The Time of Internet - Glossary
Acronyms, conventions and terms used in time metrology and referenced
in these pages.
- 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.
- CEST: (Central European Summer Time) day-light saving time, 1
hour in advance on CET time.
- 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.
- 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 .
- 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).
- 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.
- Zero time zone: time zone centered on the Greenwich meridian
in England. This time zone adopts UTC time.
- GMT: (Greenwich Mean Time) see UT time.
- 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 .
- Meridian: the meridian of a place on the Earth surface is the
half circle passing through that place and ending at the poles.
- Zero meridian: meridian of the Greenwich
astronomical observatory in England.
- MET: old name for CET time.
- 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.
- 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.
- 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
- 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.
- UT1: see UT time.
- 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 .
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