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Interesting thing about apparent size...

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#1 ngc 9999

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Posted 25 December 2012 - 07:47 PM

In one occasion I was observing m57 with my gone garrett 11x56 and then I compared this object size with an 1 arcminute double star because I see that the quoted apparent size for m57 is 1 arcminute, and what I found was that M57 was smaller than the separation of Albireo which is 34 arcseconds.

Does this happened because of the resolving power that says that the eye resolves worse an extended object at night?

The same thing happened recently while observing m52, listed apparent size of 5 arcminutes, but when comparing with stars of the same width, m52 appeared smaller in size.

Is there a formula to know what is the smallest galaxy that can be detected at a given power binocular?

#2 GlennLeDrew

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Posted 26 December 2012 - 12:16 AM

Good observation! Indeed, less-bright objects appear smaller than they are. And the lower the surface brightness, and the lower the contrast, the more pronounced the effect. For example, from a really dark site M31 is a good 2.5 degrees in length. I *know* this beyond doubt, but I have to convince myself that it would take only two such galaxies, end to end, to fill the gap between the pointer stars in the Big Dipper's bowl.

As alluded to above, the minimum size detectable depends pretty sensitively on both object surface brightness and contrast. I recommend you peruse my Gallery (link below, in sig) and look for an illustration I cooked up (with a detailed 'user guide') about brightness and contrast in deep-sky observing. It should provide an understanding of at least the fundamental aspects, and perhaps answer specific questions. At least you'll develop a feel for the range of size required for detection as conditions vary.

#3 Jon Isaacs

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Posted 26 December 2012 - 07:44 AM

Indeed, less-bright objects appear smaller than they are.



Just to add to what Glenn has said:

One reason objects appear smaller than the listed sizes is that you are not seeing the entire object. Part of the object is too faint/too small to be seen in a particular instrument and may require larger aperture/greater magnification/darker skies/narrow band filter or even a photograph to be detected by the eye.

Another thing to be aware of is the listed sizes vary depending on the source. Sky Tools 3 lists M31 at 2.6 deg x 1.1 degree whereas Cartes du Ciel lists it at 3.1 deg x 1.0 degree..

Jon

#4 KennyJ

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Posted 26 December 2012 - 08:37 AM

When one considers how large the full moon or setting sun APPEAR to the naked eye on the horizon,then contemplate M31 having an apparent diameter some five or six times LARGER,it takes some believing,but it's true.

Kenny

#5 ngc 9999

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Posted 26 December 2012 - 08:40 PM

So it seems that the ''size illusion'' is applied for fainter objects because you are not seeing the faintest outer edge of the object.

#6 GlennLeDrew

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Posted 27 December 2012 - 10:54 PM

If even if you're seeing the 'full' extent, a faint object always appear smaller than it is. The bottom line us that whatever portion you do see, it seems smaller than reality.

Here's a different slant. Look at the Big Dipper's pointer stars. As honestly as you can, estimate how many full Moons, arrayed edge to edge, could fit between these two stars? in spite of knowing how many, I always have to convince myself of the number. This is not as severe a test of apparent size disparity, but I'd bet most will underestimate by a factor of two, if not more (especially with less experience.)

#7 Tony Flanders

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Posted 28 December 2012 - 06:45 AM

Here's a different slant. Look at the Big Dipper's pointer stars. As honestly as you can, estimate how many full Moons, arrayed edge to edge, could fit between these two stars? in spite of knowing how many, I always have to convince myself of the number.


Right, the Moon seems much bigger than it really is, because it's so bright.

An even more startling fact is that the Moon fits inside the "bowl" of the Pleaides. It's a tight fit, but it makes it, as this photograph demonstrates.

You can easily hide the Moon with the tip of your little finger held at arm's length. Try it!






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