics.
• Chronometer: A precise timepiece is crucial for determining your longitude accurately. A traditional chronometer is expensive, but a highly accurate quartz wristwatch synchronized with a reliable time source (like a GPS signal before you leave) is sufficient. It is very important to keep accurate time and to sync your timepiece before taking measurements.
• Nautical Almanac: This book contains detailed celestial body positions (Greenwich Hour Angle and Declination) for each day of a given year. Be sure to obtain an almanac for the correct year.
• Sight Reduction Tables: These tables (e.g., Pub. No. 229) simplify the calculations needed to convert sextant readings into a geographical position. They provide pre-calculated altitudes and azimuths for various celestial bodies and assumed positions. Alternatively, nautical tables or a scientific calculator and sight reduction formulas can be used, though these are more complex.
• Nautical Charts: Charts of the region you are navigating are needed to plot your position. Use the appropriate scale chart for your area and the level of detail required.
• Compass: A reliable compass is essential for maintaining your heading and aligning your sextant readings with geographic directions. A magnetic compass with a bearing compass is recommended.
• Plotting Tools: A parallel rule (or rolling plotter) and dividers are needed to plot lines of position on your chart. A sharp pencil and eraser are also essential.
• Sight Reduction Worksheet (Sight Form): A pre-printed worksheet or form helps organize your observations, corrections, and calculations. This is vital for avoiding errors.

II. Basic Techniques of Celestial Navigation

Mastering celestial navigation involves understanding and applying several fundamental techniques.

Taking Sightings

This involves measuring the angle between a celestial body and the horizon:

1. Preparation: Choose a celestial body (sun, moon, planet, or star) based on visibility and time of day. Pre-calculate its approximate altitude and azimuth to aid identification.
2. Sextant Handling: Hold the sextant firmly and level. Adjust the filters to reduce glare. For the sun, always use filters to protect your eyes!
3. Finding the Horizon: Look through the sextant’s telescope and find the visible horizon. This can be challenging in rough seas. Focus the telescope and ensure the horizon is clear.
4. Bringing Down the Body: Use the sextant’s micrometer drum to “bring down” the celestial body until its lower limb (edge) touches the horizon.
5. Rocking the Sextant: Gently rock the sextant back and forth to ensure you are measuring the lowest possible altitude at the point of tangency with the horizon.
6. Reading and Recording: Note the sextant altitude reading and the exact time from your chronometer. Record these values immediately on your sight reduction worksheet.

Correcting Sextant Altitude

Sextant altitude requires several corrections to get the true altitude:

1. Index Error (IE): Determine the index error by sighting the horizon and adjusting the micrometer drum until the horizon appears continuous. The reading on the drum is the index error. Record whether it’s “on the arc” or “off the arc.” Apply the correction to your sextant reading.
2. Dip: Dip is the correction for the height of your eye above sea level. Use a dip table (found in the Nautical Almanac) to find the correction based on your height of eye. This correction is *always* subtracted.
3. Refraction: Refraction is the bending of light as it passes through the atmosphere. Use a refraction table (in the Nautical Almanac) to find the correction based on the observed altitude. This correction is *always* subtracted.
4. Parallax: Parallax is a correction for the Earth’s radius and is significant for the Moon and planets. Use the parallax correction tables in the Nautical Almanac.
5. Semi-Diameter (SD): For the Sun and Moon, you measure the lower limb (edge) rather than the center. A semi-diameter correction is needed to account for the difference. This value is provided in the Nautical Almanac and *added* to the observed altitude for the Sun, *added or subtracted* for the moon depending on whether the lower or upper limb was used..
6. Total Correction: Sum all the corrections (with their correct signs) to obtain the total correction. Apply this total correction to your sextant altitude (Hs) to get the observed altitude (Ho).

