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At the moment (01-04-2020 09:36) the Moon is situated in Gemini:

"The Moon is currently in the constellation of Gemini. The current Right Ascension is 06h 37m 56s and the Declination is +23° 42' 09”." theskylive.com, 2020.

If there are 12 houses (zodiac) and Right Ascension is in a 24hr format how is the moon calculated to be in Gemini?

The calculation i'm using has it in Cancer. This is obviously a mistake but here's how i'm getting it:

  • Aries 0h 0m 0s
  • Taurus 1h 51m 39s
  • Gemini 3h 51m 16s; <- (Moon is actually in Gemini)
  • Cancer 6h 0m 0s; <- (here's where ive got the moon at 06h 37m 56s)
  • Leo 8h 8m 44s;
  • Virgo 10h 8m 21s;
  • Libra 12h 0m 0s;
  • Scorpio 13h 51m 39s;
  • Sagittarius 15h 51m 16s;
  • Capricorn 18h 0m 0s;
  • Aquarius 20h 8m 44s;
  • Pisces 22h 8m 21s.

(Courtesy of https://astronomy.stackexchange.com/a/27488)

How have I misunderstood right ascension and how would I derive Gemini from RA 06h 37m 56s?

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    $\begingroup$ The right ascension boundaries given in the other question apply only to the ecliptic itself and thus only work for the Sun. For other bodies, you want to find the ecliptic longitude and use that to find Zodiacal house. The ecliptic-to-right-ascension boundaries are different away from the ecliptic. $\endgroup$
    – user21
    Apr 1, 2020 at 15:33
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    $\begingroup$ I’m voting to close this question because astrology != astronomy $\endgroup$
    – peterh
    Dec 8, 2020 at 11:57
  • $\begingroup$ @peterh-ReinstateMonica - both these questions are about astronomy, just couched in astrological terms. $\endgroup$
    – Rory Alsop
    Dec 8, 2020 at 18:25

2 Answers 2

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Zodiac signs are equal 30° divisions of the ecliptic. In the Western tropical zodiac, these are linked to the equinoxes and solstices, and named after the nearest constellations at the time Ptolemy wrote about them. The equinoxes precess westward 1.4° per century, so the signs currently lie 25° to 30° west of their namesake constellations. There are also sidereal zodiacs, which account for precession and keep signs aligned with stars.

A celestial object's ecliptic longitude of date determines its zodiac sign. If the lunar RA and Dec in question are for epoch J2000, then the Moon's epoch 2020.25 ecliptic longitude is 99.0°, in the tropical sign of Cancer (longitudes 90° to 120°). Relative to the IAU boundaries, drawn along lines of RA and Dec for epoch B1875, this is in the constellation Gemini.

Approximating ecliptic longitude by right ascension alone can err as much as 2° in the case of the Moon, whose orbit is inclined 5° to the ecliptic. For example, at 2020-04-06 19:40 UT, the Moon is at equatorial RA 12h4m (1° east of 12h0m) but ecliptic longitude 179° (1° west of 180°). Since the ecliptic is oblique to the celestial equator, lines of ecliptic longitude are oblique to lines of right ascension except at the solstices.

Houses are completely different, dividing the sky in reference to the local horizon. The precise definition depends on the particular astrological tradition; roughly, a celestial object enters the 12th house when it rises and the 6th house when it sets, decrementing house number every 2 hours or so. The most closely related astronomical coordinate is hour angle.

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You are trying to do astrology by using astronomical means. And you discover one of the facts which show face-on why astrology is anything but a science.

Astrological 'houses' or zodiacal constellations don't have any relation to constellations any longer due to precession changing the time of the vernal equinox over centuries and millenia - and in part also constellations were a term subject to interpretation and individual separation until defined unambiguously by IAU. Conversely you can use the difference to estimate the time when the astrological context was fixed - about 1700 to 3000 years ago depending on how you assume precession behaved exactly.

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  • $\begingroup$ Thank you, precession is why my calculation is wrong. $\endgroup$
    – Matt Jameson
    Apr 1, 2020 at 10:24
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    $\begingroup$ Downvote: questions that discuss astrology in an astronomical way ARE valid here. Also, there's no such thing as "actual constellations". I realize the IAU has defined constellation boundaries, but there's no way ancient astrologers/astronomers were using those unless they were even more prescient than some claim. $\endgroup$
    – user21
    Apr 1, 2020 at 15:34
  • $\begingroup$ @barrycarter where do you read that I say or imply the question is off-topic? You might be interpreting more in my reply than there is written. And 'actual' might be a false friend word I carry over from my mother tongue... 'currently defined' might be more appropriate and will change that $\endgroup$
    – planetmaker
    Apr 1, 2020 at 21:01
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    $\begingroup$ Yes, precession is a key argument given that astrological "Aries" (for example) actually points to astronomical Pisces. However, if you're going to dismiss the scientific basis of astrology, it's best to avoid making ignorant and incorrect statements. Astrological houses are based on the diurnal rotation of the Earth and never had anything to do with constellations. Also, modern Western astrology defines each zodiac "sign" as a specific 30º segment of the ecliptic, entirely decoupled from actual stars or IAU-defined constellations. $\endgroup$
    – Chappo Hasn't Forgotten Monica
    Apr 1, 2020 at 23:04

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