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RE: [OM] Get your umbrellas ready....

Subject: RE: [OM] Get your umbrellas ready....
From: "Daniel" <danrich@xxxxxxxxxxxxx>
Date: Mon, 4 Aug 2003 20:00:23 -1000
Thanks John
I've been into astronomy for years and never took the time to understand
this.
After I got the Celestron NX-11 with GPS I got lazy, letting the scope
finds it's was around.
Mars really stands out here in the middle of the Pacific Ocean (Maui).
Look forward in taking some picture of the war planet tonight with a
Tamron 400mm + a Zuiko 2X for a power of 16X, I'll try a 1 sec to 4 and
see what I get.
Daniel

-----Original Message-----
From: owner-olympus@xxxxxxxxxxxxxxx
[mailto:owner-olympus@xxxxxxxxxxxxxxx] On Behalf Of John A. Lind
Sent: Monday, August 04, 2003 7:29 PM
To: olympus@xxxxxxxxxxxxxxx
Subject: RE: [OM] Get your umbrellas ready....

At 11:49 PM 8/4/03, you wrote:
>OK you did your cut and paste but what the hell is
>magnitude of -2.9 and will appear 25.11 arc seconds wide?
>Daniel

Magnitude is the brightness of a celestial object.  It is based on star 
brightness.  Counter-intuitive, the lower the magnitude number the
brighter 
the object.  The brightest stars are magnitude -1.  It is 2.512 time 
brighter than a magnitude 0 star.  Thus, a magnitude of -2.9 means it
will 
be roughly six times brighter than the brightest star (-1 magnitude).

The "magnitude" system dates to Hipparcus and Ptolemy when they assigned

magnitude numbers of 1 through 6 to the stars they could see with the 
unaided eye.  They assigned first magnitude to a set of about 20 of the 
brightest stars they could see from their location.  Sixth magnitude
stars 
were just barely visible to the unaided eye under the most favorable 
conditions (very dark sky).  Today the magnitude system assigns -1 to
the 
brightest star(s).

An arc-second is an angular measure.  There are 360 degrees to a circle,
60 
minutes to a degree, and 60 seconds to a minute.  To distinguish the use
of 
minutes and seconds of angular measure from time measure, they are
referred 
to as arc-seconds and arc-minutes.  25.11 arc-seconds is 0.006975 
degrees.  Angular measure is used to describe the diameter of a
celestial 
object (typically planet or planetary moon) as viewed from Earth.  That
and 
its distance can be used to determine its absolute diameter using simple

trigonometry.  The utility of describing size by angular measure is in 
informing someone who wishes to observe the object how much
magnification 
will be required (if any) to make it visible, and how much magnification

will be required to make it appear a specific size.  If only provided
with 
absolute diameter (e.g. in miles), one would have to know its distance
to 
calculate how big it will appear.  It's much easier with direct
information 
about how big it appears to an Earth observer without regard to 
distance.  Furthermore, in very early astronomy it was much easier and
more 
immediate to measure size through a calibrated telescope using a reticle
in 
terms of angular measure (and still is).  Measuring distance of an
object 
requires several measures of object location against the most distant 
starfield over time as the Earth orbits the sun (e.g. one at the Vernal 
Equinox and another at the Autumnal).

-- John


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