🚀 go-pugleaf

Mathematically:

The shortest distance between two points in a EUclidean universe is said to
lie on the one and only line that those two points have in common.  Therefore
one needs only a chalk string and two tacks.  :)

After that we can tie the ends of the string together and draw an ellipse.

Physically:

It is much easier to draw a vertical line.  The field lines for the local
gravitaional pull of the earth are straight.  Take the chalk string and
hang a plumb on the end of it.  For thousands of years this was how buildings
were determined to stand straight.

Now the inaccuracies in the above answers will be caused by ignorable local
curvature effects.  I say ignorable because the correction terms are so small
that we can safely ignore them.  For example, deviations from the local
vertical by earth's grvitational field lines result in about as much 
inaccuracy for a straight plumb line as considering the Sun a point mass
instead of extended body will result in deviations in computing earth's 
orbit....(I admit this is a guess)

-- 
If you blow fire against the wind, take care to not get the smoke in your eyes.
      Big & Growly Dragon-monster        |       bafta@cats.ucsc.edu
     ------> shari brooks <-------       |       bafta@ucscb.ucsc.edu
                             All opinions are my own.