[LEAPSECS] comments on DRR TF.460-6

[ashtongj]

Tony asked "Are there any requirements for mean solar time other than 
astronomy and celectial navigation?"


Land surveyors still use sun and star sights to find azimuth. While GPS 
can also be used for this function, the total station (theodolite and 
laser distance meter combined) is an indispensable piece of capital 
equipment. Smaller firms and single-person firms can avoid the expense 
of GPS equipment by doing sun and star shots and not bidding on projects 
where GPS is essential. Land surveying GPS requires a site relatively 
free of trees. Trees of course can also interfere with astronomical 
observations, but there could be gaps that would be sufficient for 
astronomical observations but inadequate for GPS.

The tree cover problem can be overcome through the time and expense of 
setting up a GPS line of known direction and traversing to the wooded area.

The problem of not being able to obtain DUT1 directly from audible 
shortwave time broadcasts could be overcome by obtaining delta UT1 from 
the internet. I don't know if any of the software surveyors use to 
reduce their results limit delta UT1 to < 0.9 s or not.

Observe that the Astronomical Almanac still publishes Polaris tables, 
suggesting that someone out there is still obtaining directions from 
Polaris. I understand, although I haven't done the math myself, that 
depending on the position of Polaris, the time accuracy required to 
obtain an azimuth accuracy of 1 arcsecond ranges from 4 seconds to a few 
minutes. One could easily argue that by the time the error becomes great 
enough to create real problems, star sights will be totally abandoned. 
But the time accuracy requirement for other bodies is stricter, and it 
may not be possible to observe Polaris because of local obstructions.

Gerry

On 2010-09-21 8:26 AM, Tony Finch wrote:

> On Mon, 20 Sep 2010, Robert Seaman wrote:

>>

>> To say that "leap seconds were devised to keep the UTC time scale in

>> close alignment with earth time making UTC useful for celestial

>> navigation" is to suggest two unsupported assertions.  First that no

>> other requirements for "earth time" exist, and second that UTC is

>> responsive only to the evolving needs of those who used to have a

>> requirement for celestial navigation.

>

> Are there any requirements for mean solar time other than astronomy and

> celectial navigation?

>

> Tony.