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The penumbral shadow of Phobos on Mars on September 1 1999, imaged by Mars Global Surveyor. The center of the shadow was at approximately 14°N 236°W (map) at 20:49:02.4 UTC Earth time.

Note that the timestamp "20:13:05" printed on the photo does not correspond to the actual time that the shadow was imaged, rather it represents the "image start time" of a vertically much larger original image that took nearly an hour to acquire. M07-00166 (red) and M07-00167 (blue).

Mars Global Surveyor orbits Mars in a sun-synchronous polar orbit with orbital period 117.65 minutes, moving from south pole to north pole, and continuously points its camera straight down. The result is an image in the form of a very long thin vertical strip, where the pixels in the top part of the image are imaged nearly one hour after those in the bottom part of the image. However, "downtrack summing" usually reduces the vertical size of the image by a considerable factor by merging multiple lines into one.

In this case the image start time is 20:13:04.69 UTC, the line integration time is 80.48 milliseconds, and the downtrack summing factor is 27. The shadow is about 8 pixels high, centered at 993 pixels from the bottom of the original 1600-pixel-high image. We add (993 * 0.08048 * 27) = 2157.75 seconds = 35 minutes 57.75 seconds to get a time of 20:49:02.4 UTC for the center of the shadow.

• NASA press release (November 1 1999)
• MOC Global-Map Images, Subphase M07
• Mars Global Surveyor Mars Orbiter Camera Image Gallery