春分 - 太陽直射赤道 Three facts 1. In each year when Earth orbits around the Sun, There are two moments that the Sun is positioned directly over Earth's equator, called as equinoxes. This happens in March and in September. March equinox is also called Vernal Equinox 每年當地球環繞太陽公轉時，有兩時刻太陽垂直照射在地球赤道。三月為春分，九月為秋分。 2. 1st day of astronomical spring ( northern hemisphere)天文學上的春季第一天(北半球) 3 . 二十四節氣中的第四節氣 當太陽到達視地心視黃經apparent geocentric ecliptic longitude為0度的時刻（ geocentric, not topocentric) is 春分点是黄道相对天赤道的升交点，是现代天文学上标记和度量天体位置和运动的两个重要坐标系——天球黄道坐标系和赤道坐标系的计量零点，在天文学上有重要意义。 View from Space, Earth's rotation axis pointing vertical 太陽垂直照射在地球赤度 (sub solar point) Showing the change of terminator's inclination wrt Earth's rotation axis 從太空上看地球, 春分的時刻 地球的 solar terminator陰影 與地球轉軸是同向，Earth's rotation axis is directly perpendicular to the Sun-Earth line, tilting neither toward nor away from the Sun，南北两半球获得相等的光照（illumination) View from plane of Earth's orbit 傳統上會形容春分為晝夜平分,是否正確呢？ 史書： 《明史·历一》说：...