Why doesn’t the Sun fill the sky on Mercury?

  • I’ve seen a lot of photos showing Mercury in front of the Sun when it passes by and you can see just how tiny it is in comparison.



    Here’s a great example:



    enter image description here



    So I’d expect if I was travelling towards Mercury in that photo that as the planet got bigger in my view so would the Sun and eventually I wouldn’t be able to see the edges of the sun because it’s so big at this distance... makes sense that an object gets bigger the closer you get to it!



    However what confuses me is that if you were on the surface of Mercury... this is the view you’d get where it’s about a 6 times larger than when viewed from Earth.



    enter image description here



    So how is this so small when it’s taking the whole view up when we’re viewing it further away from mercury in the first photo? But we’re closer in the second photo but now the sun is smaller... How is the first photo possible (it’s an actual photo taken by NASA) why isn’t it taking up the whole sky in the second photo when you’re actually on the planet?


    The Earth would also look tiny with an image like the first one.

  • James K

    James K Correct answer

    2 years ago

    From where we stand on Earth, Mercury is pretty small about 13 arcseconds across at most. The sun, by comparison is about 1800 arcseconds across, so if you are to see Mercury as a disc, you need to magnify your image a lot. And that makes the sun appear very very big. It only appears very big because it has been magnified.



    But if you are on Mercury, you don't need to magnify the image of the sun. The sun on Mercury is about 5000 arcseconds across. Big, but not filling the sky. That is just because it hasn't been magnified


    So the simple answer would be "because the first photo is edited"?

    Or in other words, had the first photo not been edited, the sun would have looked much smaller (maybe slightly bigger than what it looks like from the Earth and slightly smaller than what it looks like in the second photo) and Mercury would not have been visible at all?

    @user17915 not really 'edited', but 'shot with much smaller FOV'

    @user17915 using the zoom on a camera is not editing. Otherwise I would technically be editing reality every time I look through a telescope :o

    If i remember correctly, this is isometric protection, when sizes scale down depending on field of view and distance. And there is also orthogonal projection, which does not scale object image. In the second one you would cover Mercury with the Sun entirely. So what you need to do, is to pick focus of isometric projection big enough for it to become more orthogonal-like.

    -1 because you don't mention _field of view_. This is the important photographic principle that is at the heart of the effect in question. This is also how you get those wolf-howling-at-the-moon photos where you have a wolf or something in the foreground and a giant Moon behind it. These are taken by positioning the camera some distance away and then zooming in on the wolf to get it back to "normal" size. The small FoV magnifies the distant background enormously.

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Content dated before 7/24/2021 11:53 AM

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