Figure: Astronomical seeing illustrated and explicated

Caption: "The twinkling (AKA scintillation) of the brightest star in the night sky Sirius (AKA α Canis Majoris, binary) (apparent magnitude in V band of −1.46), shortly before upper culmination on the celestial meridian at 20° above the southern horizon. In 29 seconds, Sirius moves 7.5 arcminutes from left to right." (Slightly edited.)

Click on the image and on the next image to see the silent 0:29 second film of the twinkling.

Further Explication:

1. You are NOT seeing the Airy diffraction pattern for the star because of turbulence in the Earth's atmosphere.

   The turbulent motions constantly distort the wavefronts of light or, from geometrical-optics perspective, the paths of the light rays.

   This distortion from the ideal Airy diffraction pattern is called twinkling (AKA scintillation).

   Twinkling is one of the 3 major factors governing astronomical seeing (or seeing for short).

   The other 2 major factors are cloud cover and light pollution.

   Seeing can be used as a synonym for twinkling if you know what you mean.

2. The long exposure image of a star is called the seeing disk and its angular diameter (defined as per Wikipedia: Astronomical Seeing: The full width at half maximum (FWHM) of the seeing disc) is a measure of the goodness of the seeing: the smaller, the better.

   This angular diameter is also approximately the angular resolution which for stars is roughly minimum angular separation for distinguishing two stars.

3. Great seeing is ∼ 0.4 arcseconds (abbreviated '') (which is available at high-altitude mountaintop observatories) and good seeing is ∼ 1 ''.

4. In cities, seeing is often much poorer (i.e., larger) than ∼ 1''.

   It has been claimed---by Diane Pypher Smith?---that ∼ 4'' seeing is available near the Las Vegas Strip. Yours truly will believe it when yours truly resolves Castor AB which currently have angular separation of 3.8''.

5. Because of seeing, the practical angular resolution of telescopes without special techniques is limited by the size of the seeing disk rather than Rayleigh criterion above really very modest primary diameters.

6. We can inverse the Rayleigh criterion to get a formula for the primary diameter (in inches) above which practical resolution (without special techniques) saturates at about the angular diameter of the seeing disk:

     D_in(saturation) = (4.952'')*[(λ_μ/0.5 μm)/θ_seeing_disk('')]  , 

   where D_in(saturation) is the saturation diameter in inches, λ_μ is wavelength in microns (μm), and θ_seeing_disk('') is the seeing disk diameter in arcseconds.

   This formula means you can make the resolution better by using a bigger primary, but only up primary diameter D_in(saturation).

   If θ_seeing_disk('') ≅ 1'', then any primary greater than 5 inches give no better resolution.

   If θ_seeing_disk('') ≅ 0.4'', then any primary greater than 12 inches give no better resolution.

7. As far as angular resolution goes (without special techniques), giant telescopes are no better than relatively small ones (e.g., the Celestron C8 telescope).

   However, light-gathering power grows as the square of the primary diameter, and so yes, the bigger the telescope, the better for going deep (i.e., for observing faint objects).

   That's why we keep making bigger telescopes (e.g., extremely large telescopes (ELTs) such as TMT, GMT, and EELT).

8. What about the human eye?

   Its angular resolution is typically about 1 arcminute (') = 60'', but some sharp-eyed people may be able to do better (see Wikipedia: Naked-eye astronomy).

   Taking the human pupil as the circular aperture, one can use the Dawes limit to estimate human eye angular resolution.

   There is considerable variation between individuals, but typically for scotopic vision the pupil diameter is in the range 4--9 mm ≅ 0.15--0.35 in (see Wikipedia: Pupil: Optic effects). With these values, the Dawes limit formula,

     θ_DL('') = (4.56'')/D_in  , 

   suggests that human eye angular resolution as small as 13'' is possible.

   However, the Dawes limit and the Rayleigh criterion are probably only part of the story in determining human eye angular resolution.

   It seems human eye angular resolution is the angular resolution limitation for naked-eye astronomy unless the seeing is awful: i.e., much greater than about 1''.

9. The seeing limit to angular resolution can be evaded with special techniques as aforementioned.

   These techniques include lucky imaging, speckle imaging, and, the creme de la creme, adaptive optics.

   None of these are currently available in UNLV's intro astronomy labs.

10. Another way to evade the seeing limit is to observe from space using space observatories (e.g., Hubble Space Telescope (HST)).

    Actually, adaptive optics have gone a long way to reduce the need for space astronomy, but there are still many observations that can only be done from space.

    There is no immediate replacement for the HST when it dies probably sometime after 2030 (see Wikipedia: Hubble Space Telescope: Orbital decay and controlled reentry).