国际本初子午线大会于1884年把横过英国格林尼治天文台的一条经线定为参考线，在该线上经度为零。
The Greenwich meridian was adopted in 1884 by the International meridian Conference as the line with zero degrees of longitude.
格林尼治国际标准以英国伦敦格林威治皇家天文台为准，也就是零经度为准。
Greenwich mean time (GMT) is mean solar time at the Royal Greenwich Observatory in Greenwich, London, England, which by convention is at 0 degrees geographic longitude.
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