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J2000.0

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J2000.0是在天文学上使用的历元，前缀“J”代表这是一个儒略纪元法，而不是一个贝塞耳纪元。

它指的是儒略日期 TT时2451545.0，或是TT时2000年1月1日12时，即相对于TAI的2000年1月1日，11:59:27.816或UTC时间2000年1月1日11:58:55.816。

因恒星赤经和赤纬会因岁差（与恒星的自行）改变，所以天文学家们经常指定某一特定的纪元作参考点。早期采用的纪元标准是B1950.0纪元。

在J2000时刻的天赤道与二分点用来定义天球参考坐标系，该参考坐标系也可写作J2000坐标或简单记为J2000，但更合适的，应该如下使用国际天球参考系统。

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