FreeLists / az-observing / [AZ-Observing] Re: R: 2008 All Arizona Messier Marathon - results

[Jeff Hopkins <phxjeff@xxxxxxxxxxx>]

As I said, the 1/cos was an approximation and indeed the closer to 90 
degrees the more inaccurate, but as you say even at half that value 
it is extremely large.

Tom, try following a 9th magnitude star to the horizon and see how 
far you can still see it even with a 10" scope. I suspect if you look 
away for a moment and then try to find the star even a few degrees 
above the horizon you will fail.

Jeff

At 10:39 -0700 04/11/2008, Tom Polakis wrote:
>Did Lorenzo really view M77 at astronomical twilight, when its 
>altitude was only 1 degree, or did he pick it up some minutes before 
>then?  When several of us spotted M74 on March 29, 2003, the time 
>was about midway between nautical and astronomical twilight.  The 
>sun was around 15 degrees below the horizon.
>
>Jeff, since the Earth is not flat, the 1/cos(zenith angle) 
>approximation for airmass breaks down at low altitudes.  Check out 
>the plot in this Wikipedia article.
>
>http://en.wikipedia.org/wiki/Airmass
>
>The plane-parallel curve does indeed go to  57 airmasses at 1 
>degree, but a more realistic model would give 26 airmasses -- still 
>a huge number.  My guess is that Lorenzo found M77 when it was a few 
>degrees higher, and it was extinguished by 10 or 15 airmasses (only 
>3 or 4 magnitudes).  Since M77 is a Seyfert galaxy with a very 
>bright core, perhaps he saw only the core.  That would count!
>
>Tom
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-- 
Jeff Hopkins
HPO SOFT
Counting Photons
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Hopkins Phoenix Observatory
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Phoenix, Arizona 85033-2439 U.S.A.
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