A solar-based calendar is one based on the apparent movement of the Sun across the sky as we orbit around our star. More than 2,500 years ago, various mathematicians and astronomers were basing a solar year on the equinoxes (when the Sun’s direct rays are on the equator—or beginning of fall and spring) and solstices (when the Sun’s direct rays are on the latitudes marked Tropic of Capricorn [winter in the Northern Hemisphere; summer in the Southern Hemisphere] or Tropic of Cancer [winter in the Southern Hemisphere; summer in the Northern Hemisphere]).

As the measurement of the solar (and lunar) cycle became more accurate, calendars became increasingly sophisticated. But no calendar dominated until the last few centuries, with many cultures deriving their own calendars—some even combining lunar and solar cycles in a type of moon-sun or luni-solar calendar. This is why although there is one “standard calendar” used by most countries around the world, certain cultures still use their traditional calendars, including the Chinese, Jewish, and Muslim calendars.