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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » CN II/Glossary/Dip

Author Topic: CN II/Glossary/Dip

 - posted February 06, 2019 10:35 AM      Profile for RWJ           Edit/Delete Post 
In the Glossary of your Celestial Navigation text (a fine book) I note the definition of Dip contains the statement: " The Dip correction expressed in miles is numerically equal to the distance from the observer to the last visible point on the horizon". As you state, the Dip correction = -0.97 x sq rt of ht of eye in ft. However, the Dist to Horizon formula in Bowditch is 1.17 x sq rt of ht of eye in ft. Am I missing something?
From: New Jersey
David Burch

 - posted February 06, 2019 11:17 AM      Profile for David Burch           Edit/Delete Post 
Thanks for pointing this out. We need to clarify the dip definition. The first part of the glossary definition is correct, but the last sentence should change "numerically" to "approximately." We have now made that change to the online glossary.

The dip formula (0.97) is pure geometry. The Bowditch distance to the visible horizon (1.17) includes refraction.

The dip factor should be precise for a spherical earth; the Bowditch distance factor is uncertain. It depends on temp, pressure, and wind, and sea state. Earlier Bowditch used 1.14. Then they switched sometime after the 1977 edition.

In our coastal nav course we approximate it with 1.0, as the slightest chop affects this.

From: Starpath, Seattle, WA

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