Ross SRG-4400, User Manual

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5–32 • Menu System SRG-4400 User Manual (Iss. 06)
Additional Information for the Leap Second Event (UTC)
If the menu line “Leap Second” in the UTC DateTime menu is set to User Defined, the time and
date fields of this menu can be edited. If the menu line “Leap Second” is set to Off, editing is
disabled.
The menu for Leap Second Event (UTC) indicates the time and date for the next addition or
subtraction of a Leap Second. At the indicated time and date, a second will be added to or
subtracted from UTC Time. See previous paragraphs regarding editing of the available menu
lines.
Leap Second Event
The Leap Second event is not regular; indeed it is usually only decided a few months in advance
when a Leap Second is to be added. For this reason it is not possible for all versions of the
SRG-4400 software to have the time and date of the next event coded into the software.
Historically the Earth's spin is slowing down but some scientists think that global warming may
make the Earths spin increase. In this case, Leap Seconds would occasionally need to be
subtracted.
When a positive leap second is added at 23:59:60:00 UTC, it delays the start of the following
UTC day (at 00:00:00:00 UTC) by one second, effectively delaying the UTC clock.
Announcement of Leap Seconds
The International Earth Rotation and Reference Systems Service (IERS) announces the insertion
of a Leap Second whenever the difference between UTC and UT1 approaches 0.6 s, to keep the
difference between UTC and UT1 from exceeding 0.9 s. IERS publishes announcements every
six months, detailing whether Leap Seconds are to occur or not, in its “Bulletin C”. Such
announcements are typically published well in advance of each possible Leap Second date -
usually in early January for June 30 and in early July for December 31. Because the Earth's
rotation rate is unpredictable in the long term, it is not possible to predict the need for them more
than six months in advance. Leap Seconds have been added since 1972:
Table 5.9 Leap Seconds
Year June 30 December 31 Year June 30 December 31 Year June 30 December 31
1971 -- -- 1986 0 0 2001 0 0
1972 +1 +1 1987 0 +1 2002 0 0
1973 0 +1 1988 0 0 2003 0 0
1974 0 +1 1989 0 +1 2004 0 0
1975 0 +1 1990 0 +1 2005 0 +1
1976 0 +1 1991 0 0 2006 0 0
1977 0 +1 1992 +1 0 2007 0 0
1978 0 +1 1993 +1 0 2008 0 +1
1979 0 +1 1994 +1 0 2009 0 0
1980 0 0 1995 0 +1 2010 0 0
1981 +1 0 1996 0 0 2011 0 0
1982 +1 0 1997 +1 0 2012 +1 0
1983 +1 0 1998 0 +1 2013 0
1984 0 0 1999 0 0 2014 0
1985 +1 0 2000 0 0 2015