It may be obvious and this may be a bit pedantic, but the “actual size” and “size compared to the moon” part is confusing, as comparing the size of two objects is usually comparing two objects at the same scale.
If someone was completely clueless, they would think Andromeda is a teeny tiny galaxy. But it’s actually 110,000 light years wide — quite a bit bigger than the moon. :)
The title show just be something along the lines of “Andromeda in the sky if it were brighter, next to the moon”. Or just “Andromeda if it were brighter”.
I say this because I’ve seen other “X compared to Y” articles on HN (one compared planets and the sun to the moon, where the moon was a pixel), and these are all at-scale comparisons. So I was expecting something similar here from the title.
Andromeda Galaxy is visible at its actual brightness, but it appears much smaller than in that image because only the central area is bright enough to be seen. Thus the "actual size" part. I don't think anyone was going to demand a 100,000 light-year wide image based on that phrasing.
> as comparing the size of two objects is usually comparing two objects at the same scale.
It is actually quite common to hear that something in the night sky is xFullMoons distant/apart. Of course, that is a relative measurement of area of sky covered from a human's viewpoint while standing on terra firma. It's just a size/distance that is understandable by most people. Telling someone that one object is 3° from another object means nothing. Knowing that the width of the moon is roughly 0.5° means that it is the distance of 6 full moons is more relatable. So in this case, the width of the full moon is the scale
Slight tangent, there are other measurements that make things easier to navigate[0]. The width of your index finger ~1°, the width of 3 fingers ~5°, the full fist ~10°, the width of thumb/pinky fully extended ~25°. Knowing these helps find other objects.
It's even bigger than that: the diameter of the main galactic disk of Andromeda is 152 000 light years. There are additional stars orbiting the galaxy in a halo with a total diameter 220 000 light years.
In comparison, Earth's moon is very much smaller, with a diameter of less than 1 light year.
For some reason I really enjoy the way you describe the moon as having "a diameter of less than 1 light year.", which is also true of my cat and the shoe I am wearing. And everything else I have ever seen or will experience in my life.
Yeah, I realized that after it was too late to edit. I posted that after a quick Google search — I originally had no idea how big it was, but new it was more the 2x the moon :) — and of course the big “110,000 light years” it returned at the top was the radius, not the diameter.
You'd have no trouble understanding that andromeda is a large number of magnitudes bigger than the moon even if your entire astronomical education was from that Star Wars intro scroller. It's a total non-issue.
What I find surprising (really unexpected!) is how close this
"appears bigger than the moon" observation makes our neighbor galaxy: considerably less than 100 "galaxy diameters"! My intuitive understanding of the vastness of space would have expected far more emptiness. (objectively, that's certainly more than made up by the insane amounts of emptiness we already have within each galaxy, but still, so close!)
If someone was completely clueless, they would think Andromeda is a teeny tiny galaxy. But it’s actually 110,000 light years wide — quite a bit bigger than the moon. :)
The title show just be something along the lines of “Andromeda in the sky if it were brighter, next to the moon”. Or just “Andromeda if it were brighter”.
I say this because I’ve seen other “X compared to Y” articles on HN (one compared planets and the sun to the moon, where the moon was a pixel), and these are all at-scale comparisons. So I was expecting something similar here from the title.