# NavList:

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

**Zn determination and proper dip short distance**

**From:**Steve E. Bryant

**Date:**2016 Mar 14, 16:44 -0700

Dear NavListers,

Is there some means where by one can determine Zn of the Sun, at sun set, given only the date and the latitude/longitude of position. I assume that that Zn could be determined by use of a planisphere or the 2102-D star finder but (if it is possible using the planisphere) it has been such a while since I used mine last, I’d likely need to burn the midnight oil to figure out how to use it again. I am willing to do that; but, I want feedback first.

I may have opportunity to take sights from the deck of a boat later this summer and I know from which mooring I would like to do that. I also know that the Western horizon lies unobstructed between two islands, one to the north and the other to the south of my line of sight. I’m guessing that the arc of clear horizon scribed between the tips of the islands, as seen from the mooring, is close to 5 degrees.

I’d like to be able to determine the Zn of the position of sun at the moment of set, on any given day, with respect to that mooring ball.

I am also going to assume that the suggested solutions to this question will be implicit in a similar question regarding sun rise; please advise.

Best regards,

Steve

PS-[As I am here now formulating this question, I have come to realize that to capture a sight of the sun at a useful altitude, a clear and unobstructed view to the horizon will not be possible from that mooring ball as the sun will still be at a declination south of the mooring ball as it makes its way from east to west. Also please note that there will be an island obscuring the distant/unobstructed horizon when a sight of the sun can be taken at an altitude greater than 15 degrees. Of course, this should not be an insurmountable problem by use of the dip short. It does, however, bring to mind another question to wit:

Given that the distance to the horizon in statute miles is equal to the square root of the height (11 feet in this case) of eye in feet times 1.144 and that expression will all together equal 3.8 statute miles, if the actual distance to the shore against which a sight is taken is actually six miles away, then the only value to use in the calculation of the dip short correction is 3.8 statute nautical miles; is that correct?]

So my questions are these:

1. How to calculate Zn given only a time, a date, and a lat/lon ... and a nautical almanac?

2. What is a proper dip short correction to use?

From the outset, I thought this question was going to short and straight forward. Nevertheless, I'm certain your responses will help to clarify and solidify the vague understanding I have to these processes.

I am looking forward to the answer(s).

Thank you,

Steve