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Thread: precession

  1. #1
    Yokel's Avatar
    Yokel Guest

    Default precession

    "Paul Forsdick" <> wrote in message
    | Seeing the previous posts mentioning Precession prompted me to wonder what
    | is the actual lowest star we can see from a certain location over the full
    | precession cycle.
    | for example I know that Vega gets close to the pole in the cycle so I
    | this much mean that stars around the same Right ascension would be higher
    | the sky than now and thus lower stars that are currenly too far south to
    | rise may do so, for example Theta Scorpio at -43% may be visible and other
    | stars we currently see may then be too low to see.
    | so if you have a latitude of 52% what would be the lowest star
    | it could be that I am completley wrong about this assumption.

    Let's think it out.

    Precession moves the equinox and solstice points around the entire ecliptic
    once in the precession cycle. So every point on the ecliptic will at some
    time be north of the celestial equator by the inclination of the earth's
    axis (currently just over 23 degrees, but it does vary a degree or so either

    At the moment this "furthest north" point lies near the border between
    Gemini and Taurus. So we in the Northern Hemisphere currently get the best
    view we can of the area around Orion, and for a significant part of the
    precession cycle the brightest star - Sirius - would be invisible from the
    British Isles. But by a similar argument for most of the cycle Scorpio
    would be further north than it is now and we currently get an unusually poor
    view of it.

    If you are at 52 N, you can see every star less than 38 degrees south of the
    celestial equator (90 degrees - your latitude). Seeing as every part of the
    ecliptic will at some time be 23 degrees north of the celestial equator, it
    follows that at some time in the precessional cycle every star less than 23
    + 38 = 61 degrees south of the *ecliptic* will come into view. In some
    parts of the sky this will be out a bit due to "North" and "South" being at
    various times not exactly where they are now - I thought I'd better mention
    it before someone else does. But if you have a star atlas with the South
    Ecliptic Pole marked, or "planetarium" software which will show an ecliptic
    coordinate grid, to a reasonable approximation you will (eventually) be able
    to see every star more than about 30 degrees from the South Ecliptic Pole.
    This is actually over 90% of the sky. [If there were no precession or the
    poles were at right angles to the ecliptic place this figure would be about
    80% - what we currently see throughout the year. This may seem surprising as
    we are so far from the equator, but spherical geometry means that half of
    the sky is within 30 degrees of the celestial equator and 71% within 45
    degrees, plus we will see half of the remainder as being round the pole
    nearest us.] Move 29 degrees nearer the equator so you reach the tropics,
    and everywhere within the tropics you can at some point in the precessional
    cycle see every star in the sky. And, of course, at the equator you see all
    the stars in the sky at some time each year.

    So in about 10 000 years time we will get a wonderful view of Scorpio, which
    will take the place Taurus and Gemini currently occupy. At other times we
    will be able to see the Southern Cross and various of the "Southern Birds".
    But the area around Argo we will never see much better than we do now - we
    currently get almost the best view we can of this part of the sky. And if
    you want to see the Magellanic Clouds, I am afraid it is either a visit to
    places south of here or a very long wait for them to "orbit" to our side of
    the Galaxy...
    - Yokel -

    "Yokel" posts via a spam-trap account which is not read.

  2. #2
    Yokel's Avatar
    Yokel Guest

    Default precession

    "Paul Forsdick" <> wrote in message
    | It looks like we can would be be able to see Achernar in about 6000
    | and alpha and beta Centauri in about 11,000 years so it would just be
    | Canopus which never rises of the brightest stars

    Sounds reasonable to me. Canopus is close to the Magellanic Clouds and both
    of those are too close to the South Ecliptic Pole to ever be seen from these

    There is a far better way to see Achernar, etc. which does not involve a
    wait of 6000 years. A few years ago I booked a holiday to the wild and
    remote places of Central and NW Australia. Until you have seen the stars
    from there or a similar remote location (if your night vision is up to it,
    the stars shed so much light you can walk around purely by starlight), you
    cannot really know how wonderful the night sky is. I can remember waking up
    in the middle of the night in Purnulu (also known as the "Bungle Bungles")
    and seeing even the patchy clouds glowing by starlight - this was also the
    first time I saw Achernar with my own eyes. Another night in Windjana Gorge
    (on the road between Derby and Fitzroy Crossing) you could make out the
    pattern of grass patches and sand on the desert floor by the stars. If you
    ever get the chance, don't miss it - it beats running the planetarium

    The one thing that did puzzle some of my travelling companions who had come
    from Sydney is why we couldn't see the Southern Cross during the evenings -
    we were there in October. The answer being that the Southern Cross is far
    enough from the current South Celestial Pole that it sets for part of each
    day from tropical Australia, and in October that happens to be the evenings.
    But at that time of year Scorpio was a wondrous sight in the evening western
    sky, especially as at the time I was there Venus and Mars were also on
    display to the west with Jupiter and Saturn also visible.
    - Yokel -

    "Yokel" posts via a spam-trap account which is not read.

  3. #3
    Sjouke Burry's Avatar
    Sjouke Burry Guest

    Default precession

    Paul Forsdick wrote:

    Top posting??

    Anyway, I wil start worrying about that in about 9000 years.



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