deletions | additions       diff --git a/The Earth.tex b/The Earth.tex  index 0b69a7f..22c9dc8 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} \begin{align}  a &=& &=  b \\ b &=& &=  c \end{eqnarray} \end{align}  \item {\it What is the mass of the Earth?}