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# ls5 - AST 3722C summary for lecture on tuesday january 22...

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AST 3722C summary for lecture on tuesday january 22, class #3. warning: this is not supposed to be a substitute for reading the textbook. Lect Part A: finishing chapte 9 recall from last time: 2 kinds of second -- earth-spin second and the atomic-clock second. time systems based on earth's spin: GMT=UT,UT1,local civil time, sidereal times time systems based on atomic clocks: UTC <--- the one astronomers use TAI, ET, TT, TDT, TDB 1 earth-rotation clock second is approximately 1.000 000 03 true (atomic clock) seconds but it varies!!! so it runs slow -- and we must have a leap second every so often. (handout #8.) but note leap second goes to UTC, not UT1, even though UT1 is one that's off .... TAI - UTC = 33 seconds now. with every leap second, it grows by 1 second. TAI does not have leap seconds added to it. There are a few other time systems that have been in use ET, TT, TDT. These are atomic systems -- they don't depend on funny business with earth rotation. TT is still in use - it is simply a known offset from TAI of 32.184 seconds. TT = TAI + 32.184 sec. So it has this leap second offset from UTC too. (right now TT = UTC + 65.184 sec.) handout #9 TDB. barycentric time. has to do with the fact that clocks on earth as we have them are in a gravitaional potential well, and so by General Relativity will run at a different rate than a clock in a different gravitational field. The barycenter of the solar system (SS) is the SS's center of mass -- so is a good reference point to have time measured at if you're worried about finding motions of SS objects, since all SS objects orbit this barycenter. In practics, TDB is only different from TT by a couple milliseconds. why should we care about time at this level? pulsar timings ss objects, orbits anything time variable and quasi-periodic OK, some definitions of years: How long _should_ the year be? well based on the way we live our

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lives, the "tropical" year is probably best. That is time it takes for the Sun to get back to some fiducial point on the ecliptic. But wait -- that time interval is different depending on what your starting point is. (e.g. that interval is some time if you go from vernal equinox to vernal equinox, and it's a different time if you measure it from summer solstice to summer solstice.) why? precession and nutation, changes in earth's orbit. wobbling of earth's axis. so the "tropical year" is really the "mean tropical year", averaged over all possible strating points. 1 mean tropical year = 365.24219 days. note that this is SI days -- so uses the true seconds, not the earth-spin seconds. This is 365 days, 5h 48m 45s. Old days: Julian yr: 365.25 days exactly. 11m 15s diff from tropical year per year. how bad was the julian calendar? after several hundred years of use, it was off by many days. problematic for having e.g. Easter at the right time of year vis-a-vis the vernal equinox.
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