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    Re: Refraction and humidity
    From: Alexandre Eremenko
    Date: 2007 Mar 28, 09:29 -0400

    The most advanced text on astronomical refraction
    I have is the chapter of Chauvenet where he explains
    the work of Bessel, and attaches tables computed by Bessel's
    formula with refraction in fractions of a SECOND.
    Bessel's formula does not take into account color or
    humidity. Chauvenet says that Bessel's tables give
    satisfactory results, and he does not mean observations
    with a sextant to a fraction of a minute, but
    he means real astronomical observations with instruments
    of high precision.
    That's why I assume that color and humidity can be safely neglected
    in the type of observations we do, as well as temperature and
    pressure in most cases.
    On Tue, 27 Mar 2007, Bill wrote:
    > Alex
    > Following up on Sunday's conversation on refraction for star-to-star angular
    > separation, the almanac states mean refraction is calculated for a
    > temperature of 10C, pressure of 1010 mb, humidity of 80% and wavelength of
    > 0.50169um.
    > That would indicate my recollection that humidity is a component of
    > refraction is correct, and my belief that the color of a star/body may
    > affect refraction.  I do not know if dust in the air will affect refraction
    > like humidity.
    > The important point in context is how much refraction will be affected,
    > especially for lower bodies (20d to 30d).  I don't have a clue.  Perhaps the
    > list can be of help.
    > Bill
    > >
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