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Sign up to join this community [ano] Anybody can ask a question [ano] Anybody can answer [an] The best answers are voted up and rise to the top Astronomy 1. Home 2. 1. Public 2. Questions 3. Tags 4. Users 5. Unanswered 3. 1. Teams Stack Overflow for Teams - Start collaborating and sharing organizational knowledge. [teams-illo-free-si] Create a free Team Why Teams? 2. Teams 3. Create free Team Stack Overflow for Teams is moving to its own domain! When the migration is complete, you will access your Teams at stackoverflowteams.com, and they will no longer appear in the left sidebar on stackoverflow.com. Check your email for updates. Teams Q&A for work Connect and share knowledge within a single location that is structured and easy to search. Learn more about Teams Is this photocomposition of Andromeda's galaxy and the Moon accurate? Ask Question Asked 4 years, 11 months ago Modified 4 years, 11 months ago Viewed 43k times 28 $\begingroup$ I recently saw the below image circulating around twitter/facebook/ reddit. It is titled "Andromeda's actual size if it was brighter" enter image description here One of the comments provides a link to another comparison provided by APOD, and the following comment mentions that the first one I linked seems a bit off. Is the first image accurate in term of relative sizes? What is the relative angular size of Andromeda's galaxy and the Moon? * the-moon * galaxy Share Improve this question Follow edited Nov 3, 2017 at 18:14 Federico asked Nov 2, 2017 at 10:01 Federico's user avatar FedericoFederico 36511 gold badge33 silver badges1111 bronze badges $\endgroup$ 4 * $\begingroup$ The biggest difference in images like this comes from different exposure times. Since the brightness of galaxies falls off with the distance from the center, longer exposure times capture an increasingly larger part of the galaxies' light. That is, the longer you expose, the larger the galaxies look. $\ endgroup$ - pela Nov 2, 2017 at 13:58 * $\begingroup$ When it's dark enough to see M31 with a naked eyeball, it looks lie a fuzzy patch about as wide as the moon in its longest dimension. These pics with sensitive digital cameras or telescopes make it appear larger, by bring out the fainter stars around the galaxy's edges. That pic looks like a high res image of M31, imposed on a near twilight lunar sky. If real, you'd see a lot more stars from within the Milky way. $\endgroup$ - Wayfaring Stranger Nov 2, 2017 at 14:53 * $\begingroup$ I wish I could figure out where this got linked recently $\endgroup$ - Federico 16 hours ago * 1 $\begingroup$ @Federico in case you're still wondering: news.ycombinator.com/item?id=33281682 $\endgroup$ - Eduardo Ivanec 14 hours ago Add a comment | 1 Answer 1 Sorted by: Reset to default [Highest score (default) ] 33 $\begingroup$ The Andromeda Galaxy's angular size is about 178x63 arc-minutes and the Moon is about 31 arc-minutes wide. Relatively speaking, the Andromeda Galaxy's apparent size is nearly 6x2 Moons. So from the photo, their relative sizes in the sky appears to be very roughly accurate. The galaxy appears to have been superimposed from another image just to show what the galaxy might look like in the sky if it were brighter. Share Improve this answer Follow edited Nov 2, 2017 at 11:43 answered Nov 2, 2017 at 11:17 user10106user10106 $\endgroup$ 3 * $\begingroup$ Ok, I see. (I don't fully agree on the accurateness though. Roughly looking at the pixels, the image depicts Andromeda with ~319 pixels on the major axis, and the moon with a diameter of ~40 pixels, that's roughly 8 times, vs the actual 6 you mention. $\endgroup$ - Federico Nov 2, 2017 at 11:27 * 3 $\begingroup$ @Federico I find, like you, that in that image, Andromeda is about 8 times wider than the moon. However, you have to take into account that the angular size of Andromeda is not well defined because there is no clear cutoff for where Andromeda ends. If you look at wikipedia, you'll find a quoted size of 3.167 x 1 degree which is slightly wider than Kozaky's source. I'll admit that Andromeda seems to be slightly too big, but it's still mostly accurate. Note the moon also changes size up to 14% as it orbits. $\endgroup$ - zephyr Nov 2, 2017 at 15:30 * $\begingroup$ Now that is interesting. I always knew the Andromeda galaxy was big but dim, but I didn't appreciate just how big. $\endgroup$ - John Duffield Nov 4, 2017 at 15:33 Add a comment | You must log in to answer this question. Not the answer you're looking for? Browse other questions tagged * the-moon * galaxy . * Featured on Meta * The 2022 Community-a-thon has begun! * Mobile app infrastructure being decommissioned * Do we want to change the link about MathJax help? Related 2 Is this a real picture of the moon? 10 face-on galaxy and edge-on galaxy 8 How accurate and complete is the Hubble sequence diagram? 3 Why is this moon red? 2 How to understand this multi-day exposure photograph of Jupiter and the Moon 4 How accurate should sun\moon ephemeris be for calculating eclipses? 4 Why were two galaxies named "The Sagittarius Galaxy"? 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