Lecture 4 - Time Systems Flashcards
What is time?
A measure in which event can be ordered from the past through the present into the future.
A measure of durations of events and the intervals between them.
What causes time?
Motions and forces.
List commonly-used time systems.
Sun, tides, atomic clock, solar time, sidereal time, universal time, atomic time, coordinated universal time, dynamic time, GPS time.
Discuss ‘time reference’.
Requirements: Motion is continuous and periodical, the period of motion is stable steady, repeatability of motions period.
Uses the spin of Earth about its axis. The spin rate appears to be constant. Use sun as reference: solar time.
What is solar time?
Time kept or measured by the sun; and its basic division, the day, has been recognized since the dawn of history.
What does solar time measure?
The sun’s apparent motion along the daily course that it appears to trace out in the sky from east to west (the daily rotation of the Earth about its polar axis). Measured by the apparent position of the sun on the celestial sphere. Local time dependent on the longitude of the observer.
What are 2 types of solar time?
Apparent solar time and mean solar time.
What is sidereal time?
Means ‘star time’. Sidereal time of a location on the surface of the Earth is the time elapsed since the transit of location at vernal equinox.
Are solar time and sidereal time the same time?
No.
Why is a sidereal day about 4 minutes shorter than a solar day?
On average, 1 sidereal day lasts 23 h 56m. Earth does not actually spin around 360 in one solar day. The Earth is in orbit around the sun, over the course of one day, moves about 1 degree along its orbit (360 degrees/365.25 days for a full orbit ~ 1 degree/day). Therefore, the Earth has to spin 361 to make the Sun look like it traveled 360 degrees around the sky.
What is atomic time? What are 2 types?
The duration of 9192631770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the Cesium 133 atom.
Dynamic atomic time: atomic clocks that move in a gravitational field.
Coordinate atomic time: atomic clocks that do not move and are not in a gravitational field.
What is GPS time?
US Naval Observatory monitors timing of GPS to provide a reliable and stable coordinated time reference for GPS satellite navigation system
GPS time is given by its composite or ‘paper’ clock consisting of all monitor station and satellite operational frequency standards.
What is space-time continuum?
Due to high accuracy positioning for a variety of applications, traditional 3D coordinate system has been expanded into 4D
What is mean solar time?
The hour angle of the mean Sun +12 hours.
