deletions | additions       diff --git a/The Earth.tex b/The Earth.tex  index ede8c13..fee0da8 100644  --- a/The Earth.tex  +++ b/The Earth.tex 
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My most recent frequent-flyer statement says that there are 3000 miles from Boston (BOS) to Los Angeles (LAX). Since planes travel along great circles, this distance $D_{\rm US}$ represents a segment of the Earth's circumference. There are four time zones (three intervals) from Boston to Los Angeles, out of 24, so $D_{\rm US}$ represents 1/8 of the Earth's circumference.  \begin{eqnarray}  \label{cearth}   C_\oplus = 8 D_{\rm US} &=& 2\pi R_\oplus \nonumber \\  R_\oplus &=& \frac{8 D_{\rm US}}{2 \pi}  \label{cearth}  \end{eqnarray}  \item {\it What is the mass of the Earth?}