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?

  • 4
    $\begingroup$ I'm voting to close this question as off-topic because this is a question of astrology. $\endgroup$
    – James K
    Apr 6, 2020 at 23:28
  • $\begingroup$ Does this answer your question? Does the astrology of constellations relate to its position during the year? $\endgroup$
    – user24157
    Apr 7, 2020 at 9:07
  • 2
    $\begingroup$ Voting to reopen. I think this question is a valid version of astrological vs astronomical constellations. The mathematical study of astrology is valid here, and I believe this qualifies. $\endgroup$
    – user21
    Apr 7, 2020 at 15:07
  • 1
    $\begingroup$ The revised second paragraph amounts to a new question. $\endgroup$
    – Mike G
    Apr 7, 2020 at 15:24
  • 2
    $\begingroup$ 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 $\endgroup$ Apr 7, 2020 at 17:08

2 Answers 2


The key coordinate is the Sun's geocentric ecliptic longitude: 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 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 than at aphelion.

Signs and constellations are not the same. Over the centuries, 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

Zodiac 180-360

Schedules of 30 or 31 days per sign are based on the 30° ecliptic longitude marks. Some astronomers promote another set of dates 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.


What is the astronomical logic in it?

Based largely on what Wikipedia says about the history of the zodiac 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.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .