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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@iStar.ca
> Gadus Associates,                                 Office(902)
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>

   

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