The three-shot ruleJan 1, 2003
One of the hallmarks of a competent celestial navigator is the ability to take quality sights under almost any conditions. However, even the most practiced navigators recognize that often a sight that seems of high quality when first taken may not yield good results when the LOP is laid on the chart. So, to increase the odds of a good result, a useful practice for all navigators is to take multiple sights when conditions allowwhat I'll call the three-shot rule.
The three-shot rule is a practical tool as well as an excellent training aid. In practice, taking three sights of any celestial body takes no more than a few extra minutes on deck. The small amount of additional time spent in sight reduction is well worth it. As a training aid, three shots gives the novice a chance to hone skills and see the results of improved technique over a much shorter time span.
Let's take an example of a sun sight taken through a fast-moving cloud deck, where waiting for the absolutely perfect shot may be a futile exercise. When the navigator arrives on deck to this situation, the temptation is to either grab a quick sight, hoping for the best, or put the sextant back in the box. A better strategy, especially if the weather is closing in, is to get settled in a comfortable position, experiment with the various shade combinations necessary for alternating sunlight and clouds, and take a series of at least three sights to average out any errors.
Figure 1 shows a sample plot of three sights of the sun taken within two to three minutes of each other. While two of the LOPs are nested together, the third LOP stands alone. The navigator would likely discard the outlying sight and use one of the other two. An alternate strategy would be to take all three sights and average them together, yielding an LOP somewhere in the middle (this resulting LOP does reflect any possible errors in the outlying sight, however).
Another situation for which three shots is recommended is when the vessel is experiencing rough seas, making it difficult to trust any one sight. In this situation, multiple sights are highly advisable, no matter how uncomfortable it may be on deck. The errors induced by the conditions can be averaged out over several sights, yielding an LOP that could be valuable if the alternative is a long period of dead reckoning.
When applying the three-shot rule, the navigator should attempt to take the sights as close together as possible. It is not necessary to zero out the sextant after each shot. A common strategy is to record the first sight (time and altitude), relax the arms for a few seconds, and then take the second sight, repeating the procedure for the third, all within the space of two to three minutes. This way, the sextant is preset for the second and third shots, and all that is necessary is to swing the arc and make a final adjustment of the micrometer drum.
The three-shot rule can also be used with stars and planets provided the navigator is speedy in getting a solid round of shots. (This may only be possible in higher latitudes where twilight lasts longer than a few minutes.) An example of when this might be a valuable strategy is any time a thin haze obscures the horizon, making clear star shots especially difficult. In such conditions, selecting the brightest star or planet for three shots allows the navigator to better the odds on what would otherwise be a low-quality fix, even if the remaining stars yield questionable results. A planet such as Venus, quite visible in the early evening sky (usually well before civil twilight) during the midsummer months, is a perfect candidate for multiple sights, both before and after the navigator shoots a round of stars.
When the navigator gets down to the business of sight reduction, a columnar form set up for multiple sights can help. Since many elements of the calculations will be the same for each sight (e.g., dip, index error, hours of GHA, and others), one look up in the tables can serve for all three sights. (Remember, though, that each sight has it's own assumed position and resulting Zn and intercept!) Use of a calculator, such as a Celesticomp, will speed up the reduction process of multiple sights.
If one is honing skills, taking three shots is one way to get feedback on improving skill. I have seen students increase their accuracy measurably in just a 30-minute session of steady sight taking. Without the worry of reducing the sights until later, the student is better able to concentrate on the mechanical aspects of the sight. The results usually show increased accuracy as the student masters the technique. This process can be accelerated if the teacher is taking sights along with the student, comparing sextant readings along the way.
Other times when the three shot-rule is useful include testing a sextant's accuracy, and moon sights for which determining an accurate limb is difficult. Additionally, three shots just make sense whenever navigational accuracy is needed, such as nearing landfall or gauging the effect of a current.
Although most navigators rely on GPS for accurate information when entering a current such as the Gulf Stream, celestial works quite well and provides an excellent navigational challenge. Figure 2 shows the plot of three sights taken to confirm the effect of a favorable current (like what would be encountered crossing the Gulf Stream on a passage to Bermuda). One LOP shows a negligible current while the other two confirm the expected presence of a cold eddy. Discussions among navigators after recent Marion-Bermuda races have shown that the celestially navigated boats often have as much information about Gulf Stream features as the electronics-class boats.
The discipline of the three-shot rule is tough to maintain when all seems to be going well: the air is clear, the boat is steady, and the eyes are rested. The navigator can easily get lazy during these times, confident that each sight is perfect. But, the proof of the rule's value is found upon plotting the LOPs, for that's when the LOP from the first sight of three, thought to be excellent when first taken, is actually seen to be a few miles away from the other two. Because the accuracy of a sight is never certain until the LOP is laid on the chart and compared to the DR, the three-shot rule is just good navigation.