2025年5月30日
Mars in the Loop
Image Credit & Copyright: Tunc Tezel (TWAN)
Explanation: This composite of images spaced a weather-permitting 5 to 9 days apart, from 2024 September 19 (top right) through 2025 May 18 (bottom left), faithfully traces ruddy-colored Mars as it makes a clockwise loop through the constellations Gemini and Cancer in planet Earth’s night sky. You can connect the dots and dates with your cursor over the image, but be sure to check out this animation of the Red Planet’s 2024/25 retrograde motion. Of course Mars didn’t actually reverse the direction of its orbit. Instead, the apparent backwards motion with respect to the background stars is a reflection of the orbital motion of 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. In this case Mars’ apparent eastward motion began to reverse around December 8, when it seemed to linger near open star cluster M44 in Cancer. After wandering back to the west, under Gemini’s bright stars Castor and Pollux, Mars returned to pose near M44 by early May. At its brightest near opposition on 2025 January 16, Mars was a mere 96 million kilometers away.
Tomorrow’s picture: afterimage
火星逆行
影像提供与版权: Tunc Tezel (TWAN)
说明: 这幅组合了间隔5至9天（受限于天气）数据的影像，呈现了在2024年9月19日（右上）至2025年5月18日（左下）期间，锈红色的火星于地球的夜空中，在双子座与巨蟹座逆时钟转圈的景观。你不妨用滑鼠沿着日期对影像中的点做连连看的操作，当然，更不要错过这部呈现2024/25年火星逆行运动的动画。想当然耳，火星并没真的在轨道上倒着走，我们所见到它相对于背景恒星的视运动，只是地球的轨道运动所造成的效应。由于地球在较近太阳的内侧轨道上公转，轨道速度比外侧的行星快，因此每次追上与超车外行星时，就会发生外行星逆行的现象。在上面这个例子里，火星东移的视运动在12月8日开始反向，接下来看似“留”在巨蟹座的疏散星团M44附近。于向西逆行的运动中，火星看似于双子座亮星北河二（Castor）与北河三（Pollux）下方又反向东移，并在5月初回到M44附近。火星于2025年1月16日、最明亮也最靠近地球的火星冲之时，与地球的距离仅有9千6百万公里。
明日的图片: afterimage