2025年6月21日
Two Worlds, Two Analemmas
Image Credit: (left) Copyright: Tunc Tezel (TWAN) – (right): NASA/JPL/Cornell/ASU/TAMU
Explanation: Sure, that figure-8 shaped curve you get when you mark the position of the Sun in Earth’s sky at the same time each day over one year is called an analemma. On the left, Earth’s figure-8 analemma was traced by combining wide-angle digital images recorded during the year from December 2011 through December 2012. But the shape of an analemma depends on the eccentricity of a planet’s orbit and the tilt of its axis of rotation, so analemma curves can look different for different worlds. Take Mars for example. The Red Planet’s axial tilt is similar to Earth’s, but its orbit around the same sun is more eccentric (less circular) than Earth’s orbit. As seen from the Martian surface, the analemma traced in the right hand panel is shaped more like a tear drop. The Mars rover Opportunity captured the images used over the Martian year corresponding to Earth dates July 2006 to June 2008. Of course, each world’s solstice dates still lie at the top and bottom of their different analemma curves. The last Mars northern summer solstice was May 29, 2025. Our fair planet’s 2025 northern summer solstice is at June 21, 2:42 UTC.
Tomorrow’s picture: just a bowl of spherules
二颗行星、二种日行迹
影像提供: (左图)版权: Tunc Tezel (TWAN) – (右图): NASA/JPL/Cornell/ASU/TAMU
说明: 上图的8字形曲线称为日行迹，是在地球上，每天于相同地点与时间记录天空中太阳的位置所得到的图案。左侧的地球8字形日行迹，是由2011年12月到2012年12月一个整年期间拍摄的广角数位影像组合而成。日行迹的形状，会因行星的轨道离心率和自转轴的倾角而异，是以在不同行星上记录的日行迹，形状也会不一样。以火星为例，火星的转轴倾角和地球相近，但绕太阳公转的椭圆轨道离心率较大（轨道较扁长）。而从火星表面看出去，所见到的日行迹是形似泪滴的右图。此图则是由机遇号火星车，摄于2006年7月到2008年6月期间的火星表面影像组合出来的。可想而知的，这二颗行星的至点都是位于各别日行迹的顶点和底端。刚过去的火星北半球夏至，出现在地球历2025年5月29日，而地球2025年的北半球夏至，会在6月21日世界时2:42 UTC降临。
明日的图片: just a bowl of spherules
