Not Expensive. a Good Davis Plastic Sextant Costs $120

CAPT CHRIS’S TIPS ON USING A SEXTANT

1. Plastic versus metal sextant. A good quality plastic sextant will do the job but has inherent deficiencies that must be dealt with.

On the positive side: - Not expensive. A good Davis plastic sextant costs $120. - Light weight. Will not tire your arm and wrist as will a heavy, brass sextant. - Accurate enough to produce a celestial fix. - Because you know it is not expensive, you will not be afraid to actually use it, and under conditions in which you might hesitate to expose an expensive metal sextant. - If you drop it over the side, you will not be tempted to dive in after it.

On the negative side. - Not expensive. You might be tempted to knock it around which you would not do with an expensive metal sextant. - Mirrors need to be checked (and, if necessary, aligned) before using for a series of observations. See alignment procedures below. - Some reduction in fine accuracy.

Conclusion. Buy and use a plastic sextant until you determine you need something better then buy a metal sextant.

2. Know what you are going to observe. If you are observing stars and planets, have a perfect knowledge of their identity in the sky. The best way to ensure this is to identify them in the sky the evening or morning before you intend on observing them – perhaps doing so before getting underway and while still at the dock or at home before leaving. You can download the night sky on the internet and go outside to identify the first stars or planets that show up at dusk. Once you identify the first stars that appear, you will find yourself using them over and over while sailing. It is certainly not difficult to learn a few stars in the night sky for each season of sailing.

3. When determining what to observe consider the boat’s course and tack. It is much easier to observe heavenly bodies to the south when on a starboard tack heading east than on a starboard tack heading west; Clearly, it is preferable to have an unobstructed view of the heavenly bodies rather than to be peaking around and under the mainsail as you try to take a shot.

4. A heavenly body that is climbing or descending is a better target than one approaching the meridian at the peak of its apogee across the sky. While climbing or descending at medium altitude, the heavenly body will be moving very distinctly from one observation to the next while a heavenly body at the peak of its arc across the sky will seem to float at its apogee giving confused and contradictory sextant angles.

5. Pick heavenly bodies that will make a convenient triangulation. They should ideally be between 45 and 60 degrees apart. Remember that two stars 180 degrees apart serve no useful purpose when used together. And, remember that if you are using PUB NO. 249, Vols 2 and 3, everything you shoot must be south of 29 declination. Otherwise you must use one of the recommended stars from Vol 1.

6. Prepare the following for a sextant observation:

- clipboard with an extra clip on the bottom to hold a observation record sheet firmly down from both ends, otherwise, an unattached end will constantly blow up just as you are trying to write on the sheet.

- listen to a short wave broadcast of universal coordinated time (UTC) and record any time error of your digital watch that serves as your chronometer. - Place an observation record sheet on the clipboard with columns for heavenly body, UT date and time, sextant height, DIP, course, speed and DR position at the time of the observation.

- Have a good mechanical pencil and eraser.

- Have a penlight if shooting in the early morning or, particularly, in the evening twilight. I prefer a head-mounted LED that I can turn on and off.

- Prepare your sextant by checking the mirror alignment. If you intend on observing from outside the cockpit, suspend the sextant around your neck with a camera strap should you need your hands free.

7. The best (and safest) place to use a sextant is while sitting in the cockpit of your boat. If you are alone you can place your clipboard beside you on the cockpit seat, take a shot with both hands, glace at your digital watch, then place the sextant in your lap and record the time and sextant angle on the record sheet firmly attached to the clipboard. If you have an assistant to record the time and sextant height, you have the luxury of standing in the shrouds to make an observation. The assistant can sit in the cockpit while you wrap an arm around a shroud and shoot an observation standing up. But remember - moving around on the deck of pitching, rolling sailboat with a sextant in your hand is a prescription for disaster. It is better to plan so that you can shoot everything while safely seated in the cockpit – even with an assistant.

8. Standard procedure - Bring the heavenly body down to the horizon - Swing the arc - Adjust as necessary - Stop and glance at your digital watch for the time. - Secure the sextant by sitting it in your lap - Pick up the clipboard - Record the time. - Read the sextant height off the sextant. - Record the sextant height. - Pick up the sextant - Move the same heavenly body up from the horizon and bring it down again to the horizon. - follow steps above again. - After you record three sights of the same star, you can average them or pick the one you feel most confident about and proceed to the next heavenly body.

9. Shooting the Sun.

- Put the necessary shades in place and use a low-power scope in the eyepiece. - Determine if you will shoot the upper limb or lower limb. Always shoot lower limb unless it is obscured. - Follow steps in #8 three times.

10. Shooting the Sun at Local Apparent Noon (LAN) for latitude.

- Using the Nautical Almanac, determine the exact moment of noon for your DR longitude and latitude. - Watch the compass for the Sun to approach 180 degrees – assuming it is south of you. - Begin taking observations 15 minutes prior to noon. Take an observation every three minutes and record each one. When the Sun stops climbing in the sky, take observations every minute until it begins to descend. Set the sextant to the same angle as your first observation (Hs) and continue shooting the Sun until it again reaches the same Hs. - If possible graph out the observation angles and determine the moment of noon and the peak Hs. - By averaging the first Hs with its time and the last Hs with its time, you can determine the exact moment of LAN. 11. Finding a dim star in the sextant eyepiece and bringing it down to the horizon can be difficult. Rather than find the star, find the horizon. When shooting a star, carefully turn the sextant upside down and using both eyes, sight the star in the eyepiece then adjust the arm to bring the horizon up to the star. Turn the sextant over and continue.

12. Learn to use Pub No. 249, Volume 1; it will save you time and effort. You will need to plan your sights but that is not difficult.

- Determine the moment of civil twilight for your latitude and longitude as explained in Simple Celestial. - Use the Sight Planning Form to determine the LHA of  as explained in Simple Celestial. - Extract the recommended stars from Volume 1. - Starting to the East, set your sextant to the first star’s Hc and complete your observations. - Follow the guide in Simple Celestial to reduce your sights and plot your fix.