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From: Bob Young <YoungBob2@aol.com>
Date: Tue, 22 Sep 1998 12:33:22 EDT
Fwd Date: Tue, 22 Sep 1998 17:03:22 -0400
Subject: Re: A Research Question


>From: Bruce Maccabee <brumac@compuserve.com>
>Date: Sat, 19 Sep 1998 20:08:16 -0400
>Fwd Date: Sun, 20 Sep 1998 09:44:12 -0400
>Subject: Re:  A Research Question

>>From: Scott Carr <sardonica@erols.com>
>>To: UFO UpDates - Toronto <updates@globalserve.net>
>>Subject: A Research Question
>>Date: Wed, 16 Sep 1998 16:14:59 -0400

>>Hello List->

>>I wonder if anyone might help me out with a bit of information
>>for a UFO piece I am working on. Does anyone know from how many
>>miles away you can (easily, with the naked eye) see the glow of
>>Las Vegas on the horizon?

<snip>

>>I would actually like to know the maximum distance that the Las
>>Vegas glow can be seen, which I suppose would be on the
>>clearest of nights.

>Since this is supposedly a naked eye observation during clear
>sky conditions I'll hazard a guess that 50 miles would be an
>upper limit.

>However, this is based on "easy" to see as oppesed to a barely
>visible glow. That is, sensitive, dark adapted eyes might detect
>a glow farther away.

>On the other hand, it might be that 50 miles is an upper bound
>for easy detection and 25 miles could be a better estimate. But
>I do think it would be visible 25 miles away as a glow on the
>horizon.

>Of course, I could be wrong!  Such a thing would, in principle,
>be possible to calculate.... but much easier to "measure."

Hello, Bruce Maccabbee, Scott Carr and List:

A formula used by astronomers to estimate urban sky glow is
called "Walkers Law". It can be used to estimate the sky glow at
an observing site, looking at an angle 45 degrees from overhead,
toward an urban source d kilometers away.

The formula is

I = 0.01Pd(superscript-2.5)

where I is the increase in sky glow level above the natural
background and P is the population of the city and d is the
distance to the center of the city in km.

More information can be found at the following site of the
International Dark Sky Association, which fights "light
pollution":   http://www.darksky.org/ida/ida_2/info11.html

Using an estimate of 365,000 as the population of Las Vegas,
(this 1995 figure must now be higher), and the fact that the
above formula gives underestimates for very large urban areas,
and that the intense outdoor illumination used at Las Vegas is
certainly atypical, one can come up with a _minimum_ figure of
50 miles as the distance at which the city's lights would be
visible as a glow 10% above the natural sky brightness level.
The greatest distance at which a glow from Vegas can be detected
is very likely much higher.

May you have clear, dark skies,

Bob Young