The zodiac, the 12 signs listed in a horoscope, is closely tied to how the Earth moves through the heavens. The signs are derived from the constellations that mark out the path on which the sun appears to travel over the course of a year. In principle, dates in a horoscope should correspond to when the sun passes through each constellation. But they don’t, much of the time. As the Earth orbits the sun, the sun appears to pass in front of different constellations. Much like the moon appears in a slightly different place in the sky each night, the location of the sun relative to distant background stars drifts in an easterly direction from day to day. It’s not that the sun is actually moving. The motion is entirely an illusion caused by the Earth’s own motion around our star. Over the course of a year, the sun appears to be in front of, or “in”, different constellations. One month, the sun appears in Gemini; the next month, in Cancer. The dates listed in the newspaper’s horoscope identify when the sun appears in a particular astrological sign. For example, March 21 through April 19 are set aside for the sign Aries. But your astrological sign doesn’t necessarily tell you what constellation the sun was in on the day you were born.