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    Re: The Orion North Arrow
    From: David Pike
    Date: 2017 Feb 12, 03:02 -0800

    I’m afraid I’ve changed my mind yet again.  I now believe the usefulness of the Orion Arrow for field navigation diminishes the further north you happen to be.  I was going to paint a perfectly good globe matt white and draw on it to prove it, but decided to try to draw the situation in a two view drawing instead (see below).  To keep things simple, forget the eight degree misalignment and assume Mintaka has declination zero.

    The Arrow always points to the North Pole, true, but the projection of its meridian onto the observers northern horizon moves from west of north to east of north throughout the evening.  Looking at the lower projection, a plan view of the observers horizon, imagine that the North pole can slide horizontally towards or away from the observer and the Arrow’s celestial meridian shown once just after rising in the  east and again before setting in the west is a leg with a slot along it.   With the observer at the Equator the Pole is on the northern horizon, so there is no movement of the Arrow’s meridian along the observer’s northern horizon throughout the night.  However, as the Pole moves towards the observer the leverage alters and the movement of the Arrow’s meridian on the observers northern horizon increases the further north the observer is situated. 

    So yes the Arrow always points to the North Pole, but its usefulness for navigation in the field at night diminishes the further north you happen to be.  DaveP


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