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Starting from Curiosity Observes Sunset Eclipse: Sol 2358 I found three images in the sequence PIA23133, PIA23134, and PIA23135 which I thought would mean (but don't know for sure) that they are associated with the same event. Several things confuse me, including:

  1. Why are the eclipsing bodies in PIA23133 and PIA23134 so different in angular size compared to the solar disk? If they are separate events, why are the PIA numbers adjacent?
  2. Why is it that it's mostly a chunk of the right half of the sky that darkens during the eclipse in PIA23135? per suggestion I've moving this part to Astronomy SE: Why is only a narrow chunk of sky darkened in this amazing Curiosity solar eclipse GIF from Mars?

For this particular question, please include some specific information on these images with a link if possible, rather than just a best-guess answer. Thanks!

Enquiring minds want to know!

fyi in case you are wondering, per @MarkAdler's ultra-concise answer

They use neutral-density filters to look at the Sun, which reduce the light by a factor of 100,000. The two Pancam cameras each have one neutral-density filter, with the left one filtering blue and the right one filtering red.

From https://photojournal.jpl.nasa.gov/archive/PIA23133.gif and

From https://photojournal.jpl.nasa.gov/archive/PIA23134.gif

photojournal.jpl.nasa.gov/archive/PIA23133.gif photojournal.jpl.nasa.gov/archive/PIA23134.gif

From https://photojournal.jpl.nasa.gov/archive/PIA23135.gif

photojournal.jpl.nasa.gov/archive/PIA23135.gif

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First, I was unable with a cursory search to find out what PIA stands for. It is a common acronym across many NASA missions it seems. (PIA14922, for instance, refers to an image from the Cassini mission)

The first image, PIA23133 is Phobos transiting (or eclipsing, depending on your definitions) the Sun. It was taken on March 26 this year. The second image, PIA23134 is the smaller and further away moon Deimos transiting the Sun. It was taken on March 17 this year, eight days before the first picture.

The third image, PI23135 is a shot of the shadow of Phobos passing over and darkening the sky. It is the same event as PIA23133, just shot from a different camera.

Considering the picture numbers are out of sequence wit the chronology of events, these numbers probably just refer to the order in which the images were released to the public.

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  • $\begingroup$ Thank you for your well-sourced and thoughtful answer. I'm still having trouble understanding why it's only such a narrow chunk of the sky that appears to get darkened in PIA23133. Is that phenomenon seen in solar eclipses on Earth? $\endgroup$
    – uhoh
    Apr 5, 2019 at 3:17
  • $\begingroup$ I can only give a speculative answer: Mars' atmosphere has a scale height of 10.8 km (planetary-science.org/mars-research/martian-atmosphere) which is 80% higher than earth's. This means the atmosphere extends much further out, which is probably why you see a partial shadow instead of a full darkening of the sky. $\endgroup$
    – Ingolifs
    Apr 5, 2019 at 3:24
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    $\begingroup$ slightly related, asked just now: Where does the sky's blueness come from; at what altitudes is it being produced? $\endgroup$
    – uhoh
    Apr 5, 2019 at 3:41
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    $\begingroup$ Might as well. We're currently in the SE graveyard shift, and it'll be a few hours before the americans wake up and see your post. $\endgroup$
    – Ingolifs
    Apr 5, 2019 at 3:48
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    $\begingroup$ Yeah. I'm a new zealander and you're in Taiwan, which is pretty much why I always seem to be the first to answer you. $\endgroup$
    – Ingolifs
    Apr 5, 2019 at 3:52

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