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I realize this may be the wrong site for this question... I apologize if you consider it inappropriate but hope this community knows the answer.

We just experienced the spectacular conjunction of Jupiter and Saturn (yay!). When I look at the sky, I see these two planets on the edge of Capricorn, almost in Sagittarius. Yet astrologers claim the conjunction is “in Aquarius”.

Jupiter in Capricorn

What is different about their (astrology’s) coordinate system? Any (historical) perspective would be appreciated!

PS I have no interest in astrology as a tool for predicting the future - just trying to understand where this discrepancy comes from.

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Precession of the equinox, caused by the cyclical change in the direction of the Earth's tilt, causes the position of the vernal equinox (the point where the path of the sun crosses the celestial equator) to move over time. When the constellations were first described, the vernal equinox was in Aries, and so it is still called "The First Point of Aries". Now it is in Pisces and moving towards Aquarius. Astrologers use "signs" named according to the position of the vernal equinox when the constellations were first described. So if a planet is at the vernal equinox (and therefore in Pisces, close to Aquarius) an Astrologer in the Western tradition will say that planet is "in Aries".

So for an astrologer, "Aries" is not a pattern of stars in the sky nor the region of the sky surrounding those stars, but the strip in the sky between 0 and 30 degrees of longitude, even though that region no longer contains the stars of the constellation Aries.

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  • $\begingroup$ That’s really helpful thank you! It fits very well with the observation - since the precession is about 2160 years “per 1/12” does that imply the current “baseline” of astrology traces back about 2000-3000 years? $\endgroup$ – Floris Dec 23 '20 at 1:35
  • $\begingroup$ I am not interested in astrology, but do they have at least something cyclical or periodic, or totally invented rules? Just the seasons (they seem to assign a sign depending on birth date)? $\endgroup$ – Alchimista Dec 23 '20 at 11:17
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    $\begingroup$ @Alchimista Well yes, the rules of astrology are invented, but horoscope calculations are based on the actual positions of the Sun, Moon & planets. As James K mentions, Western astrology uses ecliptic longitude (& latitude), with the vernal equinox point as the origin. When casting a horoscope the standard practice is to calculate & record longitude to the nearest minute of arc (assuming a sufficiently precise birth time). Each sign covers 30° exactly. If your astrological (Sun) sign is Aries, then the Sun had ecliptic longitude in the range 0° to 30° at the moment of your birth. $\endgroup$ – PM 2Ring Dec 23 '20 at 15:10
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Part of Zodiac around longitude 300

Astronomers and astrologers use different boundaries. In this image:

  • The blue lines mark the IAU constellations, standardized in the late 1920s along the equatorial coordinate grid of 1875 and now linked to the positions of several quasars.
  • The red lines mark the tropical zodiac signs, equal 30° divisions of the ecliptic linked to the equinoxes and solstices, described by Ptolemy ~150 CE but dating back to the Babylonians ~400 BCE.
  • The white ☌ symbol marks the Jupiter-Saturn conjunction, near the western (right) edge of the constellation Capricornus and the sign ♒ "Aquarius."

Ecliptic coordinates in astronomy and astrology have the same basis but are expressed in different ways, e.g. 300.5° longitude vs. "0.5° Aquarius."

Precession of Earth's poles and equator has these effects:

  • The equinoxes drift westward 1.4° per century. If the signs were named after the nearest constellations 2000 years ago, they are now offset by 28°.
  • The constellation boundaries are slightly crooked relative to the equatorial coordinate grid of the present date.
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