While astrology is widely disproven as a form of science in our modern age, it has its origins in the beginnings of computational astronomy. As explained by David Lindberg in his book The Beginnings of Western Science,
“By the end of the fifth century B.C., Babylonian celestial divination had expanded to embrace horoscopic astrology, which used planetary positions at the moment of birth (or near the date of birth for such exceptional phenomena as lunar eclipses) to predict individual fortunes.”
By the time the Greeks inherit the Babylonians’ methodology, astrology and astronomy are inseparable.
The specific idea of the 12 zodiacs signs (although a 13th one actually exists as well) comes from the positions not of the planets, but of the Sun. As Earth orbits on the ecliptic, the Sun is positioned “within” a specific zodiac constellation during a specific time in the orbit. For example, between July 23 and August 22, the Sun appears to be within the constellation Leo. This is why my horoscope sign would be Leo, since I was born on August 7th.
However, because of precession, these horoscope signs are actually a lie. These signs are based on the positions of the Sun in relation to these constellations from almost 2000 years ago, when astrology began. Earth’s axis has since moved about 1/13th of the way through its precession cycle in that time, so horoscopes are off by about a month from the actual position of the Sun today.
So instead of being a Leo, I guess I have to start embracing my inner Virgo!