Yesterday's post discussing Dr. Bronner's Magic Soap and some of the issues surrounding Dr. Bronner and his soap also mentioned the wonderful Redwood Trading Post, which always had racks of fascinating books on all sorts of outdoor-related subjects. I always looked forward to browsing through those books whenever we visited the Trading Post.

One of these books which we actually purchased was the old Air Force survival manual, "Survival -- Training Edition," AF Manual 64-3. I pored over it for hours on end growing up, and it could rightly be included in the list of influential books which was started back in this previous post.

That edition was dated 15 August 1969, and superseded a previous edition of 64-3 dated 16 July 1962. The edition I have contains an update dated August 1978, as well as a US Government Printing Office date stamp from 1983, so I guess we didn't purchase it before then! Apparently, the old 64-3 has now been superseded by a newer Air Force survival manual 64-4 dated July 1985, all of which you can read online here. The newer edition contains much of the material from the edition I grew up with, as well as new illustrations and material, but it also omits some very useful information as well.

One bit that is present in both the newer version and the older version is a helpful discussion of the constellations that point to the south celestial pole. While the north celestial pole is easily identified by the North Star, Polaris (which is within one degree of the actual point of the celestial north pole), the region of the south celestial pole is so devoid of stars compared to the sky around it at night that it is known as the "coal sack."

The diagram below is found on page 2-102 in the older 64-3, and can also be seen in the online version of 64-4 linked above on page 349 out of 580.

The directions instruct the observer to follow a line from the long axis of the Southern Cross (a line marked "A" in the above diagram) towards the celestial south pole. To know where along this line the actual celestial south pole is located, use another imaginary line extending perpendicular to the midpoint of the "Pointers" indicated in the diagram, which are two bright stars to the east of the Southern Cross (a line marked "C" in the diagram above). This intersection will not be exactly at the celestial south pole, but "within a few degrees" of it.

As discussed in the previous post on the amazing celestial navigation of the Polynesian Voyaging Society, the stars in their courses create circles in the sky as the earth turns on its axis. The largest of these apparent circles is the celestial equator, and they grow progressively smaller towards the celestial north pole and towards the celestial south pole.

In the diagram included on that page, it will be clear that these circles will be "tilted" for an observer who is located between the terrestrial poles and the terrestrial equator, and they will be vertical for an observer on the terrestrial equator itself. It should be fairly clear that an observer at the terrestrial north pole would be able to observe stars located between the celestial equator and the celestial north pole (stars with declinations between 0° and +90°, which designate the celestial equator and the celestial north pole by convention as described in that previous post as well). For that observer, the celestial equator would correspond to his own horizon, and he would be unable to view stars below that line (stars with negative declinations).

As an observer proceeded south from the terrestrial north pole, the celestial north pole (with Polaris) would begin to sink in the sky, and the celestial equator rise up from the horizon, such that by the time he had proceeded 10° south of the terrestrial north pole (to L. N 80° or 80° north latitude) he would be able to observe stars moving on the circles with declinations up to 10° south of the celestial equator (although a star moving on the circle at exactly -10° would only just touch the horizon at its highest point on the circle, and thus would not really be visible, and stars close to that southerly declination would be similarly difficult to make out due to the thickness of the atmosphere at that angle, the glare on the horizon, and any buildings or terrain features that might rise above the theoretically flat horizon).

Thus, by a process of working through this same sort of mental exercise, we can determine how far south a traveler would have to proceed before he could even see the stars of the Southern Cross, let alone the region of the sky containing the south celestial pole. The first of the stars of the Crux that could be seen is Gamma Crucis (or Gacrux), with a declination of -57°. It would briefly crest above the southern horizon for observers in the northern hemisphere at latitudes of N 33° or lower, although to be really visible an observer would want to proceed a few degrees further south than that.

The brightest star of the Southern Cross, Alpha Crucis (or Acrux), at the other end of the long axis from Gacrux, has a declination of -63° and so cannot be seen by observers until they pass south of latitude N 27°. Observers in Hawaii, centered on latitude N 19° can see the Southern Cross, as can observers at Machu Picchu at latitude S 13°, where there is a rock in the shape of the constellation which points to the Southern Cross once a year in May (see image here -- scroll almost all the way to the bottom of the page).

The old 64-3 edition of the Air Force Survival manual contained numerous other useful diagrams related to finding your location using the stars in a survival situation. It is quite a helpful reference and one well worth acquiring.