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    Seeing stars and planets in the daytime.
    From: George Huxtable
    Date: 2004 May 29, 00:19 +0100

    In recent discussions on the brightness of images seen in a telescope, we
    concluded that a telescope did not increase the brightness of extended
    objects seen in its field of view: it just made the object bigger, not
    brighter. That applied to picking out objects in a dark anchorage, and also
    to the view of the Sun.
    
    However, it does NOT apply to the view of a star. Because any star, viewed
    in any telescope, is no more than an infinitesimal point of light, then the
    extra light collected by using a telescope with a large objective produces
    a correspondingly brighter pinpoint in the field of view. But the
    brightness of the sky background is not enhanced in the same way. That's
    why a telescope allows faint stars to be picked up earlier, after sunset.
    
    In Maskelyne's day at the Greenwich observatory, a telescope was locked
    permanently in such a direction that Sirius passed its crosswires every
    sidereal day. For half the year, this would happen at night, but for the
    other half, it would be daylight. Nevertheless, Sirius gave a precise
    time-signal, day or night, as long as it wasn't obscured by cloud. This was
    the signal by which they kept tabs on the going of the observatory's
    master-clocks.
    
    With planets, the matter is less clear-cut. Once a planet's image has been
    magnified up enough so that it appears to the eye as a disc, rather than as
    a point of light, then further magnification will make it a bigger disc,
    but not a brighter one. Nevertheless, enlarging that disc will no doubt
    make the planet more visible, against background light in the sky.
    
    George.
    
    
    >Navigators customarily cross Venus with the Sun or Moon during daylight
    >hours when above the horizon. Pre-comutation of altitude and azimuth
    >makes this an extremely easy sight with no technical jargon necessary -
    >even the lowest power telescope is usually adequate. As I have before
    >posted, good star/planet sights require pre computation of altitude and
    >azimuth which generally allows observations of the brighter stars and
    >planets within 5 to 10 minutes before/after sunrise/sunset, depending on
    >Latitude. You will never be successful in obtaining star fixes if you
    >wait to see the body with the naked eye - this of course include Polaris
    >which is also an extremely easy sight in the higher northern Latitudes,
    >although no longer as important as it was in bygone day. The important
    >point is taking these (or for that matter any) sights on the beat horizon
    >possible and as twilight advances there is usually a rapid deterioration
    >in the distinctness thereof.
    >
    >On Fri, 28 May 2004 08:55:36 +0000 "Trevor J. Kenchington"
    > writes:
    >> Doug Royer, extending standard technique from faint stars to the
    >> brightest of planets, wrote:
    >>
    >> > Try this trick also on Venus as one may view Venus in early C.T.
    >> when
    >> > conditions are right.
    >>
    >> If you know just where to look, Venus can be visible to the naked
    >> eye in
    >> full daylight. I have only tried finding it the once but, that time,
    >> I
    >> did find it. Needed some concentration to keep it in sight though,
    >> since
    >> you have to look in exactly the right place or it disappears. (Maybe
    >> it
    >> needs the higher resolution of your retina's fovea to be able to
    >> distinguish the small patch of brighter light from the general
    >> brightness of a blue sky.)
    >>
    >> Why can't we use Venus to get a day-time position line? I'm guessing
    >> that a sextant telescope doesn't help because its light-gathering
    >> power
    >> brightens the blue sky as much as the planet, meaning that it is no
    >> easier to hold the image with a sextant than to view Venus with a
    >> naked
    >> eye, hence making the observation impractical.
    >>
    >>
    >> Trevor Kenchington
    >>
    >>
    >> --
    >> Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    >> Gadus Associates,                                 Office(902)
    >> 889-9250
    >> R.R.#1, Musquodoboit Harbour,                     Fax   (902)
    >> 889-9251
    >> Nova Scotia  B0J 2L0, CANADA                      Home  (902)
    >> 889-3555
    >>
    >>                      Science Serving the Fisheries
    >>                       http://home.istar.ca/~gadus
    >>
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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