Glossary term: 秒差距
Description: 秒差距（pc）是天文學中測量距離的一個標準單位，其定義如下：想象一個半徑為一天文單位（即地球與太陽的平均距離）的圓，我們從正面觀看這個圓。在一秒差距的距離上，我們可以看到圓的半徑在視野中所占據的角度恰為一角秒。照此定義，一秒差距大約等於3.26光年，也等於206264.8天文單位，後者也恰為一弧度所包含的角秒數。這個定義的實際意義在於它與視差法所測定距離的關係。視差法的原理是，觀測者自身的位置發生一定長度的變化時，就會測量出天空中天體視位置的微小偏移，稱為天體的視差角。對於一個距離為一秒差距的天體來說，觀測者的位置變化一天文單位（通常是由於兩次觀測之間地球沿著軌道的運動），就相當於天體發生了一角秒的視位置偏移，即天體的視差角為一角秒。這就產生了一個簡單的關係：若以秒差距為距離的單位，以角秒為視差角的單位，則天體距離的數值恰好等於視差角數值的倒數。其直接的幾何含義使秒差距成為專業天文學家首選的距離度量單位，在文獻中比光年更為常見。天狼星距離我們2.7秒差距，對應的視差角為1/2.7=0.37角秒。即使是比鄰星這顆距離我們的太陽最近的恆星，其視差角也小於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
This is an automated transliteration of the simplified Chinese translation of this term
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