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    Re: Faint stars easier to find on the horizon first?
    From: Frank Reed CT
    Date: 2004 May 31, 22:19 EDT
    Trevor K wrote:
    "Why can't we use Venus to get a day-time position line? "

    We can, and it's a nice diversion and a good way to show off to your navigation buddies. Jupiter's frequently usable in daylight, too.

    And added:
    "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."

    No, a telescope helps quite a bit (probably because it's easier to see the contrast when the object has a larger apparent diameter). Venus is beautiful through a telescope in broad daylight. Back in the days when the state of the art in planetary studies was a hand drawing, astronomers generally prefered to study Venus in daylight. You can see Venus with a telescope in daylight right through inferior conjunction (not this year though!) when it becomes an amazingly thin crescent with an extent of more than 180 degrees due to refraction in the Venusian atmosphere. If you try this observation, you have to be VERY careful since a slight bump of the telescope can bring the Sun into the field of view.

    I remember an impressive sight back around 1990. There was a conjunction between Venus and Jupiter. It was easy to find them in daylight through a telescope, and once we knew where to look, both planets were plainly visible to the unaided eye. The Galilean moons of Jupiter were invisible at any magnification.

    One of the biggest difficulties with finding planets naked-eye in daylight is that it is really very hard to estimate angular altitudes. Even relatively experienced skywatchers will routinely under-estimate how high they have to look in the sky to see an object that they know from calculation is, say, 65 degrees high.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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