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Thread: Tropic of Capricorn

  1. #1
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    Tropic of Capricorn

    Attached picture shows the solar ecliptic from December to March with planets at 23 January 2008. Can anyone help me add dates over the last few thousand years next to the stars along this line marking when the sun reached the Tropic of Capricorn, ie the sun’s position at the December solstice?

    Due to precession, the solar point of the Tropic of Capricorn is now in Sagittarius. It moved through Capricorn from about 2000 BCE to 0 earth years. The spot where Venus is in Sagittarius on the picture marks the current location. It has taken about 6000 years to move along the line from where Uranus is in the picture in Aquarius.
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  2. #2
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    Yeah, and we have also the Tropic of Taurus, in the Northern hemisphere.

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    Quote Originally Posted by Argos View Post
    Yeah, and we have also the Tropic of Taurus, in the Northern hemisphere.
    Thanks Argos - I think you meant Gemini? The northern summer solstice (aka Tropic of Cancer) is still when sun is in Gemini and won't hit Taurus until ~2150 CE.

  4. #4
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    Well, yeah, but Taurus is my sign.

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    Seriously, I wonder why do we stick to this tradition. The IAU should have a word on it, no? The mere relocation of the solstice would be a punch to the astrologers stomachs. It would be a service to science.

  6. #6
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    Quote Originally Posted by Argos View Post
    Seriously, I wonder why do we stick to this tradition. The IAU should have a word on it, no? The mere relocation of the solstice would be a punch to the astrologers stomachs. It would be a service to science.
    After the Pluto thing? They wouldn't touch it with a six foot Pole.

  7. #7
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    http://homepage.ntlworld.com/mjpowel...lled-s-hem.png illustrates the sine curve of the zodiac against the celestial equator. The points of inflection of this curve are the equinoxes (including today 21-22 September) while the stationary points (horizontal tangent) are the solstices. For example, the stationary point at the lowest point of the curve between Taurus and Gemini is the Tropic of Cancer. These points precess at the rate of one degree every ~71.6 years, completing a full sidereal cycle every ~25765 years. If this rate has been constant the zodiac would have cycled about 175,000 times since the earth came into being. http://en.wikipedia.org/wiki/Precess...29#Explanation gives the science. An IAU report is at http://syrte.obspm.fr/iau2006/cm06_94_PEWG.pdf

    My question about precession of the Tropic of Capricorn was looking for advice on the science of this precessional structure, and how exactly it has been plotted. Going beyond the question of dates over the last millennium, I would be interested to know if the rate of precession (the speed of the “wobbling of the top”) can be measured. Is it slowing down?

    I think it would be a great idea to change the names of the Tropics of Capricorn and Cancer to Sagittarius and Gemini respectively. However, the current names illustrate that astrologers claim the tropical signs, based on the seasons rather than the stars, are functional scientific products of the sine curve illustrated in the attachment. They would probably fight to keep the current names as they give tropical astrology a significant place in mapping and other sciences. By comparison, (if you will forgive a mythic aside) bumping Pluto was probably okay as it suits the subterranean style of the myth not to be recognised by the scientific establishment.

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