<p>The key coordinate is the Sun's geocentric <a href="https://en.wikipedia.org/wiki/Ecliptic_coordinate_system#Spherical_coordinates" rel="nofollow noreferrer">ecliptic longitude</a>: 0&deg; at the March equinox, 90&deg; at the June solstice, 180&deg; at the September equinox, and 270&deg; at the December solstice.
The tropical zodiac subdivides each of these four 90&deg; spans into three 30&deg; blocks of ecliptic longitude.
The <a href="https://en.wikipedia.org/wiki/Gregorian_calendar" rel="nofollow noreferrer">Gregorian calendar</a> is designed to keep the equinoxes and solstices near the same dates every year, so the dates the Sun passes through each sign are roughly the same each year too.
The Sun spends 29.4 days in the sign of Capricorn and 31.5 days in the sign of Cancer because the Earth moves around the Sun faster at <a href="https://en.wikipedia.org/wiki/Apsis" rel="nofollow noreferrer">perihelion</a> than at aphelion.</p>

<p>Signs and constellations are not the same.
Over the centuries, <a href="https://en.wikipedia.org/wiki/Axial_precession" rel="nofollow noreferrer">precession</a> has moved the signs 25&deg;-30&deg; west of the constellations they were named after.
These modified Stellarium images show the zodiac in two halves, with the ecliptic and tropical sign boundaries in red, and the IAU constellation boundaries in blue.</p>

<p><img src="https://i.sstatic.net/gtYN5.png" alt="Zodiac 0-180"></p>

<p><img src="https://i.sstatic.net/gT85z.png" alt="Zodiac 180-360"></p>

<p>Schedules of 30 or 31 days per sign are based on the 30&deg; ecliptic longitude marks.
Some astronomers promote <a href="https://earthsky.org/astronomy-essentials/sun-in-zodiac-constellations-2020" rel="nofollow noreferrer">another set of dates</a> based on the IAU constellation boundaries, including the traditional twelve plus Ophiuchus.
The time the Sun spends in each constellation ranges from 7 days in Scorpius to 44 days in Virgo.
Precession shifts these dates 1.4 days later per century.</p>

CC BY-SA 4.0
2020-04-07T04:41:27.100Z
2020-04-09T07:20:34.337Z
2020-04-09T07:20:34.337Z
Mike G
<blockquote>
  <p>What is the astronomical logic in it? </p>
</blockquote>

<p>Based largely on what Wikipedia says about the <a href="https://en.wikipedia.org/wiki/Zodiac#History" rel="nofollow noreferrer">history of the zodiac</a> it seems that the choice was made by Babylonian astronomers (and astronomy and astrology were heavily interwoven at that time) sometime during the rough period 1000 BC to 500 BC.</p>

<p>Wikipedia quote :</p>

<blockquote>
  <p>Around the end of the 5th century BC, Babylonian astronomers divided the ecliptic into 12 equal "signs", by analogy to 12 schematic months of 30 days each. Each sign contained 30° of celestial longitude, thus creating the first known celestial coordinate system. According to calculations by modern astrophysics, the zodiac was introduced between 409-398 BC and probably within a very few years of 401 BC.[10] Unlike modern astronomers, who place the beginning of the sign of Aries at the place of the Sun at the vernal equinox, Babylonian astronomers fixed the zodiac in relation to stars, placing the beginning of Cancer at the "Rear Twin Star" (β Geminorum) and the beginning of Aquarius at the "Rear Star of the Goat-Fish" (δ Capricorni)</p>
</blockquote>

<p>Because the Babylonian astronomy and systems were passed to different cultures and were (presumably) the most developed, they became the foundation system for later systems from other cultures.  Like so many "standards" they became too embedded to discard entirely.</p>

CC BY-SA 4.0
2020-04-07T06:11:45.100Z
2020-04-07T06:11:45.100Z
StephenG - Help Ukraine
<p>Since many of the 12 astrological signs (zodiacal constellations) may be visible in any single night, based on what is determined that this month (for 30 days!) is Capricorn, or Aquarius or Pisces etc.? 
Based on what in astronomy is this determined? What is the astronomical logic in it? </p>

<p>I've been told that the sun 'blocks' always one star constellation, so assuming the night is longer than the day (14-16 hours), and the earth rotates 15 degrees each hour and one star constellation appears while one disappears, it means that by 14 hours 6 'new' star consolations (that couldn't be visible at the beginning of the night) now can be seen along the night. This would mean that 12 star constellations may be visible every day (once 24 hours). If my reasoning is correct, what was the astronomic basis for the claim that one star constellations (out of the specific 12) is 'blocked' for one month? </p>

CC BY-SA 4.0
2020-04-06T23:28:59.107Z
I'm voting to close this question as off-topic because this is a question of astrology.
James K
CC BY-SA 4.0
2020-04-07T11:02:23.503Z
@ Not at all. I asked something different.
Reckless Glacier
CC BY-SA 4.0
2020-04-07T14:15:11.200Z
I understand I didn't explain my question properly. I edited the question and made it more understandable.
Reckless Glacier
CC BY-SA 4.0
2020-04-07T15:24:06.630Z
The revised second paragraph amounts to a new question.
Mike G
CC BY-SA 4.0
2020-04-07T17:08:32.523Z
The purpose of down-vote is to improve the quality of the questions on this site, hence if you downvote without explanation, it'll never help to achieve the goal of this downvote. Currently I do not understand what is wrong with my question... Thank you
Reckless Glacier
CC BY-SA 4.0
2020-04-07T19:22:27.533Z
[This comment](/q/35738#comment70550_35739) lists 13 constellations with dates different from the traditional Western zodiac. If you focus on dates and remove the unrelated "how much does the Sun block" bit, I'll vote to reopen.
Mike G
<history><zodiac>
Based on what a month determined as of a certain zodiac constellation?
Reckless Glacier
# Based on what a month determined as of a certain zodiac constellation?

