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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@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
>>

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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
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