www.timekeepingsite.org How We Keep Time What Time Is It, Anyway ? Our clock and calendar measure the movement of the Sun, the Moon and the Stars as seen from Earth. That's how mankind keeps time and how we became timekeepers. A day comes from the rotation of the earth on its axis, which takes about 24 hours, and the time of day comes from the Sun appearing to travel across the sky at about 15 degrees every hour. The cycles of the Moon's phases is used to measure a Month. The moon takes 29.5 days to return to the same point in the sky, as referenced to the Sun, this is called a synodic month. There are about 7 lunar phases as observed from the Earth. One year is the time taken for Earth to complete one orbit around the Sun relative to the stars, and is called a sidereal year. However, we measure a year to be the period between two successive spring equinoxes known as the Vernal Equinox and is on or about March 21, and this period is called a tropical year. The tropical year is about 20 minutes shorter than the sidereal year. The Moon has a spin of 27.3 days which exactly coincides with one orbit around the Earth. This is the reason that we see the same face of the moon all the time, we never see the back side of the moon. That is because the Moon is in synchronous rotation with the Earth. The Moon rotates on its own axis one time every 27.3 days. The Moon takes 27.3 days to orbit the Earth one time, this is known as a sidereal month. The Moon takes 29.5 days to return to the same point on the celestial sphere (stars) as referenced to the Sun because of the motion of the Earth around the Sun is known as a synodic month (moon-th). •How fast is the Earth spinning? about 1040 miles/hr - 1670 km/hr - 0.5 km/sec •How fast is the Earth revolving around the Sun? about 18.5 miles/sec - 30 km/sec - 30 km/sec •How fast is the Earth, along with the Solar System moving around the Milky Way Galaxy? about 155 miles/sec - 250 km/sec •How fast is our Milky Way Galaxy moving through space? about 185 miles/sec - 300 km/sec Right now the Official U.S. Time is ...... Go Here:

The Second The Second is the basic unit of time measurement in the International System of Units. For many years, until 1956, the second was defined as 1/86,400 of the mean solar day. Advances in physical science in the years following World War II made necessary a more precise definition, and the second was redefined as 1/31,556,925.9747 of the tropical year, as standardized at zero hours, minutes, and seconds on December 31, 1899, by international agreement in 1956. This definition was accepted until 1967, by which time the need for a still more precise and unvarying standard of measurement had become apparent. Scientists abandoned the use of larger, changing bases and decided to redefine the second in terms of electromagnetic wavelengths, that is The Atomic Clock. The Second is now established as 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of cesium-133. The Atomic Clock, with an accuracy of better than one second in six million years, known as the NIST-7, (in service from 1993 to 1999) differs from its predecessors in that the process used for selecting and detecting atomic states involves laser-manipulation rather than magnetic-deflection methods. This was the first major change in design for cesium-beam frequency standards. NIST F-1 Atomic Clock is referred to as a fountain clock, it is the new cesium atomic clock at NIST's Boulder, Colorado Laboratories and it is one of the most accurate clocks in the world, LEAP SECONDS "Civil time is occasionally adjusted by one second increments to ensure 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." For more detailed information from USNO Go Here The Minute The Minute in timekeeping equals 60 seconds. The Hour The Hour in timekeeping is 60 minutes. AM and PM A.M. or AM -- Before noon; ante meridiem or ante meridian. Latin = "before midday." The portion of the day between midnight and the following noon. Also written as a.m. or am P.M. or PM -- Afternoon. Latin = post meridiem or post meridian. Also written as p.m. pm

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