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    Re: Precession
    From: Frank Reed
    Date: 2019 Dec 21, 09:33 -0800

    The "poles" don't move. The stars move relative to the poles. Well, of course, it all depends on how you look at it. :) The key error to avoid is that the geographic north pole isn't going anywhere. The Earth's geographic pole will be just about where it is now in 10 or 20 or 30 thousand years. The Earth's orientation to the celestial sphere rotates around due to precession.

    In any case, if you want to visualize this, you need to find the north ecliptic pole (or south, but north for now). This is the spot that is directly "above" the Earth's orbital plane. It's in Draco at RA 18h00m and Dec 66.5° (that's 90°-23.5°). In the 26000-year precession cycle, the northern stars, including Polaris, will make an orbit around the north ecliptic pole. How big is that circle? Well, it's the same size as the Arctic Circle on a globe of the Earth, which has a length of something like 8600 nautical miles. Make sense?

    By the way, the Earth's axial tilt, equal to the obliquity of the ecliptic, varies cyclicly with a period of about 41,000 years in a range from about 22.0° to 24.5° so the precession cycle doesn't really repeat in 26,000 years, but it's fairly close. 

    A bit of trivia. If the north/south ecliptic pole is at your zenith, what does the sky look like? Well, the ecliptic itself is 90° from either ecliptic pole, so if that spot in Draco is at your zenith, that would imply that the Sun is on the horizon (actually a bit above due to normal refraction). Therefore the sky is blue. It's necessarily just after dawn. You can never see the stars of central Draco at the zenith (unless you are above most of the Earth's atmosphere!).

    Frank Reed

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