From the fourth to the second millennia BC, Taurus contained the Sun on the first day of spring. The position of the Sun against the background of fixed stars on the first day of spring is the vernal equinox. However, in the time of Hipparchos (2nd century BC), the Sun was in Aries at the vernal equinox. Its position then became known as "The First Point of Aries." Hipparchos discovered that the equinoctial point had moved westward from its earlier location in Taurus, sliding along the ecliptic, in a so-called "precession of the equinoxes."
The equinoctial points have continued to shift, by about 1 degree every 72 years, so that in roughly 26,000 years it would move all the way around the ecliptic and return to the same location. At present the vernal equinox is located about 30 degrees west of the First Point of Aries, in Pisces near its border with Aquarius. Similarly, at the solstices the Sun is now located in Taurus and Sagittarius instead of Cancer and Capricorn.
Because astrologers still calculate from the First Point of Aries instead of the current March equinox location in Pisces, all modern horoscopes have shifted out of phase with the actual stars. That is, the actual zodiac of constellations now differs from the zodiac signs or houses as used by astrologers, because the zodiac of signs refers to the location of the Sun in the epoch of Hipparchos, disregarding precession. For example, if a contemporary of Hipparchos' were born in early March, the Sun would then have been in the constellation of Pisces, and his astrological sign would have been Pisces. But someone born in early March in our time would still be given the sign Pisces by an astrologer, despite the fact that the Sun was actually in the constellation of Aquarius at the time of birth. Those who read horoscopes are one sign out of phase with the constellations!
Sign Containing the Sun (modern vs. late antiquity).