# NavList:

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

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Re: shooting indoors
From: Greg Rudzinski
Date: 2010 Mar 24, 14:54 -0700

Scott,

If your basement window is less than 6 feet above lake level then you are good to go without a dip short correction ;-) Otherwise try the following formula:

D= .416d + .566 h/d
D= dip short in minutes of arc (subtracted from sextant altitude Hs)
d= distance to horizon in nautical miles
h= height of eye above lake in feet

Example for a 30 foot height of eye with a 2.75 nautical mile distance to the horizon:

D= (.416)(2.75) + (.566)(30)/(2.75) = 1.144 + 6.18 = 7.3' (minutes of arc to be subtracted from Hs)

You won't need to worry about sextant parallax with a 2.75 nautical mile horizon.

One more suggestion would be to use exactly the same section of window for all observations so any refraction errors from the window will be consistent. A window corner would flex less than the window center.

Now send us some results :-)

GRudzinski
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