On Fri, Jan 20, 2012 at 9:30 AM, Bob Camp <[email protected]> wrote: > Hi > > 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 http://www.nofs.navy.mil/about_NOFS/telescopes/fastt.html -- Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
