# NavList:

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

**Re: Was Bowditch Table 15, now confused**

**From:**Bill B

**Date:**2005 Jan 28, 23:09 -0500

OK. Then my understanding of the explanations/graphic representation is of sensible horizon is in sync. >>> As it happens, measurements >>> using a pool of oil or mercury are relative to the Sensible >>> Horizon. The "relative to sensible horizon" portion is where I begin to lose it. Relative is, well, relative. Observations of a body from plate of mercury/oil at sea level, on deck, on the ground at 700 ft above sea level, or on a stool at 703 ft above sea level will give the same results (noting the approx. 700 feet is insignificant relative to the radius of the Earth and resolution of the system). Correct? Stated in my words (don't you just love active listening) if a pool of mercury/oil is 6 feet above water level, the plane it describes is the sensible horizon--same as an eye at 6 ft. Now the wrinkle. If an eye is 6 ft above sea level, I apply a dip correction as I can see farther over the horizon--put another way the observed object appears higher than it would from sea level. Yet I do not apply dip just because the a blue water tide is up 2 feet--the observed horizon went up along with my eye level (noting again the 2 feet is insignificant relative to the radius of the Earth and resolution of the system). When using an artificial horizon the liquid is my sea level and my horizon (observed angle is 2X when the observed body is factored in) so I do NOT apply dip. Elevation of the pool of liquid and actual sea level is insignificant for practical purposes as noted above. Which brings us to a bubble level fitted to a sextant. If I calibrate it so the bubble is centered while a natural sea horizon is aligned on both sides of the horizon mirror at almanac STP (discounting index error) while it is 6 ft. above the water level, I would have to take dip into account in subsequent observations when using it on land. Yes no? Mostly correct? Thanks Bill > How about this. > > Stand and look out to sea, but look above the sea/sky interface, on a line > perpendicular to the line that your body makes from feet to head: you are > looking out along the imaginary sensible horizon. Then "dip" your eyes down > to the sea/sky interface, which is the visible horizon. The angle traversed > by your eyes as they dip is the "Dip". > > The "horizon" that we are intuitively used to calling a horizon from > childhood (the sea/sky interface) is not part of the Horizon(tal) Coordinate > System, which is made up of imaginary lines. The sensible horizon is, > however, part of the Horizonal Coordinate System. The Horizontal Coordinate > System's horizons are perpendicular to a line from the center of the earth > through the point of the surface of the earth on which you are standing, to > a point on the celestial sphere above your head. That frame of reference > moves as you move. > > Since you can see only the visibile horizon, then you have to apply the Dip > and Refraction corrections to convert your sextant measurement to the > Horizontal Coordinate System's frame of reference, in order to reduce a > sight. > > The Horizontal Coordinate System connected to the Terrestrial and Celestial > Coordinate Systems, and hence to the data in the Nautical Alamanc. > >>> Perhaps a clearer way to put it is that the Sensible Horizon >>> is perpendicular to the vertical. As it happens, measurements >>> using a pool of oil or mercury are relative to the Sensible >>> Horizon. >> >> Not clear how to interpret that. >> >> Is the sensible horizon is a plane perpendicular to the vertical (line >> through center of the Earth and viewer), AT THE VIEWER'S HEIGHT >> OF EYE, as I >> understand Jim's drawing? >> >> General instructions for a pan of oil etc. instruct the user to >> place it on >> the ground. No dip correction. If it relates to the sensible horizon and >> my understanding of the definition is true, then dip correction would be >> required if the pan were placed on a stool? That doesn't seem >> right to me. >> >> I do clearly understand I am deeply confused. >> >> Bill >> >>