Basic Terms of Time Services

Time Scale of Measurement

A time scale is a system of assigning dates to events. There are two widely used time standards. One is based on astronomy and the other is the frequency of atomic oscillations.

Apparent Solar Time

Apparent solar time is the time derived from the sun's apparent position. A sundial indicates the apparent solar time directly.

Universal Time (UT) and Greenwich Mean Time (GMT)

UT0

Since the earth's orbit around the sun is not a perfect circle and its speed changes with its distance from the sun, apparent solar time cannot be a uniform time scale. The length of the solar day is also affected by the inclination of the earth's spin axis to the plane of the earth's orbit. To correct for the non-uniformities, astronomers calculated the effects of the earth's non-circular orbit and the polar inclination on apparent solar time. Mean solar time is apparent solar time corrected for these two effects. Universal Time (UT0) is the mean solar time at the Greenwich Meridian and is called Greenwich Mean Time (GMT).

UT1

There are two refinements of UT0 called UT1 and UT2. As better clocks were developed, astronomers began to notice a discrepancy in UT0 measured at different locations. This was later found caused by a wobble in the axis of the earth. After careful measurements at various observatories throughout the world, the effect of the wobble was eliminated by the introduction of a new time designation UT1.

UT2

With further improvements in the precision of clocks, it was discovered that UT1 had periodic variations caused by the seasonal fluctuation in the rate of rotation of the earth. Corrections were made to remove this effect and an even more uniform time UT2 was obtained.

International Atomic Time (TAI)

In 1967, the Thirteenth General Conference of Weights and Measures adopted a resolution to replace the astronomical definition of the second - the second is the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of Caesium-133 atom. The International Atomic Time (TAI) is the international reference scale of atomic time based on the seconds as defined above in the International System of Units (SI).

It is in the form of a continuous scale, i.e. in days, hours, minutes and seconds from the origin 1958 January 1 d 0 h 0 min 0 s. The atomic time scale is accurate to a few billionths of a second as compared to a few thousandths of a second per day in Universal Time.

Coordinated Universal Time (UTC)

Coordinated Universal Time (UTC) is a compromised time scale introduced in 1972. To ensure that UTC is kept within 0.9 seconds of UT1, provision was made for the introduction of " leap second" in the UTC scale whenever necessary. Thus, UTC differs from International Atomic Time (TAI) by an integral number of seconds. The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS) in Paris.

Leap Year

Gregorian calendar

The Gregorian calendar, which is currently used in most countries of the world, was derived from the Julian calendar. In the Julian calendar, a mean length of 365.25 days for the year is adopted. The calendar year is adjusted to this mean value by inserting an intercalary day every four years. The intercalary or leap year has 366 days and each of the other three years has 365 days. The year with a numerical designation divisible by 4 would be a leap year. The mean Julian calendar year exceeds the length of the tropical year (365.2422 days) by about 11 m 14s.

In 1582, Pope Gregory XIII revised the Julian calendar and adopted a different rule for leap year by omitting the intercalary day in century years that are not divisible by 400. In the Gregorian calendar, there are 97 leap years (366 days a year) and 303 ordinary years (365 days a year) for every 400 years so that the mean Gregorian calendar year is 365.2425 days, quite close to the length of the tropical year of 365.2422 days.

Is the Year 2000 a leap year?

According to the leap year rules of the Gregorian calendar, 2000 is divisible by both 4 and 400 and thus the Year 2000 is a leap year. However, the century years 1700, 1800 and 1900 are not leap years as the figures are not divisible by 400.

Leap second

Coordinated Universal Time (UTC), the basis for our civil time, is occasionally adjusted by one second called leap second to insure that the difference between a uniform time scale defined by atomic clocks does not differ from the Earth's rotational time by more than 0.9 seconds.

Since the first leap second in 1972, all leap seconds have been positive.

Click here for the historical list of leap second.

Julian Date (JD) and Modified Julian Date (MJD)

Julian Day Number is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C. The Julian Date is the Julian day number followed by the fraction of the day elapsed since the preceding noon. Modified Julian Date (MJD) is defined as: MJD=JD-2400000.5

Julian Date (JD) and Modified Julian Date (MJD)

MJD

Local Time

The world has been divided into 24 time zones, each centred on lines of longitude at 15 degrees intervals so that every country falls within one or more agreed time zone. The Greenwich Meridian (also called Prime Meridian) lies at centre of the first zone. Places west of this are one or more hours behind while those east of this would be in front of. In most places, local time differs from UTC by a whole number of hours, depending on the local time zone.

Hong Kong Standard Time (HKT) = Coordinated Universal Time (UTC)+ 8 hours.

Click here for the International Time Zone Map