 # NavList:

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

Message:αβγ
Message:abc
 Add Images & Files Posting Code: Name: Email:
Re: now off topic of navigation
Date: 2013 Feb 8, 17:13 -0500
The problem statement is that the string length is 1 inch longer than the circumference of the earth.  That is 1577727361 inches (24901 miles * 63360 inches per mile)

In the attached spreadsheet, we see the angle, in yellow, at which the string departs from the surface of the earth and heads towards the pull point.  I calculate the arc length by (excluded angle)/360 * circumference.  Then I calculate the length of the two lines which are off of the surface, sqrt of the sum of the squares.  Since we have a fixed goal of 1577727361 inches, by varying the angle, we vary the length of the perimeter.  The perimeter should equal the string length.  Those two squares are in green.

Since we know the true radius at that point = 251103108 inches  and the intercept of the line 251103521.6 inches, taking the delta leaves 413.4 inches, in blue.

As an exercise, I used the angle to determine the length of the string, should the excess height be 180 feet (2160 inches).  In this case I get the string length = 1577727371.94 inches (10.94 inches longer than the problem statement)  Should you wish to do this yourself, the angle is .2376495 degrees

On Feb 8, 2013 3:13 PM, "Frank Reed" <FrankReed@historicalatlas.com> wrote:

I got 180 feet for the height and 14.3 nautical miles for the distance to the spot where the string leaves the Earth's surface, but I haven't checked my numbers.

Although it appears as if this question has drifted far from navigation, the problem is actually very close to a traditional navigation calculation, namely, what is the distance to the geometric horizon for a given height of eye? This imaginary "string" around the Earth, if it has been pulled away at just one point (more of a rubber band now) is equivalent to the sight line from an observer at some altitude h looking out to the horizon (where the "sting" re-contacts the surface of the Earth). We do not normally have a need for the direct distance from the observer's eye to the horizon since it differs from the distance measured along the Earth's surface by only a few inches. But in this case, that's exactly what the puzzle requires. That small difference should be half an inch for the puzzle as stated. If the distance measured out to the geometric horizon from the observer's eye exceeds the distance measured along the Earth's surface from the point directly below by half an inch, then how high is the observer (how high has the string/rubber band been pulled)? And how far away is the geometric horizon (the point of contact of the string) in that case? You need the angle, a, as measured at the Earth's center that satisfies:
tan(a)-a=q/R
where q is half an inch and R is the radius of the Earth. If you're working on ye olde "back of the envelope", it helps to use the series expansion for tan(a) which is tan(a)=a+a^3/3+... For the angle, a, I get 14.2 minutes of arc. Then the usual formula for distance to the geometric horizon, d=1.05*sqrt(h in feet)) will give you that height of 180 feet, or you can work it out from plane trig and the series expansion for cos(a) which is cos(a)=1-a^2/2+...

-FER

----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------

View and reply to this message: http://fer3.com/arc/m2.aspx?i=122297 Browse Files

Drop Files ### NavList ### What is NavList? ### Join NavList

 Name: (please, no nicknames or handles) Email:
 Do you want to receive all group messages by email? Yes No
You can also join by posting. Your first on-topic post automatically makes you a member. ### Posting Code

Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
 Email: ### Email Settings

 Posting Code: ### Custom Index

 Subject: Author: Start date: (yyyymm dd) End date: (yyyymm dd)