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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
    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.
    >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)
    >> 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@huxtable.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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