In the sidereal system, observable stars and constellations are the frame of reference, McDonough says. And these are not fixed but rather shift by one degree roughly every 72 years. As such, the dates for sun signs have changed over the years. Here’s where each sign falls now, plus a quick intro into what each one represents. For a deeper understanding, check out the AstroTwins’ Astrology Fundamentals class on mbg. This isn’t to say that you need to change the sign you identify with overnight, but it’s information that all the astro-acquainted should know after mastering the basics of Western astrology.