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Thread: Cosmological ?s

  1. #1
    Mike's Avatar
    Mike Guest

    Default Cosmological ?s

    True north will make its closest approach to Polaris at the end of the 21c.
    Does anyone know the exact year? And will it later head back towards Thuban
    in draco?

    Are the 'dog days' calculated by the rising of the first dog star and the
    setting of the second? That seems too easy somehow.

  2. #2
    Odysseus's Avatar
    Odysseus Guest

    Default Cosmological ?s

    Mike wrote:
    According to Burnham the closest the pole gets to Polaris will be in
    2102, at 27'31" distance from the star. The pole traces out a circle
    (about 47░ in diameter) in the heavens, so it doesn't really "head
    back" but always goes round in the same direction. The precession
    cycle will return the pole to the vicinity of Thuban in another
    21,000 years or so.

    It doesn't sound that easy to me! Every star that's visible from a
    given latitude rises and sets every day, so using these phenomena to
    define a period of the year requires that they be related to the
    position of the sun. And what do you mean by "the first dog star" and
    "the second"? The "Dog Star" is Alpha Canis Majoris, AKA Sirius. Are
    you thinking of Procyon (which name means "before the dog"), Alpha
    Canis Minoris, sometimes called the "Little Dog Star", as the other?
    Depending on the observer's latitude, it rises and sets each day
    somewhat under an hour before Sirius does.

    Anyway, I don't believe there's an official definition for the "dog
    days". Some authors say that a period of about forty days, beginning
    three weeks before the annual sun-Sirius conjunction and ending three
    weeks after, constituted the "dog days". But in general the use of
    the term has more to do with hot summer weather -- which we now know
    is *not* caused by the star! -- than with its astronomical origins.


  3. #3
    Mike's Avatar
    Mike Guest

    Default Cosmological ?s

    "Odysseus" <> wrote in message

    I would have imagined it closer.

    Is it round or oval? Does it go clockwise?

    So it must have been closer to Kochab and Pherkad some 10,000 years ago.

  4. #4
    Odysseus's Avatar
    Odysseus Guest

    Default Cosmological ?s

    Mike wrote:
    Like most circles, it's round. It goes counterclockwise from the
    point of view I described; looking down on the earth's north pole
    from space the 'wobble' would be clockwise. See


    (scroll about halfway down) for an animation showing about ten
    thousand years of precession as it might be seen from the North Pole.

    Not as long as that, more like 3500 years -- and not very close
    either, about the width of the Little Dipper's bowl from Kochab. Ten
    thousand years ago I guess it would have been somewhere between
    Draco's head and Bo÷tes.


  5. #5
    Mike's Avatar
    Mike Guest

    Default Cosmological ?s

    "Odysseus" <> wrote in message

    Way cool. So the point will be near Errai in 5000ad and Alderamin in
    8000ad. The circle is bigger than I presupposed.

  6. #6
    Odysseus's Avatar
    Odysseus Guest

    Default Cosmological ?s

    Mike wrote:

    One way to grasp the size of the circle is to picture the noonday sun
    at the summer solstice, imagining it to be located at the top of the
    circle. Then the position of the sun at noon on the winter solstice,
    47░ lower, would be at the 'bottom'. This won't work if you live in
    the Arctic or Antarctic, though, because the latter position will be
    below your horizon.




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