Since actual solar days vary throughout the year, a mean solar day of 24 hours is used to denote one revolution. Determinations of the rotation of the earth relative to the sun are made by observing mean sidereal rotation of the earth and converting it to mean solar rotation by ephemeris tables based on the accumulated data of many astronomical observatories. Mean solar rotation derived from uncorrected astronomical observations is denoted UT0.
Annual variations occur in the speed of rotation of the earth and are probably due to seasonal changes in the wind patterns of the Northern and Southern Hemispheres. There is also a semiannual variation due chiefly to tidal action of the sun, which distorts the shape of the earth slightly. The cumulative effect of these variations is that the earth is late about 30 milliseconds or 0.45 arc second near June 1, and is ahead about 30 milliseconds or 0.45 arc second near October 1 each year. When UT0 is corrected for these periodic variations, it is denoted UT1.
Irregular variations in the speed of rotation of the earth also occur. These may be due to turbulent motions in the core of the earth. In addition, friction of the ocean tides causes a decrease in speed of about one millisecond per century. Observations of these effects throughout the world are reported to the Bureau International de l'Heure at Paris, which issues corrections to UT1 to establish UT2. "Standard Times" are based on UT1 or on UT2.
The world is divided into 24 zones, each 15 degrees of longitude, or 1 hour angle, apart. The meridian of Greenwich, England, is the center of the zero zone, which extends to 7.5 degrees east and west. Proceeding eastward from Greenwich, the zones are numbered 1 to 12 with the prefix "plus" to indicate the hour angle to be added to universal time to obtain local "standard time". Proceeding westward, the zones are numbered 1 to 12 with the prefix "minus" to indicate the hour angle to be subtracted from universal time to obtain local "standard time". For example, Washington, DC, at longitude 77 degrees West, is in time zone -5.
Actual boundaries of time zones are defined by law or custom and generally do not coincide with the theoretical zone, even in some places at sea. In many areas, local legal "standard time" differs by 60 or 30 minutes from theoretical standard time.
Note the above does not take into account "daylight savings time" or "summer hours". These are purely set by custom or law, and vary from country to country.
The 24-hour system is used. Numbering starts with 0000 for midnight at the Greenwich Meridian (longitude zero). The first two figures give the hour, and the last two figures give the number of minutes past the hour when the next .8 second tone begins after the announcement.
Prior to 1/1/72, time signals broadcast from WWV and WWVH were kept in close agreement with UT2 (astronomical time) by making adjustments of 100 milliseconds as necessary. On December 31, 1971, the UTC (NIST/NBS) scale was retarded 0.01076 second to give it an initial difference of exactly 10 seconds late with respect to the International Atomic Time (TAI) scale as maintained by the Bureau International d l'Heure (BIH).
Since the new UTC rate (effective January 1, 1972) is no longer adjusted periodically to agree with the rotation rate of the earth, UTC departs more rapidly than before from earth rotation time (UT1), gaining about 1 second per year. Corrections to UTC are now made in step adjustments of exactly 1 second (called a leap second), as directed by BIH. The leap second adjustments ensure that UTC signals as broadcast never differ from UT1 by more than about +0.7 second. Corrections no longer relate to UT2.
P.R.S.G. Notes: 99% of all Shortwave Listeners use GMT time. You should learn how to use GMT correctly, REMEMBER TO ADVANCE THE DATE AFTER 2359!