vernal equinox

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The vernal equinox, however, occurs when the center of the sun crosses the Equator.
儅太陽中心橫過赤道之時，便是春分。
It issued the Easter Rule which states that Easter shall be celebrated on the first Sunday that occurs after the first full moon on or after the vernal equinox.
這個理事會制定了複活節的標準：複活節應該在第一個滿月後或是春分後的第一個星期進行慶祝。
Therefore, on the vernal equinox day, the daylight hours are actually longer than the length of time between when the sun crosses the horizon at dawn and when the sun crosses the horizon at sunset.
於是，在春分之日，太陽從黎明經過地平線生氣之時，到黃昏經過地平線落下之時，兩者間的時長確實沒有我們所看見的白天長。

Well, obviously Florida and California are two states where one can ride 12 months a year but for motorcyclists in less temperate climes it can be hard hanging up your helmet until the vernal equinox.
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the time in the spring when the sun crosses the equator, and when night and day are of equal length

the time at which the sun crosses the plane of the equator towards the relevant hemisphere, making day and night of equal length. It occurs about March 21 in the N hemisphere (Sept 23 in the S hemisphere)