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Re: [OM] Terrestrial vs. astronomical infinity

Subject: Re: [OM] Terrestrial vs. astronomical infinity
From: "R. Lee Hawkins" <lhawkins@xxxxxxxxxxxxxxxxxxx>
Date: Mon, 21 Sep 1998 19:00:06 -0400
Cc: lhawkins@xxxxxxxxxxxxxxxxxxx
In your message dated: Mon, 21 Sep 1998 22:43:09 +0200 you write:
>
>To infinity I found in an other book some interesting remarks: If the
>stars would be realy "infinty" we couldn´t focus on them and see them,
>because the result where points of no extension. Actualy "infinity" is

This author obviously doesn't understand diffraction.  The stars *are*
point sources for our eyes.  However, any "point source" has a point
spread function due to the aperture of the detector (our eyes, about 7mm
when dark adapted) and (for those of us not on the Shuttle) the
distortions of the atmosphere.

>where our eyes could not decipher any diference between two points of
>different distance from us (like DOF), just a question of resolution.

Which is exactly what the definition of a "point source" is.  The
closest star (other than the Sun) to the Earth is Proxima Centauri.  At
its distance of some 4 light years, our eyes can't discern the
difference from a point of light on one side of the star from the other.
Thuse that star (and all others, since all others are farther from us)
look like point sources.  And since they look like point sources, they
are for all intents and purposes at infinity.

Cheers,
--Lee

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