NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: CelNav without sextants
From: Frank Reed CT
Date: 2004 Nov 7, 22:36 EST
From: Frank Reed CT
Date: 2004 Nov 7, 22:36 EST
A few days ago, I wrote:
"Another related game: are there circumstances under which you can navigate celestially without a sextant, without a timepiece, AND without an almanac (of any type)? I think the almanac is the "technological artifact" that is most difficult to do without..."
And Alex has been dying of curiosity <g> assuming that I meant this as a riddle. No, actually, I meant it as more of a game. And the real point was to emphasize the necessity of having at least some form of almanac.
Nonetheless, let's "play the game". Latitude is no serious problem assuming we can remember simple rules for generating the daily declination of the Sun or assuming that we know the declinations of a handful of stars from memory (Polaris being the obvious case). But what about longitude? Since antiquity astronomers and geographers have understood that you need a clock in the sky -- a clock that reads some "absolute time" that we can compare against the apparent time given by the positions of the Sun and stars relative to the local observer's horizon. What absolute clock can we use if we have lost everything (no sextant, no almanac, no telescope)? Nature hasn't provided us with anything that would let us read the time to the second or even to the nearest minute, but there is a clock which beats out the time regularly, its alarm going off like clockwork every 2 days, 20 hours, 49 minutes. It's the eclipsing binary star Algol in the constellation Perseus. Visual observers with a little practice can time the middle of its eclipses to the nearest 30 minutes or so. Not great for longitude, but if you had absolutely nothing else, it would get you your longitude to the nearest 7.5 degrees. In an alternate history, that might be worth something. Note that eclipses of Algol are only visible half of the time (half occur in daylight), and they are not observable at all for some six weeks around May 20th.
How would you use Algol in practice to get longitude? First, you would need to know how to find Algol and estimate its magnitude. Second, you would need the time of at least one relatively recent eclipse. Maybe you observe one just before leaving on your round-the-world voyage. Let's suppose you observe the eclipse that takes place a few days from now centered on 16:55 UT on Nov. 10, 2004. Next, you would need to know the rule for predicting future eclipses. It's very short and easy to memorize. I have split it up into very small morsels:
1) start with a known eclipse date and time (best if it's a date from August or February but any will do).
2) add 3 days.
3) subtract 3 hours 11 minutes.
4) every 7 weeks, subtract 1 minute.
5) for the four months centered on May 20, add 7 minutes. for the four months centered on November 20, subtract 7 minutes. Otherwise, do nothing.
You can generate a list of eclipses for years into the future by iterating steps 2 through 5. The predicted times will be accurate to +/- 3 minutes which is plenty good enough given the observational limits of visual magnitude estimates. When you observe an eclipse of Algol on the far side of the globe, that immediately gives you Greenwich Time. Comparing that with local time, determined by observing the stars at the zenith at the time of the observation, yields the longitude.
Please understand, I am presenting this simply as a way to "play the game" that I set out earlier. This is not real navigation. But perhaps we can fantasize about ocean navigators on the sea of a planet in some other star system.. One with a better "Algol" that can be read to the nearest minute or the nearest second... For them the problem of longitude would never have existed.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"Another related game: are there circumstances under which you can navigate celestially without a sextant, without a timepiece, AND without an almanac (of any type)? I think the almanac is the "technological artifact" that is most difficult to do without..."
And Alex has been dying of curiosity <g> assuming that I meant this as a riddle. No, actually, I meant it as more of a game. And the real point was to emphasize the necessity of having at least some form of almanac.
Nonetheless, let's "play the game". Latitude is no serious problem assuming we can remember simple rules for generating the daily declination of the Sun or assuming that we know the declinations of a handful of stars from memory (Polaris being the obvious case). But what about longitude? Since antiquity astronomers and geographers have understood that you need a clock in the sky -- a clock that reads some "absolute time" that we can compare against the apparent time given by the positions of the Sun and stars relative to the local observer's horizon. What absolute clock can we use if we have lost everything (no sextant, no almanac, no telescope)? Nature hasn't provided us with anything that would let us read the time to the second or even to the nearest minute, but there is a clock which beats out the time regularly, its alarm going off like clockwork every 2 days, 20 hours, 49 minutes. It's the eclipsing binary star Algol in the constellation Perseus. Visual observers with a little practice can time the middle of its eclipses to the nearest 30 minutes or so. Not great for longitude, but if you had absolutely nothing else, it would get you your longitude to the nearest 7.5 degrees. In an alternate history, that might be worth something. Note that eclipses of Algol are only visible half of the time (half occur in daylight), and they are not observable at all for some six weeks around May 20th.
How would you use Algol in practice to get longitude? First, you would need to know how to find Algol and estimate its magnitude. Second, you would need the time of at least one relatively recent eclipse. Maybe you observe one just before leaving on your round-the-world voyage. Let's suppose you observe the eclipse that takes place a few days from now centered on 16:55 UT on Nov. 10, 2004. Next, you would need to know the rule for predicting future eclipses. It's very short and easy to memorize. I have split it up into very small morsels:
1) start with a known eclipse date and time (best if it's a date from August or February but any will do).
2) add 3 days.
3) subtract 3 hours 11 minutes.
4) every 7 weeks, subtract 1 minute.
5) for the four months centered on May 20, add 7 minutes. for the four months centered on November 20, subtract 7 minutes. Otherwise, do nothing.
You can generate a list of eclipses for years into the future by iterating steps 2 through 5. The predicted times will be accurate to +/- 3 minutes which is plenty good enough given the observational limits of visual magnitude estimates. When you observe an eclipse of Algol on the far side of the globe, that immediately gives you Greenwich Time. Comparing that with local time, determined by observing the stars at the zenith at the time of the observation, yields the longitude.
Please understand, I am presenting this simply as a way to "play the game" that I set out earlier. This is not real navigation. But perhaps we can fantasize about ocean navigators on the sea of a planet in some other star system.. One with a better "Algol" that can be read to the nearest minute or the nearest second... For them the problem of longitude would never have existed.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
