超級月亮真的與地球太陽成一直線嗎?

有位朋友問我,為什麼NASA的網頁都是寫「Earth, Moon and sun are all in a line」?其實我七年前寫過一篇文章《月球小知識:月有陰晴圓缺》,趁現在再解釋一次。

NASA的文章可能希望讀者易於理解,把答案簡化了許多。總而言之,NASA的答案基於「假設太陽系是個二維平面」。

首先我們要想像飛出太陽系,從太陽系的「上面」或「下面」看地球、月亮和太陽,我們就會看見一個二維「平面」。基本上就是想像我們平時用相機拍照,把三維空間放進二維影像裡面一樣。

這樣的話,地球、月亮和太陽不單會在「超級滿月」時「成一直線」,三者每隔大約14天就會「成一直線」一次,因為月球繞地球公轉週期大約是28天。而且每14天就會梅花間竹發生一次日食或月食!

總而言之,當地球、月亮和太陽「成一直線」就會發生月食,跟月亮「超級不超級」無關。月球的繞地公轉平面(稱為白道面)與地球的繞日公轉平面(稱為黃道面),兩者之間有個大約5度的夾角,加上月球公轉軌道是楕圓形的,才會離地球又遠又近。

那麼究竟這次所謂的「超級」月球,有多「超級」?Well, 且讓我引Neil deGrasse Tyson的比喻:「如果你覺得一個16吋pizza比一個15吋pizza算得上是『超級大』的話,那麼今次月球也是『超級大』的。」

舊文《月球小知識:月有陰晴圓缺》傳送門:

https://bit.ly/3ALUUR5

By David Yu

David Yu is the father of Simba and an Astrophysicist. He obtained his PhD in Astrophysics from the Max Planck Institute of Extraterrestrial Physics, Germany. He was formerly a postdoc at the KTH Royal Institute of Technology, Sweden, and is currently a lecturer at the University of Hong Kong. He served as the host of the RTHK TV31 programme Science Night, the Depute Editor-in-Chief of the Taiwanese Science Magazine Physics Bimonthly, and the Science Consultant of Stand News. The books he authored include Dr David’s 21 Mysteries of The Universe (2021), Space – A Down-to-Earth Astrophysicist (2021), Time – A Spacetime Journey of Astronomy and Physics (2019), and Secrets from the Stars (2017).

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