Bookmark and Share Email this page Email Print this page Print

An emergency star compass

Jan 1, 2003

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 reason, it is possible to find oneself at sea without a compass. Here is a way to use the navigational stars for finding direction that will be good enough to get you to a place where your compass can be fixed.

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 table 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 40° N will rise at an azimuth of 050° and set at an azimuth of 310°. Except for stars with a declination exactly on your latitude, the star will move to the north or south as it makes its transit of the sky. But viewing the star within a few degrees above the horizon provides an azimuth with reasonable accuracycertainly 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 table shows the azimuth for some of the selected stars as they rise and set from five latitudes: the equator, 15° north, 30° north, 15° south, and 30° south. Some special cases are discussed below.The Southern Cross (Crux)

The paucity of stars near the southern horizon means that we must find other ways 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 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.

When the Southern Cross makes an angle with the horizon of 45° from the east, the point on the horizon directly under Gacrux bears 159°. This configuration is visible from latitudes up to 21° N, after which it becomes too low to view the bottom of the cross. When the Southern Cross is vertical, as shown in figure 1, the point on the horizon directly below the vertical (longest) staff of Crux is at an azimuth of 182°. This configuration is visible from latitudes up to 26° N, 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° from the west, an azimuth of 205° lies directly beneath Gacrux. This configuration is visible from latitudes up to 17° N, after which it becomes too low to view the bottom of the cross.Polaris

Of course, there is good old reliable Polaris, which is on an azimuth no more than 2° 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 exists: 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°. For an emergency, however, it is unlikely this precision will be required.

When you are at about 1° north latitude, Polaris is so low in the sky it just peeks above the horizon. South of that, to about 27° 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 situation is shown in figure 2.

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 shown in figure 2.Mintaka

Bowditch describes Mintaka, the northernmost star in Orion's belt, as within 0.3° of the equator and useful for finding 090° and 270°.Pleiades

An object that isn't useful for sight reduction but is easy to identify is the Messier object M45, the Pleiades. Its lack of a pinpoint of light and tabulated ephemeris data in the almanac has 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.