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    Re: Approaching A Destination by Latitude Line Sailing
    From: Brad Morris
    Date: 2014 Apr 5, 11:57 -0400

    Hello Gary

    I was thinking of the approach to a larger land mass.  For example, sailing to NYC from England.  Its hard to miss the continent, but its easy to end up either to the north or south of NYC.

    The approach you detailed is entirely correct for a small island. 

    Brad

    On Apr 4, 2014 10:52 PM, "Gary LaPook" <garylapook---.net> wrote:

    Unless you are looking for an island then your latitude must be precise enough to bring the island into sight. Traditional "latitude sailing" had you aiming to the east or west of the island, upwind, using DR for longitude and then take noon sights as you sailed due east or west taking a noon sight every day and heaving to if necessary for a day due to clouds at noon. If your ship is covering a 100 NM per day your latitude DR error in a day should be less than 10 NM so should be accurate enough to find any but the lowest lying island.

    The same technique was adapted for inflight celestial navigation in which you don't have the luxury of heaving to for a day. See my website:





    gl


    From: Brad Morris <Bradley.R.Morris---.com>
    To: garylapook---.net
    Sent: Friday, April 4, 2014 3:31 PM
    Subject: [NavList] Approaching A Destination by Latitude Line Sailing


    In a previous post, the practical nature of approaching a destination via a latitude line was considered.
    This is my understanding.
    Let us assume that the destination is precisely at N40 deg 0 min of latitude. The clever navigator will choose a latitude line just to one side of this for the following basic reason. If you sail on precisely N40 deg 0 min (or so you think, because you have been using DR for days) what happens when you hit land and the destination is not in sight. Is the actual destination to the north of you OR to the south? This is an unresolvable uncertainty. You may as well flip a coin.
    By sailing to one side of the destination latitude, say N40 deg 15 min, when land is sighted, you KNOW that you must turn to the south and sail for a few miles until the destination is observed. No need to investigate the northerly coast, the destination is clearly to the south by design.
    In modern day GPS navigation, you sail directly towards the destination because there is no uncertainty, and the only DR performed is when you aren't observing the GPS. But of course, this is a celestial navigation group, so we cannot use principles of GPS navigation.
    Brad
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