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    Re: Pilot avoids collision with Venus
    From: Gary LaPook
    Date: 2012 Apr 30, 14:50 -0700
    A better way to look at it is to determine the dip of the horizon as seen from 35,000 feet which is 3° 01' meaning the visible horizon is that far below the true horizontal. The altitude of Venus at the position and time you used was 2° 41.7'. so, if the pilots were judging the altitude of Venus by reference to the visible horizon (as when using a marine sextant) they would have measured an altitude of 5° 42.7' above the visible horizon. . But the sun would not be rising until almost 1100 Z so it was pitch black and the horizon would not have been visible, and the moon was not up either.

    But, when you fly jets you get used to the visible horizon not representing the horizontal. At low altitudes you use the visible horizon to position the nose of the plane but at high altitude (or in clouds) you must adjust the pitch attitude of the plane by reference to the attitude indicator or flight director and, in cruise at high altitude you usually have the nose about three degrees to five degrees (depends on weight and airspeed, we call this deck angle) above the horizontal, haven't you ever noticed that it is an uphill walk back to your seat from the lav in the tail? So, if the copliot momentarilly forgot about the deck angle and if he noticed Venus on the same spot on the windshield where the horizon would appear to be at lower altitudes, then Venus would appear to him to be at their same altitude. This phenomenon is well known, planes that appear to be at your altitude pass above you, and you have guard against this visual illusion.

    gl

    --- On Sun, 4/29/12, Michael Meredith <kc8ttv@gmail.com> wrote:

    From: Michael Meredith <kc8ttv@gmail.com>
    Subject: [NavList] Re: Pilot avoids collision with Venus
    To: NavList@fer3.com
    Date: Sunday, April 29, 2012, 7:22 PM

    Nice! I'm envious.

    I'm not a navigator, just someone curious and fascinated by the art and science. Thinking about the Air Canada flight again, I plugged their approximate position and time (N55W029/0700Z) into my astronomy software to get an idea of what they were looking at through the window. I'm trying to comprehend how their altitude affected the position of Venus in their field of view. To an observer on the ground at that location it looks like Venus was about 2.5° above the horizon. At 35,000' the horizon is roughly 220 nm distant. Projecting Air Canada's position 220 nm in the direction of Venus (Az 128°) takes us to approximately N52° 41' W024° 15'. At that location and time, Venus was several degrees higher. Would Air Canada's view of the sky be the same as someone's on the ground at N52° 41' W024° 15', at the same time?

    Attached are a couple of screen shots from the software I'm using.
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    Linked File: https://www.NavList.net/imgx/ACA878-N55W029.bmp
    Linked File: https://www.NavList.net/imgx/ACA878-ground.bmp

       
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