Final answer:
The zodiac sign someone refers to, such as Aries, is related to the position of the Sun at the moment of birth. However, the real constellations have shifted due to the precession of the Earth, so the astrological zodiac signs no longer match the actual constellations in the sky.
Step-by-step explanation:
The zodiac signs and associated constellations have a historical connection, but due to the precession of the Earth, they have shifted over time. The zodiac, a celestial coordinate system that divides the sky into twelve segments, originally corresponded to the positions of the sun against the backdrop of various constellations - like Aries, Taurus, and so on. Due to precession, a slow wobble in Earth's rotation, the actual stars of these constellations have moved approximately one zodiac sign over the course of about 26,000 years. For example, the astrological sign of Aries now falls within the actual constellation of Pisces.
In astrology, the sun sign indicates the zodiac sign that the sun occupied at the time of a person's birth. To determine the sun's position in the zodiac historically, ancient astronomers would observe the last constellation of the zodiac to rise before the sun, or the first to set after it, since the sun's brightness makes it difficult to observe the stars within the constellation it occupies. Thus, each month-long period was associated with a zodiac sign.