On Jun 4, 4:18 pm, Quadibloc <[EMAIL PROTECTED]> wrote: > It isn't the center of the Universe. It's just the place where all of > humanity lives and does its work.
The surface of the earth is where all the well-interconnected, well- maintained atomic clocks are. It is also where all the politicians allocating funding for the operation of those clocks are. > But people who are working on a Mars > trip certainly need a source of time centered on the Sun. People on a Mars trip, or anywhere, need to determine their needs for clock uniformity and synchronization and buy a clock suitable for that purpose. UTC, TAI, or any other time scale may not suffice for your purposes. Given the non-linear relativistic variations, neither UTC nor TAI makes much sense off the surface of the earth. > In my ignorance of these matters, I would have thought that if the Sun > were the immovable center of the Universe, and the Earth were > revolving around it in a perfect circle, time (at least at the poles, > assuming no axial tilt) would progress at a uniform rate on the Earth > as viewed from the Sun, just changed by a *fixed percentage* due to > relativistic effects. We don't live on that Earth, and even if we did the answer is still No, because of tides. Again I'll refer to this article. http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_59/iss_3/10_1.shtml Clocks tick slower in the daytime than at night because they are deeper in the sun's gravity well in the daytime. The effect has an amplitude of about 2 ps. The first team to measure it will win a Nobel prize. > The Sun's proper motion is approximately linear, so its reference > frame is inertial, meaning its motion changes nothing. (Disregarding the possible use of an ensemble of millisecond pulsars) there are no precision clocks anywhere but on the (diurnally tidally heaving) surface of the earth. There is little point defining a practical convention based on something for which there is no means of verifying the measurements. Even getting funding to try to measure such things is difficult. > Is the problem due to the fact that the Earth's orbit is elliptical? > If that's the only cause, the effect must be a rather small one. For measuring pulsar time arrivals that effect is relevant. Small enough is a matter of taste -- de gustibus non disputandem -- or, more practically, a matter of your need for precision and uniformity of timekeeping. To within some engineering limit you can throw money at the problem and buy or build a better clock for your use. At the picosecond level your laboratory will never agree with someone else's laboratory. TAI is a best effort, statistically-based, politically acceptable, practical implementation of a coordinate time based on current levels of human cognition. UT1 is the same sort of measure of earth rotation. When clocks reach the picosecond level the globally- averaged TAI may not suffice for your needs. Mean solar time is already accurate enough at a level of 1 second. In either case this may or may not be big enough to cause problems for you, and someone else's needs will almost certainly be different. The only national government which has expressed support for UTC continuing to match mean solar time is the UK. Every other government's position is either to abandon them, or silence. _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
