Mention lunar timescale effort

* theory.html: Mention the ESA’s effort on lunar timescales.
(Thanks to Tim McBrayer.)

theory.html

@@ -1361,7 +1361,15 @@ They sometimes disagree.
 </section>
 
 <section>
-  <h2 id="planets">Time and time zones on other planets</h2>
+  <h2 id="planets">Time and time zones off Earth</h2>
+<p>
+The European Space Agency is <a
+href='https://www.esa.int/Applications/Navigation/Telling_time_on_the_Moon'>considering</a>
+the establishment of a reference timescale for the Moon, which has
+days roughly equivalent to 29.5 Earth days, and where relativistic
+effects cause clocks to tick slightly faster than on Earth.
+</p>
+
 <p>
 Some people's work schedules have used
 <a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars">Mars time</a>.