charles 2009-3-23 11:43
Z形的火星行跡
Z is for Mars
[img]http://i5.tinypic.com/2hd9kj4.jpg[/img]
影像版權與提供: Tunc Tezel
[color=navy]Explanation: [/color]
[color=dimgray]This composite of images spaced about a week apart - from late July 2005 (bottom right) through February 2006 (top left) - traces the retrograde motion of ruddy-colored Mars through planet Earth's night sky. On November 7th, 2005 the Red Planet was opposite the Sun in Earth's sky (at opposition). That date occurred at the center of this series with Mars near its closest and brightest. But Mars didn't actually reverse the direction of its orbit to trace out the Z-shape. Instead, the apparent backwards or retrograde motion with respect to the background stars is a reflection of the motion of the Earth itself. Retrograde motion can be seen each time Earth overtakes and laps planets orbiting farther from the Sun, the Earth moving more rapidly through its own relatively close-in orbit. The familiar Pleiades star cluster lies at the upper left. [/color]
[color=navy]說明:[/color]
[color=dimgray]這幅影像是由每張相隔一星期的照片組合出來的，它呈現了淡紅色火星在地球夜空 逆行的情景，攝影的期間為2005年7月底至2006年2月止。2005年11月7日，火星運行到"衝"的位置，此時太陽和火星分別在地球的兩側。這一天在上面這系列影像的中點，當時火星最靠近地球也最明亮。當然火星不是真得在公轉軌道上倒著運行，以掃出這道Z字形的軌跡。火星相對於背景恆星會發生逆行，只是反映地球的公轉運動而已。因為地球離太陽比較近，在軌道上運行的速率比火星快，當地球追上並超越火星時，就會出現火星逆行的景象。影像左上方的是我們很熟悉的昴宿星團。 [/color]
[size=2]資料來源: Scientific American
Department of Physics, NCKU[/size][color=white][size=1]
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