自行(英文Proper motion),又叫自動、固有運動或者正動,係天文現象,指天球上,恆星之類嘅位置,相對於周圍星體,年年向一方移行運動。
所謂自行,係指恆星自身嘅運動。但係地球上睇出來嘅,都唔完全由星自己嘅運動。皆因太陽系都會郁。而遠處嘅星,由於光速有限,根本度唔到佢家下自行嘅速度,只可話古時發光一刻,星嘅自行速度。
一七一八年,哈雷氏發明赤經、赤緯,度恆星位置。但後人發現,星嘅位置同家下唔同。原因除咗歳差、章行差、章動等,自行都其中之一。
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