<p>Zodiac signs are equal 30&deg; divisions of the <a href="https://en.wikipedia.org/wiki/Ecliptic" rel="nofollow noreferrer">ecliptic</a>.
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&deg; per century, so the signs currently lie 25&deg; to 30&deg; west of their namesake constellations.
There are also sidereal zodiacs, which account for precession and keep signs aligned with stars.</p>

<p>A celestial object's ecliptic longitude of date determines its zodiac sign.
If the lunar RA and Dec in question are for <a href="https://en.wikipedia.org/wiki/Epoch_(astronomy)" rel="nofollow noreferrer">epoch</a> J2000, then the Moon's epoch 2020.25 ecliptic longitude is 99.0&deg;, in the tropical <em>sign</em> of Cancer (longitudes 90&deg; to 120&deg;).
Relative to the IAU boundaries, drawn along lines of RA and Dec for epoch B1875, this is in the <em>constellation</em> <a href="https://commons.wikimedia.org/wiki/File:Gemini_IAU.svg" rel="nofollow noreferrer">Gemini</a>.</p>

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

<p><a href="https://en.wikipedia.org/wiki/House_(astrology)" rel="nofollow noreferrer">Houses</a> 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 <a href="https://en.wikipedia.org/wiki/Hour_angle" rel="nofollow noreferrer">hour angle</a>.</p>

CC BY-SA 4.0
2020-04-02T04:33:26.687Z
2020-04-02T04:33:26.687Z
Mike G
<p>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.</p>

<p>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.</p>

CC BY-SA 4.0
2020-04-01T10:24:04.610Z
Thank you, precession is why my calculation is wrong.
Matt Jameson
CC BY-SA 4.0
2020-04-01T21:01:02.800Z
@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
planetmaker
CC BY-SA 4.0
2020-04-01T23:04:45.427Z
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](https://en.wikipedia.org/wiki/House_(astrology)) 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.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2020-04-01T09:21:18.267Z
2020-04-01T21:31:12.633Z
2020-04-01T21:31:12.633Z
planetmaker
<p>Its very simple. You ASSUME that the right ascension of aries start point is 0 but its not. Its more than that. Its more like 40 deg or so. Aries WAS the starting point at 0, but about 2000 years ago. Precession of the equinoxes has changed this. But I get why the confusion because even now the right ascension 0 is called the first point of Aries even though its in Pisces. Nomenclature traditions in conjunction with poor science caused that.</p>

CC BY-SA 4.0
2024-03-04T18:31:53.957Z
2024-03-04T18:31:53.957Z
Roving Eye
<p>At the moment (01-04-2020 09:36) the Moon is situated in Gemini:</p>

<p>"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.</p>

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

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

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

<p>(Courtesy of <a href="https://astronomy.stackexchange.com/a/27488">https://astronomy.stackexchange.com/a/27488</a>)</p>

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

CC BY-SA 4.0
2020-12-08T11:57:09.973Z
I’m voting to close this question because astrology != astronomy
peterh
CC BY-SA 4.0
2020-12-08T18:25:49.443Z
@peterh-ReinstateMonica - both these questions are about astronomy, just couched in astrological terms.
Rory Alsop
<the-moon><right-ascension><zodiac>
Right Ascension to Zodiac House
Matt Jameson
# Right Ascension to Zodiac House

(Asked by: username_0 on 2020-04-01)

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?

**Tags:** the-moon, right-ascension, zodiac

> **Comments:**
> **username_1**: I’m voting to close this question because astrology != astronomy  
> **username_2**: @username_1-ReinstateMonica - both these questions are about astronomy, just couched in astrological terms.  

---

## 3 answers

### Answer by username_3 on 2020-04-02. Score: 2 | Accepted answer

Zodiac signs are equal 30° divisions of the [ecliptic](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/Epoch_(astronomy)) 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](https://commons.wikimedia.org/wiki/File:Gemini_IAU.svg).

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](https://en.wikipedia.org/wiki/House_(astrology)) 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](https://en.wikipedia.org/wiki/Hour_angle).


---

### Answer by username_4 on 2020-04-01. Score: 0

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.

> **Comments:**
> **username_0**: Thank you, precession is why my calculation is wrong.  
> **username_4**: @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  
> **username_5**: 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](https://en.wikipedia.org/wiki/House_(astrology)) 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.  

---

### Answer by username_6 on 2024-03-04. Score: -2

Its very simple. You ASSUME that the right ascension of aries start point is 0 but its not. Its more than that. Its more like 40 deg or so. Aries WAS the starting point at 0, but about 2000 years ago. Precession of the equinoxes has changed this. But I get why the confusion because even now the right ascension 0 is called the first point of Aries even though its in Pisces. Nomenclature traditions in conjunction with poor science caused that.


---

