LARRY W. KOEHN / SCIENCE PHOTO LIBRARY LARRY W. KOEHN / SCIENCE PHOTO LIBRARY
Optical image showing the retrograde motion of the planet Mars (brightest object at centre left). The photograph shows the path of Mars, as seen against the stars of the Milky Way, from July 17th 1988 until December 5th of the same year. The first exposure shows Mars at lower centre left; in the following exposures Mars moves upwards towards the left edge where it reaches a stationary point. After this Mars moves down towards the centre of frame and then again towards upper left drawing almost a paraboloid curve. The exposures were taken every six days at the same sidereal time, the time measured with reference to the fixed stars, to ensure the alignment of the stars.
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