[time-nuts] Determination of the placement of the first pps
albertson.chris at gmail.com
Mon Jan 23 11:43:01 EST 2012
On Fri, Jan 20, 2012 at 9:30 AM, Bob Camp <lists at rtty.us> wrote:
> The original location of the PPS was from astronomical data. That's still
> what ultimately steers UTC.
> More precisely, for the US, there's a telescope at the Naval Observatory
> that watches the sun as it comes overhead. The point it hits this or that
> mark on the telescope tells you what the official solar time is (after some
> math). The current one has been in place since the late 1800's, and it's
> still in use.
It's a transit telescope (one that looks up at the local meridian) but
they don't use the sun. It looks at every start that passes in front
of it, thousands of them every night. Then they reduce the data by
knowing the exact location of every star and the time is pasted
directly overhead. This way they get thousands of measurements every
day. If you use the Sun you get only one per day.
A transit solar scope would be a fun Time Nut project. I think a
primitive one would be a photo cell and a length of wire. Measure the
time when the shadow of the wire sweeps across the cell. The trouble
is that with only one measurements per day it would time years to
build up uSec level data. Even if you placed the cell behind a pin
hold mask and used a fine piano wire, shades and baffles and so on.
This is the telescope that is currently used to measure Earth rotation
Redondo Beach, California
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