Glossary

Acronyms, conventions and terms used in time metrology and referenced in these pages.

1. CET: (Central Europe Time) Central Europe mean Time, it is the time of the first time zone, just eastward the zero time zone, 1 hour in advance on UTC time (UTC +01:00). Italy adopts CET time since 1893.
2. CEST: (Central European Summer Time) day-light saving time, 1 hour in advance on CET time.
3. Julian Date: it is a day numbering system to avoid the wrong dating of the same day, due to the disagreement of different calendars that come one after the other in the series of the historical ages. This system has a cycle (Julian Period) of 7980 years that begins at the noon of 4713 January 1 before Christ and ends at 3267 after Christ. Days are numbered in consecutive order starting from that date, which day has number zero. For example, 1996 January 1 at 12 hours started the julian day 2.450.084.
4. Modified Julian Date: day numbering system derived from the julian date. It was introduced to set the beginning of days at 0 hours, instead than 12 hours and to reduce the number of digits in day numbering. The modified julian date is obtained subtracting 2.400.000,5 from the julian date. As a consequence, the origin of this date (day zero) begin at 1858 November 17 at 0 hours. For example, 1996 January 1 at 0 hours began the modified julian day 50.083 .
5. Day of week: the numbering of day of week begins on Monday that is considered the first day of week (ISO 8601 and UNI 7180-73 standards).
6. Time zone: part of the Earth surface where the same time is adopted by convention. Ideally, it is an area bounded between two meridians spaced by 15 degrees of longitude . Practically, for administrative and political reasons, it is often bounded by state borders that better approximate the two meridians.
7. Zero time zone: time zone centered on the Greenwich meridian in England. This time zone adopts UTC time.
8. GMT: (Greenwich Mean Time) see UT time.
9. Longitude: the longitude of a place on the Earth surface is the angle formed at poles between the meridian of that place and the zero meridian .
10. Meridian: the meridian of a place on the Earth surface is the half circle passing through that place and ending at the poles.
11. Zero meridian: meridian of the Greenwich astronomical observatory in England.
12. MET: old name for CET time.
13. Leap second: second added (or removed) to the scale of UTC time, in such a way to keep the difference between UTC time and the Earth rotational time UT1 below 0.9 second. The leap second is inserted on decision of the International Earth Rotation Service.
14. Week of year: the first week of year is the week that has at least 4 days in that year (UNI 7180-73 standard). In each year there are at least 52 weeks and no more than 53 weeks.
15. TAI: International Atomic Time, a time computed at a later time with statistical methods for error minimization from the time of about 200 cesium atomic clocks spread in about 60 metrological laboratories in the world.
16. UT: Universal Time, also called Greenwich Mean Time (GMT), a time measured with astronomical methods, corresponding to the mean solar time of the zero meridian.
17. UT1: see UT time.
18. UTC: Coordinated Universal Time, the official measure of time in the world, it is independent from the time zones. UTC time flows like the International Atomic Time (TAI) , except for the insertion of the leap seconds .