vernal equinox

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The vernal equinox, however, occurs when the center of the sun crosses the Equator.
当太阳中心横过赤道之时，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.
这个理事会制定了复活节的标准：复活节应该在第一个满月后或是春分后的第一个星期进行庆祝。
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.
于是，在春分之日，太阳从黎明经过地平线生气之时，到黄昏经过地平线落下之时，两者间的时长确实没有我们所看见的白天长。

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)