Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                              2020 September 23

                              ISS Transits Mars
                     Image Credit & Copyright: Tom Glenn

   Explanation: Yes, but have you ever seen the space station do this? If
   you know when and where to look, watching the bright International
   Space Station (ISS) drift across your night sky is a fascinating sight
   -- but not very unusual. Images of the ISS crossing in front of the
   half-degree Moon or Sun do exist, but are somewhat rare as they take
   planning, timing, and patience to acquire. Catching the ISS crossing in
   front of minuscule Mars, though, is on another level. Using online
   software, the featured photographer learned that the unusual transit
   would be visible only momentarily along a very narrow stretch of nearby
   land spanning just 90 meters. Within this stretch, the equivalent
   ground velocity of the passing ISS image would be a quick 7.4
   kilometers per second. However, with a standard camera, a small
   telescope, an exact location to set up his equipment, an exact
   direction to point the telescope, and sub-millisecond timing -- he
   created a video from which the featured 0.00035 second exposure was
   extracted. In the resulting image capture, details on both Mars and the
   ISS are visible simultaneously. The featured image was acquired last
   Monday at 05:15:47 local time from just northeast of San Diego,
   California, USA. Although typically much smaller, angularly, than the
   ISS, Mars is approaching its maximum angular size in the next few
   weeks, because the blue planet (Earth) is set to pass its closest to
   the red planet (Mars) in their respective orbits around the Sun.

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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
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