Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Basics of computing sunrise/sunset
    From: Marcel Tschudin
    Date: 2009 Jun 27, 15:48 +0300

    OK, I give it a try.
    Frank questioned whether his hypothetical model of a series of balloon
    soundings along the line of sight would allow an improved calculation
    of the refraction near the horizon and Bill B. questions whether this
    would help with thermal inversion/abnormal lapse rates causing
    refraction anomalies at my visible horizon?
    First I should mention that the abnormal refraction phenomena depend
    on the viewing angle; they appear near the astronomical horizon, i.e.
    when looking horizontally. If the position of the eyes is considerably
    above or below the the large temperature gradient they may - depending
    on the height difference - actually not even notice that there exists
    somewhere an abnormal layer, this because the angle of incidence is
    too large. It is thus important from where I observe e.g. the sun near
    the horizon.
    If the observation is done from a boat with the eyes a few meters
    above the sea, the apparent horizon is fairly close to the
    astronomical horizon; the dip can therefore be subjected to anomalous
    refraction phenomena but the sun still well above the horizon not. The
    measured angular distance between sun and horizon is erroneous because
    of the wrong position of the horizon (dip) not because of the sun's
    position. If, in the other case, the sunset is observed from a high
    point above the sea the viewing angle of the horizon is already too
    large (too distant from the astronomical horizon) to be disturbed by
    anomalous atmospheric layers. If in this case the sun is near the
    astronomical horizon and the position of the eyes close to e.g. an
    inversion the image of the sun is "disturbed". The wrong result
    obtained from measuring the distance between sun and horizon is in
    this case due to the wrong position of the sun and not the position of
    the dip. If both, sun and apparent horizon, are close to the
    astronomical horizon both can also be affected by anomalous
    atmospheric layers.
    A word on weather balloons which measure pressure, temperature and
    relative humidity of the atmosphere. These measurements are only
    available at certain selected heights, which, depending on the
    situation, may have been selected too coarse to catch certain
    anomalous features. Imagine a "S"-shaped vertical temperature profile
    which may eventually only have been measured with two arbitrary
    selected data points; depending where those two data points are one
    may obtain "any" temperature gradient. I also intend to use weather
    balloon data, but from those I try to obtain from statistical analyses
    "typical" values for a certain location at a given local time for the
    various height layers of the atmosphere.
    If you are more interested in refraction: have a look at the over 100
    Web-pages which Andrew T. Young dedicated to this subject. There are
    two entries into this labyrinth, the alphabetic one
    and the tables of content
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site