![]() (An arcsecond is 1/3600 of a degree.) That is, a single Hubble pixel spans an angle of 0.04 arcseconds. Look up its angular resolution and you'll find out that it is 0.04 arcseconds. The Wide Field Camera 3 (WFC3) was used for both of these photos. How well should Hubble see either the galaxy or Pluto? To answer this question, you need to know the angular resolution of Hubble's camera. (See, our earlier order-of-magnitude estimate of a thousand times bigger was pretty close.) Take the ratio of those two and you'll see that the galaxy appears 1300 times bigger than Pluto.For the galaxy, 50,000 light-years / 72 million light-years = 0.00069.Now that we've done that, we can plug in the actual numbers. ![]() It's important to do a reality check like this first, because when you're dealing with very large or very small numbers, forgetting to punch in one zero in your calculator can majorly affect the outcome of your calculations. So we already know that the galaxy should appear about a thousand times bigger in the sky than Pluto does!.Pluto is like a thousand wide divided by a billion away that ratio should be around a millionth.The galaxy is like a hundred thousand wide divided by a hundred million away that ratio should be around a thousandth.Before you start punching in numbers with lots of zeros, you should first do a mental reality check on their order-of-magnitude proportions. To gauge how large these things appear in our sky, we can take the ratio of these things' sizes to their distances. On the date of the Pluto-and-moons image (July 7, 2012), the Solar System Simulator tells us that Pluto was 4.675 billion kilometers from Earth.According to the " Fast Facts" published with that Hubble image of galaxy NGC 5584, it is about 72 million light-years away, and the photo spans 50,000 light-years.The basic question that we're asking here is, how large do galaxies and Pluto appear in the sky? To answer that question, we need to know their sizes and distances.
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