1. Time Michelle Houck April 28, 2008, 4:15pm EST

2. What is time? Standard by which we measure Standard by which we measure One of the fundamental units of physics One of the fundamental units of physics Broken into seconds, minutes, hours, days, weeks, months, years, decades, centuries, etc. Broken into seconds, minutes, hours, days, weeks, months, years, decades, centuries, etc.

3. What is a second? 1 second = the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom 1 second = the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom

4. Mean Solar Time We use the angle of the sun to tell time We use the angle of the sun to tell time 1 rotation of the Earth is one day, broken into 24 hours 1 rotation of the Earth is one day, broken into 24 hours Abbreviated UT1 Abbreviated UT1

5. Greenwich Mean Time Based at Royal Observatory, Greenwich, London Based at Royal Observatory, Greenwich, London Mean solar time – uses the angle of the sun to declare time, 1 rotation of the Earth = 24 hours Mean solar time – uses the angle of the sun to declare time, 1 rotation of the Earth = 24 hours Prime Meridian, all world times are based on this time Prime Meridian, all world times are based on this time Locations east are +hours and locations west are –hours from GMT Locations east are +hours and locations west are –hours from GMT

6. Time Zones Divisions of Earth with hourly displacements from GMT Divisions of Earth with hourly displacements from GMT Could divide the earth into 24 equally sized wedges, but political and geographical boundaries make some time zones irregular. Could divide the earth into 24 equally sized wedges, but political and geographical boundaries make some time zones irregular.

8. Daylight Savings Time Some time zones will move an hour ahead or an hour back twice a year to allow for the hours of sunlight to be at the most convenient part of the day Some time zones will move an hour ahead or an hour back twice a year to allow for the hours of sunlight to be at the most convenient part of the day

9. Coordinated Universal Time Abbreviated UTC Abbreviated UTC Very precise time standard kept using the average of 300 atomic clocks around the world Very precise time standard kept using the average of 300 atomic clocks around the world

10. Leap Year and Leap Second To adjust for slight differences in the solar time and standard time, we periodically add to our time. To adjust for slight differences in the solar time and standard time, we periodically add to our time. Once every 4 years we add one day (February 29) to the year Once every 4 years we add one day (February 29) to the year When The difference between UTC and UT1 becomes greater than 0.6s, a leap second is announced, then either at the end of June or the end of December, one second is added between 23:59:59 and 00:00:00 of the next day When The difference between UTC and UT1 becomes greater than 0.6s, a leap second is announced, then either at the end of June or the end of December, one second is added between 23:59:59 and 00:00:00 of the next day

11. How does my computer know what time it is? The computer has an internal battery so it can keep track of time even when it’s turned off The computer has an internal battery so it can keep track of time even when it’s turned off If you have a continuous internet connection, the computer will automatically synchronize with an internet time server regularly If you have a continuous internet connection, the computer will automatically synchronize with an internet time server regularly You can force the computer to synchronize with a time server You can force the computer to synchronize with a time server Can also set the computer to automatically adjust for daylight savings time, and other similar changes Can also set the computer to automatically adjust for daylight savings time, and other similar changes

12. What time is it? http://www.time.gov/timezone.cgi?Eastern/d/-5/java