From: Tony Finch <[EMAIL PROTECTED]> Subject: Re: [LEAPSECS] Introduction of long term scheduling Date: Wed, 3 Jan 2007 17:38:35 +0000 Message-ID: <[EMAIL PROTECTED]>

> On Wed, 3 Jan 2007, Magnus Danielson wrote: > > > > Assuming you have corrected for another gravitational field, yes. The > > current SI second indirectly assumes a certain gravitational force, we > > is assumed to be "at sea level" whatever level that is. > > Wrong. The SI second is independent of your reference frame, and is > defined according to Einstein's principle of equivalence. Good point. Thanks for reminding me. > What *does* depend on the gravitational potential at the geoid is TAI > (and TT), since a timescale (unlike a fundamental unit) is relative to a > reference frame. When comparing two realizations of an SI second, compensation of the difference in the reference frame needs to be done. To build up TAI, difference in gravitational force do need to be compensated out. > > We still depend on geophysics to some degree. > > Note that the standard relativistic transformations between TT, TCG, and > TCB is (since 2000) independent of the geoid. So although the realization > of these timescales is dependent on geophysics (because the atomic clocks > they are ultimately based on are sited on the planet) the mathematical > models try to avoid it. Naturally. Cheers, Magnus