Time is a very complex subject that you can devote your entire life to. Today we’ll cover a few of the basics and enough to get your interest up! We’ll see that it’s difficult to know what a second is and how long relative times are, but absolute time is even messier! We also discuss dried coffee and tetris!
Importance of Time (and why it’s on a geology show)
- It synchronizes the world and our human interactions (need minutes - hours accuracy generally)
- It allows us to talk about events in a common coordinate system
- Allows synchronization of scientific measurements and comparison of data sources. This is really important for seismometers for EQ location!
- Let’s us use GPS! 1 billionth of a second (nano second) error in 1 GPS satellite, GPS receiver is +/- 1 ft to satellite, which is 2–3 feet on Earth.
- Burning candles in marked cases
- Water powered clocks
- Pendulum clocks Galileo and Huygens (fancy temperature compensation as well)
- Video on Galileo
Modern Time Keeping (Atomic Clocks)
- First clock was ammonia maser at National Bureau of Standards in 1949, but it really wasn’t all that accurate. It was more of a proof on concept device
- First cesium clock was in 1955 at the National Physical Laboratory (UK)
- Leads us to the definition of the SI second he duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the caesium–133 atom
- The NIST-F2, a cesium atomic fountain clock, is good to one second in 300 million years. How F2 works is a combination of feedback control loops, lasers, and really cold atoms.
- Remember, atomic clocks tick away seconds, they say nothing about the hours, minutes, seconds notation we use to write time. We just define a frequency
- Can’t predict them far into the future because of irregularities in Earth’s rotation
- Announced ahead of time in a bulletin by the International Earth Rotation and Reference Systems Service
- 25 leap seconds since 1972
- Next leap second is this year! June 30, 2015
- Real problem in computing, has caused software and GPS hardware crashes/issues before
- Google smears the second out over a period prior to the leap
There are TONS of time standards, we’re only going to touch on a few. Most are known with highest precision in retrospect!
- Exactly what you would think, it’s about using the sun’s position as a time source. There is the sundial time (apparent solar time) that changes throughout the year, and the mean solar time which is like a clock time.
- The equation of time represents the difference between the mean and apparent solar day
- Star clock
International Atomic Time (TAI)
- A measurement of proper time (it’s a relativity thing)
- Weighted average of over 400 atomic clocks
- If there is an error, it isn’t corrected. This makes it into terrestrial time.
Universal Time (UT)
- This is what we used to call GMT!
- Based on Earth’s rotation w.r.t different bodies (why there is UT0,UT1,UT1R,UT2,UTC)
- UT1 is really mean solar time at the equator
Coordinated Universal Time (UTC)
- Formalized in 1960
- Adjustments were accommodated by leap seconds starting in 1972
- Generally considered to be GMT, but GMT isn’t defined/recognized by the scientific community
- This comes from TAI by accounting for leap seconds!
Epoch time (Unix Time)
- Epoch time is the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970
- No leap seconds by definition, but implementation is actually messy
- Stored is an integer (32-bits) meaning that it will run out and roll over on Tuesday 2038–01–19 One second after 03:14:07 UTC, it’s the year 2038 problem.
- Coffee rings and coffee disks: Physics on the edge
- Particle shape controls movement during drying
- The can be applied to surface design, paints, and more
Shannon Dulin - @ShannonDulin