(Asked by: username_0 on 2020-04-06)

Since many of the 12 astrological signs (zodiacal constellations) may be visible in any single night, based on what is determined that this month (for 30 days!) is Capricorn, or Aquarius or Pisces etc.?
Based on what in astronomy is this determined? What is the astronomical logic in it?

I've been told that the sun 'blocks' always one star constellation, so assuming the night is longer than the day (14-16 hours), and the earth rotates 15 degrees each hour and one star constellation appears while one disappears, it means that by 14 hours 6 'new' star consolations (that couldn't be visible at the beginning of the night) now can be seen along the night. This would mean that 12 star constellations may be visible every day (once 24 hours). If my reasoning is correct, what was the astronomic basis for the claim that one star constellations (out of the specific 12) is 'blocked' for one month?

**Tags:** history, zodiac

> **Comments:**
> **username_1**: I'm voting to close this question as off-topic because this is a question of astrology.  
> **username_0**: @ Not at all. I asked something different.  
> **username_0**: I understand I didn't explain my question properly. I edited the question and made it more understandable.  
> **username_2**: The revised second paragraph amounts to a new question.  
> **username_0**: The purpose of down-vote is to improve the quality of the questions on this site, hence if you downvote without explanation, it'll never help to achieve the goal of this downvote. Currently I do not understand what is wrong with my question... Thank you  
> **username_2**: [This comment](/q/35738#comment70550_35739) lists 13 constellations with dates different from the traditional Western zodiac. If you focus on dates and remove the unrelated "how much does the Sun block" bit, I'll vote to reopen.  

---

## 2 answers

### Answer by username_2 on 2020-04-07. Score: 2

The key coordinate is the Sun's geocentric [ecliptic longitude](https://en.wikipedia.org/wiki/Ecliptic_coordinate_system#Spherical_coordinates): 0° at the March equinox, 90° at the June solstice, 180° at the September equinox, and 270° at the December solstice.
The tropical zodiac subdivides each of these four 90° spans into three 30° blocks of ecliptic longitude.
The [Gregorian calendar](https://en.wikipedia.org/wiki/Gregorian_calendar) is designed to keep the equinoxes and solstices near the same dates every year, so the dates the Sun passes through each sign are roughly the same each year too.
The Sun spends 29.4 days in the sign of Capricorn and 31.5 days in the sign of Cancer because the Earth moves around the Sun faster at [perihelion](https://en.wikipedia.org/wiki/Apsis) than at aphelion.

Signs and constellations are not the same.
Over the centuries, [precession](https://en.wikipedia.org/wiki/Axial_precession) has moved the signs 25°-30° west of the constellations they were named after.
These modified Stellarium images show the zodiac in two halves, with the ecliptic and tropical sign boundaries in red, and the IAU constellation boundaries in blue.

![Zodiac 0-180](https://i.sstatic.net/gtYN5.png)

![Zodiac 180-360](https://i.sstatic.net/gT85z.png)

Schedules of 30 or 31 days per sign are based on the 30° ecliptic longitude marks.
Some astronomers promote [another set of dates](https://earthsky.org/astronomy-essentials/sun-in-zodiac-constellations-2020) based on the IAU constellation boundaries, including the traditional twelve plus Ophiuchus.
The time the Sun spends in each constellation ranges from 7 days in Scorpius to 44 days in Virgo.
Precession shifts these dates 1.4 days later per century.


---

### Answer by username_3 on 2020-04-07. Score: 1

> What is the astronomical logic in it?

Based largely on what Wikipedia says about the [history of the zodiac](https://en.wikipedia.org/wiki/Zodiac#History) it seems that the choice was made by Babylonian astronomers (and astronomy and astrology were heavily interwoven at that time) sometime during the rough period 1000 BC to 500 BC.

Wikipedia quote :

> Around the end of the 5th century BC, Babylonian astronomers divided the ecliptic into 12 equal "signs", by analogy to 12 schematic months of 30 days each. Each sign contained 30° of celestial longitude, thus creating the first known celestial coordinate system. According to calculations by modern astrophysics, the zodiac was introduced between 409-398 BC and probably within a very few years of 401 BC.[10] Unlike modern astronomers, who place the beginning of the sign of Aries at the place of the Sun at the vernal equinox, Babylonian astronomers fixed the zodiac in relation to stars, placing the beginning of Cancer at the "Rear Twin Star" (β Geminorum) and the beginning of Aquarius at the "Rear Star of the Goat-Fish" (δ Capricorni)

Because the Babylonian astronomy and systems were passed to different cultures and were (presumably) the most developed, they became the foundation system for later systems from other cultures. Like so many "standards" they became too embedded to discard entirely.


---

