軌道交點

軌道交點是傾斜的軌道穿越參考平面的兩個點之一^[1]。包含於參考平面上的軌道（稱為無傾斜軌道）沒有交點。一般的參考平面有：

對地心軌道的參考平面是地球的赤道平面。在這種情況的無傾斜軌道稱為赤道面^[2]。
對日心軌道的參考平面是黃道。在這種情況的無傾斜軌道稱為黃道面^[2]。
對太陽系之外的軌道，參考平面是垂直於觀測者視線方向的平面，稱為天球平面^[3]^{, chap. 17.}。

交線是天體的軌道平面與參考平面交會之間的交集，它經過這兩個交點^[2]。

如果參考平面的參考方向被定義在其中的一個平面側，則這兩個交點就可以被區分。對地心和日心軌道，稱為升交點（或北交點）的是軌道上的天體運行到參考平面北側時通過點，而降交點（或南交點）則是天體運行至南側時通過的點^[4]。對於太陽系之外的軌道，升交點是天體遠離觀測者時所經過的點，降交點則是朝向觀測者接近時所經過的點^[5]^{, p. 137.}。

升交點的符號是（Unicode: U+260A, ☊），而降交點的符號是（Unicode: U+260B, ☋）。在中世紀與現代的早期，升交點和降交點分別被稱為龍頭（拉丁：龍骨，阿拉伯：ra's al-jauzahar）和龍尾（拉丁語：cauda draconis）相對應^[6]^{, p. 141;}^[7]^{, p. 245.}。同樣的，在阿拉伯語中如ganzaar、genzahar、geuzaar和zeuzahar等已經很少用到的字，都曾被中世紀的西方用來表示這兩個交點^[8]^{, pp. 196–197;}^[9]^{, p. 65;}^[10]^{, pp. 95–96.}。希臘的名詞αναβιβάζων和καταβιβάζων也被用來稱呼升交點和降交點，在英文以anabibazon和catabibazon表示升交點^[11]^;^[12]^{, ¶27.}。

交點的位置可以被做為一個參數，稱為軌道要素，用來描述軌道。這個參數被稱為升交點黃經（或者有時就稱為交點黃經）。

地球交點

月球交點

對月球環繞地球的軌道，參考的基準平面不是赤道平面而是黃道面。太陽引力對月球的作用，在軌道上產生的交點稱為月球交點，會因為月球進動逐漸西移，約18.6年遶行軌道一周^[1]^[13]。

參考資料

^ ^1.0 ^1.1 node 互聯網檔案館的存檔，存檔日期2007-03-09., entry in The Columbia Encyclopedia, 6th ed., New York: Columbia University Press, 2001–04. Accessed on line May 17, 2007.
^ ^2.0 ^2.1 ^2.2 line of nodes （頁面存檔備份，存於互聯網檔案館）, entry in The Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling, on line, accessed May 17, 2007.
^ Celestial Mechanics （頁面存檔備份，存於互聯網檔案館）, Jeremy B. Tatum, on line, accessed May 17, 2007.
^ ascending node （頁面存檔備份，存於互聯網檔案館）, entry in The Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling, on line, accessed May 17, 2007.
^ The Binary Stars, R. G. Aitken, New York: Semi-Centennial Publications of the University of California, 1918.
^ Survey of Islamic Astronomical Tables, E. S. Kennedy , Transactions of the American Philosophical Society, new series, 46, #2 (1956), pp. 123–177.
^ Cyclopædia, or, An universal dictionary of arts and sciences （頁面存檔備份，存於互聯網檔案館）, Ephraim Chambers, London: Printed for J. and J. Knapton [and 18 others], 1728, vol. 1.
^ Planetary Latitudes, the Theorica Gerardi, and Regiomontanus, Claudia Kren, Isis, 68, #2 (June 1977), pp. 194–205.
^ Prophatius Judaeus and the Medieval Astronomical Tables, Richard I. Harper, Isis 62, #1（Spring, 1971）, pp. 61–68.
^ Lexicographical Gleanings from the Philobiblon of Richard de Bury, Andrew F. West, Transactions of the American Philological Association（1869-1896）, 22（1891）, pp. 93–104.
^ anabibazon, entry in Webster's third new international dictionary of the English language unabridged: with seven language dictionary, Chicago: Encyclopaedia Britannica, 1986. ISBN 0-85229-503-0.
^ New thoughts on the genesis of the mysteries of Mithras^{[失效連結]}, Roger Beck, Topoi 11, #1 (2001), pp. 59–76.
^ Introduction to Astronomy 250, Coordinates, Seasons, Eclipses （頁面存檔備份，存於互聯網檔案館）, lecture notes, Astronomy 250, Marcia Rieke, University of Arizona, accessed on line May 17, 2007.

參見

[col-1] 1.0 ^1.1 node 互聯網檔案館的存檔，存檔日期2007-03-09., entry in The Columbia Encyclopedia, 6th ed., New York: Columbia University Press, 2001–04. Accessed on line May 17, 2007.

[darlinglon-2] 2.0 ^2.1 ^2.2 line of nodes （頁面存檔備份，存於互聯網檔案館）, entry in The Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling, on line, accessed May 17, 2007.

[3] Celestial Mechanics （頁面存檔備份，存於互聯網檔案館）, Jeremy B. Tatum, on line, accessed May 17, 2007.

[darlingasc-4] scending node （頁面存檔備份，存於互聯網檔案館）, entry in The Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling, on line, accessed May 17, 2007.

[5] The Binary Stars, R. G. Aitken, New York: Semi-Centennial Publications of the University of California, 1918.

[6] Survey of Islamic Astronomical Tables, E. S. Kennedy , Transactions of the American Philosophical Society, new series, 46, #2 (1956), pp. 123–177.

[7] Cyclopædia, or, An universal dictionary of arts and sciences （頁面存檔備份，存於互聯網檔案館）, Ephraim Chambers, London: Printed for J. and J. Knapton [and 18 others], 1728, vol. 1.

[8] Planetary Latitudes, the Theorica Gerardi, and Regiomontanus, Claudia Kren, Isis, 68, #2 (June 1977), pp. 194–205.

[9] Prophatius Judaeus and the Medieval Astronomical Tables, Richard I. Harper, Isis 62, #1（Spring, 1971）, pp. 61–68.

[10] Lexicographical Gleanings from the Philobiblon of Richard de Bury, Andrew F. West, Transactions of the American Philological Association（1869-1896）, 22（1891）, pp. 93–104.

[11] zon, entry in Webster's third new international dictionary of the English language unabridged: with seven language dictionary, Chicago: Encyclopaedia Britannica, 1986. ISBN 0-85229-503-0.

[12] New thoughts on the genesis of the mysteries of Mithras^{[失效連結]}, Roger Beck, Topoi 11, #1 (2001), pp. 59–76.

[13] Introduction to Astronomy 250, Coordinates, Seasons, Eclipses （頁面存檔備份，存於互聯網檔案館）, lecture notes, Astronomy 250, Marcia Rieke, University of Arizona, accessed on line May 17, 2007.

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