Sunday, November 19, 2006

Flight of the Navigator

Flight of the Navigator was a Disney release back in the eighties about a pre-adolescent boy befriended by an autonomous, sentient extraterrestrial craft that took him back and forth in both space and time.

Space and time. Navigation. You probably already know where I'm going with this.

>>Cue Music: Cat Steven's: "I'm being followed by a moon shadow. Moon Shadow, Moon Shadow..."

As I indicated in Cast in Bronze, I think there is ample evidence of varying levels of accomplishment among ancient peoples for both land and marine navigation. Those skills would be dependent on a small set of "enabling technologies" and on basic trigonometry or, better yet, spherical trigonometry. Any viable ancient navigation tools of that long-ago time could have been developed in one of two ways:
  • trial and error-based correction, or
  • mathematical predictions based on observation (scientific method)

    Either course of development would likely have required refinements based on lessonlearned through initial trials. This is more or less how systems of all kinds are developed yet today.

Want to use the moon to determine your longitude, but you don't own a GPS receiver or even a sextant?

Use an Old Farmers Almanac and your watch. The tables of "regional corrections" for local (real) noon and moonrise times are accurate enough to get you within maybe one quarter a degree. Greater accuracy is possible, but requires more complex math and extremely careful observation.

What's that? You don't own a watch either? Not a problem, you can still get within about a half degree.

Read on, McDuff.

From: Bowditch -The American Practical Navigator, hosted on irbs.com (I'd Rather Be Sailing)

"Nautical Almanac"
"The major portion of the Nautical Almanac is devoted to hourly tabulations of Greenwich Hour Angle (GHA) and declination, to the nearest 0.1' of arc. On each set of facing pages, information is listed for three consecutive days. On the left-hand page, successive columns list GHA of Aries(symbol not dispayable here), and both GHA and declination of Venus, Mars, Jupiter, and Saturn, followed by the Sidereal Hour Angle (SHA) and declination of 57 stars. The GHA and declination of the sun and moon, and the horizontal parallax of the moon, are listed on the right-hand page. Where applicable, the quantities v and d are given to assist in interpolation. The quantity v is the difference between the actual change of GHA in 1 hour and a constant value used in the interpolation tables, while d is the change in declination in 1 hour. Both v and d are listed to the nearest 0.1'."

But wait a minute. That method requires a chronometer. And it sounds complicated.

Yes, it is a little more complicated than reading your location directly from the face of a GPS receiver.

Yes, it does now require a good chronometer, but the chronometer used by mariners before 1797 was built into the system.

(from the United States Naval Observatory)


"Initially the almanacs provided the data required for the method of lunar distances, a technically demanding and mathematically complex method of determining longitude before the invention of accurate clocks for shipboard use."

If you want to know the nuts and bolts of moontracking to tell time and determine longitude, go read Bowdatch. But for sake of this discourse, just take my (and the US Navy's) word for it, it works.

Local noon is easy to determine (short of being deep inside the polar circles). The argument that "ancients had no means of determining longitude because they had no chronometers" doesn't wash. We don't know whether they had man-made chronometers or not, but they were adept at discerning "noon" and, after about 3,000 bce, thet were pretty darn good at making sundials.

Apparent movement of the sun through the sky is one degree every four minutes. That is a stable, reliable external (daytime) chonometer that practically anyone can learn to use. Yes, I know there are exceptions to that stability, but a couple minutes in your time observation is not going to navigate you into the wrong hemispere.

Considerably more precise chronographic instruments, such as the Heliochronometer, followed in slightly less ancient times.

Portable sundials, suitable for use by long distance travelers, have come and gone in varying forms and configurations for a couple thousand years (that we know of). I'll get into specific types later.

To quote celestialnavigation.net, "PTOLEMY WAS RIGHT!"

"The first thing you have to know about celestial navigation is that its view of the heavens is pre-Copernican. That's right - you look at the earth as the unmoving center around which the sun, moon, stars, and planets turn. In other words, we deal with the heavens as if wysiwyg (what you see is what you get). "

The ancients, particularly the Mayans and quite possibly the ancient Egyptians and Sumerians, were quite accomplished at mathematical systems analysis, especially when applied to astronomy, particularly the sun, the moon and Venus. They understood quite well the nature of the "stable system" and they noted the exceptions (comets, planets, etc.).

It is a natural extension of this depth of observation to note differences in the "stable system" when the techniques are applied in a different terrestrial location than where they made their initial observations. They assembled moonrise, sunrise, planetary transit, and even precession tables at their observatories, if the interpretations by some in the Meso-American astro-archeology field are correct, and, quite possibly, used such tables as references to determine their geolocations while they were themselves in transit. Today, we can visit their observatories and see their tables inscribed in stone, but we don't know what tools (angular measurement tools or chronometric tools) they used. Those artifacts are long gone. The "evidence" is not the artifactual tools, but rather the written record. No one doubts they made detailed, sophisticated observations. In their scope (pun fully intended) and structure, the Mayan observation records bear more than a passing resemblence to the Nautical Almanac.

Can we doubt the Mayans and other ancient peoples were sophisticated enough to apply that accumulated knowledge in a similar fashion?

Modern Celestial Navigation

The Polaris Intercontinental Ballistic Missile navigation system, although the intricate details
of which are still classified since its implementation during the Cold War, is based on real-time celestial observation. It was, after all, a US Navy weapon system. A lens system and electronic "reference table" allows the guidance system to observe certain stars and determine exact geolocation (terminal accuracy within a few hundred meters, often better by an order of magnitude). Granted, this system benefits from and is dependent on a highly stable chronometer. Anything that moves at 17,000 miles per hour needs a good clock to keep track of its movement and calculate its location. The ancient navigators operated at a much more leisurely pace, by say, 4 or 5 orders of magnitude slower.

So, that should adequately establish that the ancients could have possessed the requisite skills to master the art of celestial navigation, at least, adequately for the rates of travel we percieve they went.

So where are tools? Who used them? And where did they use them?

Holy moonbeams, Batman!! Tune in the for next exciting episode of the Oopa Loopa Cafe to find out!!

2 comments:

Michael Wood, Cincinnati said...

I'm always amazed by the arrogance of certain people who say that since THEY couldn't figure out an answer to a problem today, that no one could have figured out the answer to the same problem in antiquity.

The gentle de-bunking in your essay properly points out that earlier man could very well have routinely used solutions that a city-dweller would NOT know today.

oz said...

Hi Mike,

Sad to say, but it's not just "city dwellers" who don't understand. Even most experienced sailors today couldn't muster the skills to use the lunar distances method. They have all grown dependent upon more modern methods and more advanced technology, to whit, GPS satellites, instead of the natural satellites (even though the basic premise is the same -- just that the GPS receiver does the math for you). And the GPS is much less time consuming, more reliable, and much more precise.

As for being "gentle", it is not my intent to demean anyone, but rather merely to show that the evidence really is there even if it has been ignored or overlooked by most scholars.

Even though you are far afield, my friend, we both use the same pole star to find our way home.