[Prev][Next][Index][Thread]

Re: (meteorobs) An aside remark



psmc@ixdot netcom.com wrote:
> 
> I beleive satellites are visible through out night.  Not all satellites mind
> you. The ones in near Earth orbit are only visible in a small window around dawn
> and dusk.  Given the proper altitude, satellites can be seen throughout the
> night.

Often yes, but not always ... even the moon gets eclipsed now and then
:-)

The satellites are also "visible" during the day if you "know" where to
look.  An aid is a telescope with a "narrow" field.  Most artificial
satellites, like stars and meteors, are point sources.  As the f number
increases, ie the ration of focal length to aperture diameter increases,
the apparent brightness of the background sky glow decreases.  The
brightness of the point source is only a function of aperture.  So if
you fix the aperture, the point source becomes increasing easy to see as
you increase the focal length.

A simple comparison is look at a star near dusk/dawn with your eyes,
7x50 binoculars, and an f15 2 inch refractor telescope.

So the only remaining question is when and how bright is the satellite. 
As most satellites are reflectors, the brightness is a function of their
orientation of their surfaces with the observer and the sun.  In the
case of certain satellites, such as Iridums, the satellite orientation
to the sun is tightly controlled, so given the observer location,
calculating the magnitude as a function of time for each satellite is
straight forward.

The cutoff at mag 0 for Iridium calculations is simply to restrict the
amount of computation, the satellite doesn't "magically" extinguish
below mag 0, ie for positive magnitudes.

Hope this helps.

Regards,
John
-- 
John Ohrt *** Toronto, ON, Canada *** mailto:johrt@ultidot net


References: