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X-issue: 17.86
Date: Wed, 6 Mar 96 18:54:04 CST
From: (Joe A. Dellinger)
Subject: Leap-years and leap-seconds

	Starting in the 1970's we have had to add occasional leap *seconds*
to keep our official time in sync with the Earth's rotation. A leap second
is a 61st second in a minute (always just before UT midnight), so that some
date specifications such as "23:59:60 Dec 31 1972" are actually perfectly
valid.  This has worked reasonably well so far, but I've read predictions
that within only a couple of decades we'll first start needing leap seconds
every year, then twice a year, then even more and more often as the Earth
slows down... (Note also that the Earth doesn't only slow down, it can
sometimes speed up as well as our moment of inertia changes due to
redistributions of atmospheric mass! To my knowledge we haven't yet needed
an "anti leap-second" but it is bound to happen eventually.)

	What shall we do then? Redefine the second? Keep two separate time
standards, one with a fixed second and another with a stretchy second that
changes to accommodate the Earth? There is already something called
"Ephemeris time" that has been steadily diverging from standard time since

	Another source of confusion is technical terms that mean different
things in different fields. Geophysicists have appropriated the astronomical
terms "Julian Day" and "Sidereal Year". The only trouble is they use
"Julian Day" to mean day number in the current year, and "sidereal year"
to mean tropical year!

  [Some of you have submitted items relating to why do we need leap-days
  when we keep adding leap-seconds, and I have pointed out the difference
  between rotation and revolution to you.  I am delighted to have this
  clear exposition from Joe in RISKS.  TNX]

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