Path: utzoo!mnetor!uunet!husc6!rutgers!bellcore!faline!karn
From: karn@faline.bellcore.com (Phil R. Karn)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Leaping clocks
Message-ID: <1664@faline.bellcore.com>
Date: 2 Jan 88 07:02:28 GMT
References: <8712311716.aa28742@Huey.UDEL.EDU> <01.JAN.1988.20:20:49.LAWS@RSRE>
Organization: Bell Communications Research, Inc
Lines: 19
Summary: why there are leap seconds

Leap seconds are inserted once per year, on the average. They keep UTC
(Coordinated Universal Time, as broadcast by WWV) within 0.7 seconds of
another time scale, UT1.  UT1 is the time as determined by astronomical
observations of the rotation of the earth, corrected for an annual
fluctuation of about +/- 30 milliseconds caused by seasonal movement of
atmospheric mass and a slight annual distortion in the shape of the earth
caused by solar tidal effects.

Basically, the problem is that the second was defined to be too short with
respect to the average rotation rate of the earth, so additional seconds
have to be inserted periodically into UTC to allow the earth to "catch up".

As you point out there is also a very long term trend having to do with the
transfer of angular momentum from the earth's rotation to the moon. However,
this amounts only to about 1 millisecond per century.

References: Reference Data for Radio Engineers, Howard Sams & co.

Phil