[time-nuts] UTC and leap seconds

[jimlux]

Hal Murray wrote:
> iovane at inwind.it said:
>> I was wondering, why we assume that  Earth's rotation is slowing down,
>> instead  that clocks are speeding up? 
> 
> The quick answer is that there is a mechanism that explains why the Earth is 
> slowing down: tidal effects.  There is no corresponding way to explain why 
> atomic clocks are speeding up.
> 
> 
> How many different timekeeping mechanisms are there that are accurate enough 
> to notice changes in the Earth's rotation?
> 
> Wikipedia says 2 ms/100 years and that it was noticed by Halley in 1695 and 
> confirmed by Dunthorne in 1749.  I assume they were using the Earth's orbit 
> around the sun as their reference clock.
>

how exactly would that work?  Are they measuring the number of "days" in 
a "year"?  How would one do that? See when the sun crosses in front of a 
specific star (to get the "day") (or an equivalent measurement at night 
of some sort)