One of the most vexing problems facing the neophyte celestial navigator is estimating the height of a celestial body. There are many catch-as-catch-can methods for pre-setting the sextant, so that especially at twilights, when there is little time, the celestial object can be captured and brought to ground.
A clever means of accurately shooting a star is to reverse the technique normally employed in observing the sun or the moon. Usually a navigator will hold the sextant upright and will set the index arm to an approximate height of the celestial object and then bring the star or planet down to the horizon. At times though, due to the dimness of the star or the fact that there are many other surrounding stars, it is easier to reverse the process by instead bringing the horizon up to the sextant. Not only is this method “cool,” it works like a charm and will give the neophyte a sense of accomplishment.
This “upside down” process is accomplished by setting the sextant to 0° 0’ and then turning it upside down and aiming the sextant at the star in question. Then the index arm is moved away from the navigator until the horizon is brought close to the star. Then the sextant is turned over to its usual position, and when viewed through the eyepiece, the star should appear very close to the horizon. Fine tuning of the micrometer drum is then used so that the star is actually bisecting the horizon.
At times it is also appropriate to quickly pre-calculate the sextant angle prior to the shot. It makes it easier for the observer knowing that if Venus or Jupiter is 45° above the horizon to preset the sextant to that angle. One can estimate the sextant height by using hands, fingers and fists to measure the height of a celestial object above the horizon.
This technique used by humankind for thousands of years is based on the fact that the hands, fingers and fists can be used to measure angular distance.
The measurements obviously cannot be exact, and will change from person to person, but generally the following can be established as “rules of thumb.” This term, by the way, derives from the fact that the thumb was used in antiquity as a measuring device.
A closed fist with the thumb down held at arms length is about 10°. This can be ascertained by extending the arm and placing the bottom of the fist on the horizon. Then placing hand over hand, it should take about nine fists to reach the zenith. Again, this is not an exact science, but a close approximate means of establishing angular measurement.
For smaller angles, the outstretched palm with thumb and pinky extended is about 18° to 20°. The middle three fingers equal about 4°, the tip of the index finger about 1.25°.
The Arab navigators used the width of the thumb about 1.5° as a measurement for the Kamal. This measurement, called an issabah in Arabic, was then converted to knots tied into a piece of string. Attached to one end of the string was a piece of wood or ivory about 2 inches by 1 inch. The loose end of the string was placed in the mouth, and the wood piece was held at arms length. Usually Polaris was sighted with the lower part of the Kamal on the horizon and the upper edge on the star. The number of knots was the height of the star. This was an effective means of running down the latitude crossing the Indian Ocean.
Contributing Editor David Berson writes the Nav Problem page in every issue of Ocean Navigator. He is also the owner and operator of Glory, an electrically powered excursion boat, in Greenport, N.Y.