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Celestial Navigation Part III – Tips and Tricks on Sight Reduction

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Most books on celestial will offer a ‘simple’ sight reduction form. Don’t use it. Most of these forms are standardized for use with any celestial body, and will contain information not applicable to a sun sight. They will confuse you. Make your own forms instead (Download your sight reduction form). The form is in three parts, one for each stage of the sight reduction.

Celestial Navigation Part II – Predicting the Suns Geographic Position

Correcting the Sextant

Correct the sextant reading for index error, dip (height of eye) and observed altitude. Read your sextant’s instructions on how to correct index error – a properly maintained sextant should have zero error. If it does, it will reveal itself when the sextant is set on 0º 0′ – when pointed towards the horizon; the reflected horizon will appear slightly above or below the actual horizon. Adjust the micrometer drum until both horizons appear as one – the amount of adjustment is your index error; minus (-) if it is on the arc, and plus (+) if it is off the arc. Height of eye is simply how far off the water you were when you took the sight.

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Inside the front cover of the Almanac you will find a table giving minutes of correction corresponding to various dip measurements, in feet and meters. Refer to this, but note that on most cruising sailboats, the dip (your height of eye) will be about six feet, with the corresponding correction about 3′. The altitude correction table is found on the same page. This correction accounts for the thickness of the atmosphere through which the sun’s light must travel, and it’s being refracted because of it. Think of the grade-school pencil-in-a-glass-of-water example. Find the corresponding range of observed altitude, and record the correction on the form. Note the different tables for different times of the year. Some corrections may be negative, so always put a plus or minus sign before each number to avoid confusion. Total the three corrections, and record the resulting Ho – the observed sextant angle – on the highlighted line. You’ll need this number later.

The Sun’s GP: Your Watch & The Almanac

Perusing the Nautical Almanac, you’ll find all kinds of useful information, from the rise and set of the sun and the moon, to information on the 57 most useful navigational stars as well as the best times of the year to view the different planets. All of this information is useful to the navigator. What we need to get started is information on the Geographical Position (GP) of our celestial body – the sun in this case. Find the page corresponding to the date the sight was taken. Each date consists of two full pages of information – stars and planets on the left page, sun and moon on the right. Find the sun column. You will see two columns of information, labeled d (declination) and GHA (Greenwich Hour Angle) at the top, and a column of numbers down the side, corresponding to whole hours of GMT. Locate the hour of GMT when you took the sight, and record the values for d and GHA in their appropriate places on the form. There are two GHA slots on the form – since the ‘date page’ only includes information for the whole hour of GMT, you’ll refer to the ‘grey pages’ (at the back of the Almanac), which give figures for the minutes and seconds of GMT. Add these figures to the hourly GHA to arrive at the total GHA (recall that the sun travels a full 15º of longitude each hour – and always to the west – so the minutes and seconds make a huge difference). Declination changes only negligibly from hour to hour, so one figure here is sufficient.

Next, record your assumed position (AP). This figure is simply the closest whole degree of latitude from your dead reckoning, plus your DR longitude degrees, but with the same minutes as your total GHA. The goal is to end up with a Local Hour Angle (LHA), expressed in whole degrees. Get this figure by subtracting your AP longitude from the total GHA (by making the minutes of your AP longitude the same as the minutes of GHA, the subtraction will cancel them out, leaving a whole number). Where GHA is the sun’s position relative to Greenwich (and 000º longitude), LHA is the sun’s position relative to you (almost, anyway, – you in this case is an assumed position (AP), near to your DR but a spot on the globe with whole degrees of latitude and longitude. The sight reduction tables have been computed to include information on the bearing to the sun (azimuth), as well as what an imaginary sextant would have read based on these whole numbers. The navigator then gets his line of position by comparing this imaginary sextant reading to his own, and plotting the difference). Next month, we’ll describe exactly how to do that, and bring the celestial series to a close.

Andy Schell is a professional yacht captain. He contributes regularly to All at Sea and several other sailing publications, and is Chief Editor of the annual Yacht Essentials Portbook. He and his wife Mia recently completed an Atlantic crossing to Ireland via Newfoundland on their yawl Arcturus. 

Don’t miss the full Series on Celestial Navigation.

What is your experience with Celestial Navigation?

Of course comments or questions to the author are welcome below.

Celestial Navigation Part IV – Publication 249 and The Universal Plotting Sheet

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Andy Schell
Andy Schellhttp://59-north.com
Andy Schell is a professional sailor, writer and the event manager of the ARC Caribbean 1500. You can find him online at 59-north.com.

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