How does a constellation become a sign of the zodiac?

"signs of the zodiac" are just convenient intervals, used in astrology, to divide the ecliptic in 12 parts.

There are 13 constellations on the ecliptic (the forgotten one is Ophiuchus, the serpent bearer), but 13 is an inconvenient number.

After the Romans imposed their 12-month calendar to most of the European world, European astrologers settled on 12 "houses" which were meant to be the intervals on the ecliptic, where the Sun resided during each month. That is why there are 12 houses, while there are 13 constellations.

The constellations themselves have come to us through the Greek astronomers, who got them mostly from the Babylonians. The constellations are apparent groupings that were made roughly 3000 years ago -- they were meant to be a mnemonic trick to help one remember where the stars are placed, relative to each other.

The ecliptic is the apparent path of the Sun (and planets) on the celestial sphere. It too was already known to the Babylonians and, later, to the Greek astronomers.

Since most figures along the ecliptic represent "animals" (if we include humans in the "animal" kingdom), then the band of constellation around the ecliptic have been called the "circle of animals" ( = zodiac -- from the same root that gives us zoological garden, for a garden full of animals).

One important date, in the Roman calendar, was March 21 (date of the Vernal Equinox, when the Sun's declination is 0, and where the ecliptic crosses the celestial equator -- when the Sun is there, it marks the beginning of Spring). So each house covers the "residence" of the Sun from the 21st of a month to the 21st of the next month (approximately).

Problem is that these dates were set roughly 2000 years ago. Since then, because of precession, the position of the equinox has slowly drifted and is about to enter the constellation of Aquarius.

Signs of the zodiac are therefore the representation of the location of the "houses", which no longer match the real position of the Sun on the ecliptic.

For example, The Sun enters Ophiuchus on November 30. It then passes into Sagittarius on Dec. 18 and stays there until January 20, when it passes into Capricornus.

Compare that with the "houses" of astrology, where the Sun is deemed to be in Sagittarius from November 23 to December 21.How does a constellation become a sign of the zodiac?Ooo that's spooky.

Only the ancient Babylonians know the answer to that.

And they are all dead.

They invented the system about 3000 years ago as an aid to their scientific analysis of the sky.How does a constellation become a sign of the zodiac?The zodiac constellations are defined as those through which ecliptic passes (the path the sun traces through the sky). There are 12 zodiac signs, however the ecliptic actually passes through 13 constellations; Ophiuchus is not generally included in the list of zodiac signs.