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Thread: On the Zodiac

  1. #1

    On the Zodiac

    I'm trying to understand how the signs of the Western Zodiac are supposed to work. Here's where I need clarification:

    The IAU gives a 'solar stay' amount of days for a sign, ranging from 8.4 to 44.5. Tropical and Sidereal give each sign around 30 days each. I'm trying to figure out why there's a discrepancy. I'm also trying to figure out how the latter (traditional) signs are supposed to be determined. I mean wouldn't more than one sign actually be in the sky at a time?

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    Quote Originally Posted by Indagare View Post
    I'm trying to understand how the signs of the Western Zodiac are supposed to work. Here's where I need clarification:

    The IAU gives a 'solar stay' amount of days for a sign, ranging from 8.4 to 44.5. Tropical and Sidereal give each sign around 30 days each. I'm trying to figure out why there's a discrepancy. I'm also trying to figure out how the latter (traditional) signs are supposed to be determined. I mean wouldn't more than one sign actually be in the sky at a time?
    The traditional signs, as used by astrologers in casting horoscopes, are 30 degree expanses of ecliptic longitude, with the sign of Aries starting at the March equinox point. Precession of the Earth's spin axis has carried the constellations of the same names eastward from their positions at the time a couple of millenia ago when the signs came into common use.

    The IAU numbers are the intervals in which the Sun is within the IAU-standardized boundaries of the namesake constellations, as set down in 1930. These do not match the signs very well, even if we back-precess them to where they would have been in classical Greek times. In addition the modern southern extremity of Ophiuchus encroaches on the ecliptic, so the Sun spends some 18 days in that constellation.

  3. #3
    Quote Originally Posted by Hornblower View Post
    The traditional signs, as used by astrologers in casting horoscopes, are 30 degree expanses of ecliptic longitude, with the sign of Aries starting at the March equinox point.
    A small clarification. The tropical zodiac defines the cusps between signs from the solstices and equinoxes.

    The natural seasons vary in length due to the ellipticity of earth's orbit. As a result, the solstices and equinoxes are not exactly at ninety degrees from each other, and the tropical signs also vary in length.

    The signs average 30 degrees of ecliptic arc, but their length is traditionally determined by the season change points, so they are not exactly thirty degrees.

    http://en.wikipedia.org/wiki/Equinox shows that northern seasons, defined as the period between equinox and solstice or vice versa, have the following rounded lengths:

    Spring 93 days
    Summer 94 days
    Autumn 89 days
    Winter 89 days

    This variance is due to the position of the fastest orbital point, the perihelion, in winter at 4 January.

    In ten thousand years perihelion will be in July, summer will be the shortest season, and the summer 'signs' will be shorter in duration than the winter signs.

    The tropics of Cancer and Capricorn, defining the boundaries between tropical and temperate zones, are the most northerly and southerly positions respectively where (at the time of Christ) the sun was overhead at noon as it moved into these signs/constellations at the solstices. Due to precession these tropics now occur when the sun is in Gemini and Sagittarius.

  4. #4
    Quote Originally Posted by Hornblower View Post
    The traditional signs, as used by astrologers in casting horoscopes, are 30 degree expanses of ecliptic longitude, with the sign of Aries starting at the March equinox point. Precession of the Earth's spin axis has carried the constellations of the same names eastward from their positions at the time a couple of millenia ago when the signs came into common use.

    The IAU numbers are the intervals in which the Sun is within the IAU-standardized boundaries of the namesake constellations, as set down in 1930. These do not match the signs very well, even if we back-precess them to where they would have been in classical Greek times. In addition the modern southern extremity of Ophiuchus encroaches on the ecliptic, so the Sun spends some 18 days in that constellation.
    Quote Originally Posted by Robert Tulip View Post
    A small clarification. The tropical zodiac defines the cusps between signs from the solstices and equinoxes.

    The natural seasons vary in length due to the ellipticity of earth's orbit. As a result, the solstices and equinoxes are not exactly at ninety degrees from each other, and the tropical signs also vary in length.

    The signs average 30 degrees of ecliptic arc, but their length is traditionally determined by the season change points, so they are not exactly thirty degrees.

    http://en.wikipedia.org/wiki/Equinox shows that northern seasons, defined as the period between equinox and solstice or vice versa, have the following rounded lengths:

    Spring 93 days
    Summer 94 days
    Autumn 89 days
    Winter 89 days

    This variance is due to the position of the fastest orbital point, the perihelion, in winter at 4 January.

    In ten thousand years perihelion will be in July, summer will be the shortest season, and the summer 'signs' will be shorter in duration than the winter signs.

    The tropics of Cancer and Capricorn, defining the boundaries between tropical and temperate zones, are the most northerly and southerly positions respectively where (at the time of Christ) the sun was overhead at noon as it moved into these signs/constellations at the solstices. Due to precession these tropics now occur when the sun is in Gemini and Sagittarius.
    Okay, thanks! This answers my first question very well. Now I want to know: Are more than one sign in the sky at a time, and how is it determined which one the sun is 'in'? Let's say that the current sign is Taurus - is it the only Zodiac sign in the sky or are Aries and Gemini also visible? How long does it take for one sign to completely leave the visible sky?

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    When the Sun is "in" Taurus, that means that it is in the same area as the constellation, thus the constellation is in the daytime sky, and you can't see it at all.

    You can see roughly 180 degrees of the sky. So (depending on your latitude, horizon, etc.) you can see five or six zodiac signs at any one time at night.

    Edit to add: When you get a chance, get a sky map and study it. Then visit a dark area at night and compare the map to the night sky. Use a red flashlight so you don't spoil your night vision. This should give you some insight.

    Fred
    "For shame, gentlemen, pack your evidence a little better against another time."
    -- John Dryden, "The Vindication of The Duke of Guise" 1684

  6. #6
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    We see almost all of them each night. The one we might not see is the one the sun is in--that's the current sign, more or less (they've "slipped", as others have mentioned), that's why it's called the sun sign.

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    Quote Originally Posted by Robert Tulip View Post
    A small clarification. The tropical zodiac defines the cusps between signs from the solstices and equinoxes.

    The natural seasons vary in length due to the ellipticity of earth's orbit. As a result, the solstices and equinoxes are not exactly at ninety degrees from each other, and the tropical signs also vary in length.The signs average 30 degrees of ecliptic arc, but their length is traditionally determined by the season change points, so they are not exactly thirty degrees.

    http://en.wikipedia.org/wiki/Equinox shows that northern seasons, defined as the period between equinox and solstice or vice versa, have the following rounded lengths:

    Spring 93 days
    Summer 94 days
    Autumn 89 days
    Winter 89 days

    This variance is due to the position of the fastest orbital point, the perihelion, in winter at 4 January.

    In ten thousand years perihelion will be in July, summer will be the shortest season, and the summer 'signs' will be shorter in duration than the winter signs.

    The tropics of Cancer and Capricorn, defining the boundaries between tropical and temperate zones, are the most northerly and southerly positions respectively where (at the time of Christ) the sun was overhead at noon as it moved into these signs/constellations at the solstices. Due to precession these tropics now occur when the sun is in Gemini and Sagittarius.
    My bold. Those points are spaced exactly 90 degrees apart along the ecliptic. You are right about the time the Sun is in each zone varying according to the eccentricity of Earth's orbit.

    What are the cusps, and where are they?

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    Quote Originally Posted by Hornblower View Post
    My bold. Those points are spaced exactly 90 degrees apart along the ecliptic. You are right about the time the Sun is in each zone varying according to the eccentricity of Earth's orbit.
    Yes, wouldn't the four points be defined by the tilt of the earth's axis, and have nothing to do with its orbit?

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    Yes indeed. The ecliptic and the celestial equator are intersecting great circles, with the intersections exactly 180 degrees apart, and the solstice points midway between the intersections.

    Whether the traditional signs are exactly 30 degrees wide or are intervals of 30 or 31 days is a minor issue as I see it. The big thing is the major mismatch between them and the modern IAU constellation boundaries.

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    Quote Originally Posted by Robert Tulip View Post

    The signs average 30 degrees of ecliptic arc, but their length is traditionally determined by the season change points, so they are not exactly thirty degrees.
    By definition the signs are each precisely 30° wide. What varies is the amount of time that the Sun appears to transit a sign due to the Earth's eccentric orbit and perturbations mainly from the Moon. The various equinox and solstice points are each separated by precisely 90° of ecliptical longitude. They define the astronomical seasons which vary in duration due to the Earth's eccentric orbit and perturbations.

    Until a couple of centuries ago, both astronomers and astrologers used the zodiacal sign system to determine a body's ecliptical longitude. Of course due to precession the signs are now approximately one constellation west of a match with their namesake constellations. Besides orbital eccentricity and perturbations, the greatest factor for the length of time that the Sun appears in an official constellation is the constellation boundaries set by the IAU in 1930, which included a dipping of Ophiuchus into what had traditionally been Scorpio/Scorpius.
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  11. #11
    Quote Originally Posted by Hornblower View Post
    Yes indeed. The ecliptic and the celestial equator are intersecting great circles, with the intersections exactly 180 degrees apart, and the solstice points midway between the intersections.

    Whether the traditional signs are exactly 30 degrees wide or are intervals of 30 or 31 days is a minor issue as I see it. The big thing is the major mismatch between them and the modern IAU constellation boundaries.
    Apologies, my error, thank you for correction. I wrongly inferred from the fact that signs are not exactly one twelfth of the year in duration that they are not exactly 30 degrees of arc. So cusps are defined by thirty degree arcs of the ecliptic.

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