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Using the 'selected stars' and more for finding direction at sea

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In what situation can you imagine yourself at sea without a compass? Failure of that reliable instrument on board and no spare available? Adrift in a liferaft? Whatever the cause of the situation, it is not beyond reason imagining it happening.

For any mariner used to taking star sights, the 'selected stars' of the Nautical Almanac should be familiar. Perhaps not all of them have been used during your past experience but a sufficient number spaced around the horizon should have been seen on enough occasions to be recognizable without reference to a star chart, should you find yourself in that situation as well.

 Selected stars

Each daily page and the Index to Selected Stars (the handy tear-out sheet in the front) in the Nautical Almanac give the declination for the 57 tabulated stars. This declination, when viewed from the equator, can be used to determine the azimuth at which the star will rise in the east and set in the west. For example, a star with a declination of 40N will rise at an azimuth of 050 and set at an azimuth of 310 degrees. Except for stars with a declination exactly on your latitude, the star will move to the north or south as it makes its transit. But viewing the star within a few degrees above the horizon provides an azimuth with reasonable accuracy ... certainly enough for an emergency situation. As you move away from the equator, the stars to the east and west retain nearly the same azimuth as they do at the equator. The accompanying Emergency 'Selected Stars' Compass  shows the azimuth for the Selected Stars as they rise and set from five latitudes ... the equator, 15 degrees North, 30 degrees North, 15 degrees South and 30 degrees South. The italicized lines are special cases discussed below.

 Special cases

 The Southern Cross (Crux)

The paucity of stars near the southern horizon beg for other means of determining azimuths toward the south. Traditional navigators in the Caroline Islands use the Southern Cross (which they call Uup) in various configurations with respect to the horizon to determine some intermediate azimuths. Though they use different names for the stars, they, in fact, use the rising and setting azimuth of the star Gacrux, as displayed in the table. As Crux makes its transit around the pole, its configuration with respect to the horizon is used for other directions. We can draw on these directions used by the traditional navigators for our Emergency Compass.

Figure 1 shows the Southern Cross making an angle with the horizon of 45 degrees from the east. When in this configuration, the point on the horizon directly under Gacrux bears 159 degrees. This configuration is visible from latitudes up to 21N, after which it becomes too low to view the bottom of the Cross.

When the Southern Cross is vertical, as shown in Figure 2, the point on the horizon directly below the vertical (longest) staff of Crux is at an azimuth of 182 degrees. This configuration is visible from latitudes up to 26N, after which it becomes too low to view the bottom of the Cross.

As the Southern Cross continues to the west, and forms an angle of 45 degrees from the west, as shown in Figure 3, an azimuth of 205 degrees lies directly beneath Gacrux. This configuration is visible from latitudes up to 17N, after which it becomes too low to view the bottom of the Cross.

An advantage of these borrowed 'stars' is that at very northerly or southerly latitudes some of the selected stars become too high or low to rise and set at the horizon. As can be seen in the Emergency 'Selected Stars' Compass  for 30 degrees North and South, five of the selected stars are unusable at these latitudes.

 Polaris


Of course, there is good old reliable Polaris, which is on an azimuth no more than two degrees either side of true north. If precision beyond that is required, Polaris lies exactly above or below the celestial pole when either of two conditions exist, as shown in Figures 4-A and 4-B. These conditions are when either Epsilon Cassiopeia, the trailing star of Cassiopeia or Alkaid, the trailing star of Ursa Major are directly above Polaris. Only in these cases is the azimuth of Polaris exactly 000 degrees. For an emergency however, this precision is unlikely to be required.
 
 
 

Polaris just peeks above the horizon at about one degree north latitude. South of that, to about 27 degrees south, when the 'pointers' (Merak and Dubhe), in the constellation Ursa Major (or Big Dipper), are vertical with the horizon, they point to Polaris beyond the horizon at its northerly azimuth. This is shown in Figure 5.

Another set of 'pointers' exists in the constellation Auriga; Theta Auriga and Menkalinan. If neither set of 'pointers' is aligned with the vertical, note that lines through each set can be mentally intersected to locate a 'fix' on Polaris below the horizon. These pointers in Auriga are also shown in Figure 5.
 
 
 
 
 
 
 
 
 

Mintaka

Bowditch describes Mintaka, the northernmost star in Orion's belt, as within 0.3 degrees of the equator and useful for finding 090 and 270 degrees. This star is also included in the Emergency 'Selected Stars' Compass .

 Pleiades

An object that isn't useful for sight reduction but easy to identify is the Messier object M45, Pleiades. It's lack of a pinpoint of light and tabulated ephemeris data in the almanac have kept it out of the realm of celestial navigation. As an emergency compass direction, however, it is sufficient for determining an azimuth and is included in the table. This is another 'star' borrowed from the traditional navigators of the Caroline Islands.

Emergency 'Selected Stars' Compass

The Emergency 'Selected Stars' Compass  gives a ready reference to azimuths based on your approximate latitude - equator, 15 degrees North, 15 degrees South, 30 degrees North, and 30 degrees South. It should allow you to find your way with an azimuth on or close to your desired course, or its reciprocal, from most locations.

(NOTE: Figures were generated using SkyGlobe 2.0 for Windows, a KlassM software product, by Mark A Haney. Capture was made using Corel Photo-Paint, Version 7.373)

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