Aries is the Ram, a small constellation of three main stars, of which Hamal is the brightest. It gave its name to the First Point of Aries, the zero of celestial longitude, when the March equinox lay within it. The equinox left Aries about two thousand years ago and the name stayed. The Sun now arrives in Aries in the second half of April, just as the horoscope’s Aries is ending.
The dates above are computed for 2026 from the constellation boundaries the International Astronomical Union fixed in 1930. They move by about a day every seventy years, so they are good for a lifetime, and for the particular day you were born the calculator will check the boundary itself.