Glossary term: 秒差距
Description: 秒差距（pc）是天文学中测量距离的一个标准单位，其定义如下：想象一个半径为一天文单位（即地球与太阳的平均距离）的圆，我们从正面观看这个圆。在一秒差距的距离上，我们可以看到圆的半径在视野中所占据的角度恰为一角秒。照此定义，一秒差距大约等于3.26光年，也等于206264.8天文单位，后者也恰为一弧度所包含的角秒数。这个定义的实际意义在于它与视差法所测定距离的关系。视差法的原理是，观测者自身的位置发生一定长度的变化时，就会测量出天空中天体视位置的微小偏移，称为天体的视差角。对于一个距离为一秒差距的天体来说，观测者的位置变化一天文单位（通常是由于两次观测之间地球沿着轨道的运动），就相当于天体发生了一角秒的视位置偏移，即天体的视差角为一角秒。这就产生了一个简单的关系：若以秒差距为距离的单位，以角秒为视差角的单位，则天体距离的数值恰好等于视差角数值的倒数。其直接的几何含义使秒差距成为专业天文学家首选的距离度量单位，在文献中比光年更为常见。天狼星距离我们2.7秒差距，对应的视差角为1/2.7=0.37角秒。即使是比邻星这颗距离我们的太阳最近的恒星，其视差角也小于1角秒。
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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher The translation of this term and its definition is still awaiting approval
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