# NavList:

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

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Re: I wish I had thought to bring 'that' along with me...
Date: 2017 Jan 25, 03:07 -0500
Frank

You wrote
take the drop time in seconds to the nearest tenth of a second and multiply by 3.9. This directly yields the dip in minutes of arc.

That statement is true for the range of HoE given in the Nautical Almanac.  Well done!

That statement is not precisely true for the range of HoE given in the Air Almanac.  It's close, but not precise.

Example:   The Air Almanac specifies that for HoE of 2551feet, the dip will be 50'.
T=(2*h/g)^.5  yields drop time in seconds where h is the height and g is gravity.
T=(2*2551feet/32feet/sec^2)^.5
T=12.626 seconds , round to nearest tenth, per instructions
=12.6 seconds
Dip=T*3.9=49.1'
Close, but not 50

Example: The Air Almanac specifies that for HoE of 395feet, the dip will be 20'.
T=4.968, round to 5.0 seconds
Dip = 19.5'
Close, but not 20

Gee, this sounds terribly nit picky....it wasn't meant that way.  I just ran the equations and documented the results.

+++++++++

Frank, you also wrote

Note that throwing things over the side may get you tarred and feathered, so get permission or be discreet.

Roger that. Shipping companies are going out of their way these days to be environmentally concious.  Don't be "that guy".  Particularly in port.  Even out to sea, this may be frowned upon.

+++++

Frank and Bruce both recommended taking pictures and scaling the HoE.

A container ship is going to change displacement dependent upon load.  So the content and number of containers is going to change the HoE.  Further, as diesel is burned, the HoE is going to rise, slowly but surely.

While feasible to grab that picture in port, there will be no way to do it underway.  Even in port will be hard, as the ship will be against the wharf during loading.  They aren't exactly going to let you wander around while they are loading containers. That would be ridiculously dangerous.  If it's possible to get around to the other side of the harbor, sure, but once loaded, that container ship wants to go!

It also has to be done at each port, for each change in containers.  It should be done periodically underway, as diesel fuel is burned.

This procedure may work for ferries and the Charles Morgan, but I am not so sure about its practicality for a container ship in busy ports.  And it will never work underway.

+++

David could, of course, pretend to be a the captain of a Soviet ballistic missile submarine, and directly measure the dip using a dip meter.  This eliminates all of the uncertainty.  If only Alex could be prevailed upon to loan him one of his dip meters!!

On Jan 24, 2017 11:20 PM, "Frank Reed" <NoReply_FrankReed@fer3.com> wrote:

And if you don't have any string, but you do have your digital camera (with a lanyard!), then drop a rock over the side. Drop it from your height of eye, and record the event from drop to splash with the video mode of your camera. Or if you have an audio recorder, you can time it by saying "drop... splash". It's also possible to time this with a common stop watch if you can read it to tenths of a second or with a stopwatch app on a smart device. In any case, take the drop time in seconds to the nearest tenth of a second and multiply by 3.9. This directly yields the dip in minutes of arc. The object you drop needs to be something small and dense... in other words, a rock or maybe a large bolt (don't use the one that holds the rudder on}. Otherwise, air resistance becomes significant. Note that throwing things over the side may get you tarred and feathered, so get permission or be discreet.

Yet another approach. Take a photo of the vessel from the pier before you depart. You can measure some convenient object at your leisure on board and that gives a scale for the photo. This is easy and effective.

A fail-safe technique: take a bunch of sights (that's the plan anyway), and treat dip as a systematic error. This is especially easy if you have the exact position of the vessel (GPS coordinates) during the run of sights, but it's not essential. There will be some dip value that minimizes the systematic error. Done.

Frank Reed

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