Calculating Position

This involves using the Nautical Almanac and sight reduction tables:

1. Greenwich Hour Angle (GHA) and Declination (Dec): Use the Nautical Almanac to find the GHA and declination of the celestial body at the *exact time* of your sighting. The GHA is the angular distance westward from the Greenwich Meridian to the celestial body’s meridian. Declination is the angular distance north or south of the celestial equator. Interpolate between hourly values if necessary.
2. Assumed Position (AP): Choose an assumed position (AP) close to your estimated location. The AP is a whole degree of latitude and longitude, which simplifies calculations when using sight reduction tables.
3. Local Hour Angle (LHA): Calculate the Local Hour Angle (LHA) by subtracting your assumed longitude (West is +, East is -) from the GHA. If the result is greater than 360 degrees, subtract 360. If the result is negative, add 360. LHA = GHA + Longitude (W) or LHA = GHA – Longitude (E).
4. Sight Reduction Tables: Enter the sight reduction tables with your assumed latitude, the celestial body’s declination, and the LHA. Look up the calculated altitude (Hc) and azimuth angle (Z).
5. Azimuth (Zn): Convert the azimuth angle (Z) to true azimuth (Zn). Use the rules for converting Z to Zn based on whether your latitude is North or South and whether your LHA was East or West. The Zn is the direction from your assumed position to the celestial body.

Determining the Line of Position (LOP)

The LOP represents all possible locations where the altitude of the celestial body would be the same at the time of your sighting:

1. Altitude Intercept (a): Calculate the altitude intercept (a) by finding the difference between the observed altitude (Ho) and the calculated altitude (Hc). a = Ho – Hc. If ‘a’ is positive, the observed position is *toward* the celestial body. If ‘a’ is negative, the observed position is *away* from the celestial body.
2. Plotting the LOP: On your chart, plot your assumed position (AP). Draw a line from the AP in the direction of the azimuth (Zn). Measure the altitude intercept ‘a’ along this azimuth line. If ‘a’ is positive, measure the distance *toward* the celestial body; if ‘a’ is negative, measure the distance *away* from the celestial body. Remember, 1 minute of arc on a chart equals 1 nautical mile.
3. Drawing the LOP: At the end of your measured intercept, draw a line perpendicular to the azimuth line. This is your Line of Position (LOP). Your actual position is somewhere on this line.

Fixing Your Position

To get a fix (a precise position), you need at least two, ideally three, Lines of Position (LOPs). Each LOP provides a line along which you are located. The intersection of these lines gives you your position:

1. Multiple LOPs: Take sights of two or three different celestial bodies (or the same body at different times). Repeat steps 1-4 above for each sighting to obtain multiple LOPs.
2. Running Fix: If using only one celestial body, take multiple sights over a period of hours. Advance the first LOP along your course line for the distance traveled between sights. This gives you a “running fix.” The accuracy depends on the accuracy of your course and speed estimates.
3. Plotting the Fix: On your chart, plot all the LOPs. The point where the lines intersect is your estimated position. If the lines do not intersect perfectly, you will have a small triangle. The center of the triangle is generally taken as the estimated position. The size of the triangle gives an indication of the accuracy of your sights.

III. Methods for Celestial Navigation

Several methods can be used for celestial navigation:

• Noon Sight: Determine latitude by observing the sun’s maximum altitude at local noon.
• Position by Intercept (Marcq St. Hilaire): Use altitude and azimuth calculations to determine a line of position from any celestial body.
• Sun-Run-Sun: Combine multiple sun sights taken over a period of hours to determine both latitude and longitude.

IV. Practical Navigation

In practice, celestial navigation is often used in conjunction with other navigation methods.

• Correcting Dead Reckoning: Use celestial sights to correct a dead reckoning track, which is estimated from the vessel’s position, course, and speed.
• Redundancy: Celestial navigation serves as a vital backup in the event of equipment or electrical failure.
• Emergency Navigation: Taking sun lines a few times a day and advancing them by dead reckoning allows a vessel to get a crude running fix sufficient to return to port.

V. Learning Resources

To master celestial navigation, consider the following resources:

• Books: “The Practical Guide to Celestial Navigation” by Phil Somerville is an accessible and practical guide. Bowditch’s “American Practical Navigator” is a comprehensive reference.
• Courses: Enroll in a celestial navigation course offered by a reputable maritime academy or sailing school.
• Practice: Practice taking sights and performing calculations regularly to develop proficiency. Use practice scenarios and simulations.