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    Re: CelNav without sextants
    From: Frank Reed CT
    Date: 2004 Nov 8, 01:57 EST
    Alex E asked:
    "Which one in Perseus? Beta, Epsilon?"

    Algol = Beta Persei

    "What does it mean "eclipsing binary star"? Sometimes you see one star, sometimes two?"

    Yes, but they're so close together that they cannot be resolved in ordinary telescopes. All you see is a cyclic drop in brightness. As one star passes in front of the other, even if it's smaller, the net light from the system sent our way decreases. The star is like a light house, blinking at us every three days. Algol is the prototype of the eclipsing binary. Some eclipsing binaries visible in small telescopes have considerably sharper eclipses that would allow an observer to get UT (conceivably) within five minutes or better.

    "How long are the eclipses, and what is the exact period (you only give to minutes, this is not enough to predict for a whole year. One needs to seconds at least)."

    The rules I gave would allow you to predict eclipses to the nearest couple of minutes for some years into the future (the rule that you subtract another minute every seven weeks amounts to giving the orbital period accurate to about one part in 100,000). The eclipses last something like ten hours, if memory serves. A visual observer can get the time of minimum accurate to about half an hour without instruments. Not awful for finding longitude, but not much better than awful. Again, before some tells me this is totally impractical, yes, I know. This is in the spirit of a "game".

    By the way, the seasonal rule, +/- 7 minutes in certain months, arises from the finite speed of light. The Earth is something like 250 million kilometers further from Algol around May 20th than it is on November 20th.

    "If these eclipses are seen with a naked eye, they should be known since antiquity."

    Perhaps surprisingly, it is unlikely that Algol's highly predictable eclipses were known before 1782 though its variability was discovered earlier. Here's an article I found that discusses some of the history and the astronomy, too:

    And asked:
    "Has anyone ever proposed to use it for longitude?"

    Probably not in any formal sense! Eclipsing binaries were discovered a few decades too late for the early historical discussions of longitude (fully 15 years after the publication of the lunar distance tables in the Nautical Almanac). I would imagine the concept has occurred to many people, but I doubt there's ever been any reason to put it in print. Incidentally, some years ago, there was a proposal briefly to re-define time standards in terms of pulsar observations. This would be roughly equivalent to using variable stars like Algol to get an absolute time standard, but of an entirely different level of accuracy and requiring the highest end astronomical equipment for observations.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
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